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AP BIOLOGYGENES

www.njctl.org

September 2011Henriquez, Lageman, Satterfield

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Genes Unit Topics

· Discovery of DNA

· DNA Structure & Semi-Conservative Replication

· RNA Transcription

Click on the topic to go to that section

· Gene Expression, Central Dogma

· Three Types of RNA, Translation

· Article Discussion Day

· DNA Replication

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Discovery of DNA

Return toTable ofContents

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Recall that a DNA is a molecule that stores and transmits genetic information.

Deoxyribonucleic Acid

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DNA

To understand the "secret of life" scientists had to figure out the chemical and physical nature of the gene - the factor passed from parent to offspring that directs the activity of the cell and determines traits.

By applying basic principles of physics and chemistry and keeping up on the latest discoveries of their time, a group of remarkable scientists were able to determine the structure of DNA.

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DNA and Modern Medicine

The discovery of the structure and function of DNA has led to astounding leaps in understanding of biology, heredity, and modern medicine.

"It's impossible to overstate the importance of knowing the structure of DNA." - Francis Collins, Director of the Human Genome Project

Click Here to see a DNA timeline

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"Standing on the Shoulders of Giants"

The proof that DNA is the carrier of genetic information involved a number of important historical experiments. These include:

Griffith Transformation Experiment

Avery-Macleod-McCarty Experiment

Hershy-Chase Experiment

Contributions of Watson, Crick, Wilkins, and Franklin

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Griffith and Transformation

In 1928 British Scientist Frederick Griffith was conducting experiments with mice to determine how bacteria made people sick.

Griffith isolated two different strains of pneumonia bacteria from mice and grew the bacteria on petri dishes in the lab.

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Griffith's Colonies

One strain grew in rough colonies and did not cause disease. The other strain grew in smooth colonies and caused disease.

Did notcause disease

Causeddisease

R straincolonies

S straincolonies

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Mice and the 2 strains

When he injected the mice with the rough (R) strain, they lived. When he injected the mice with the smooth (S) strain, they died.

However, when he heated the S strain of bacteria, killing them, and then injected the heat-killed S strain bacteria into the mice, they did not die.

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Mouse Mortality

Heating the S strain killed the bacteria and prevented them from passing disease to the mice.

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Griffith Experiment Part 2

Griffith then mixed heat-killed disease-causing S strain bacteria with live, harmless R strain bacteria and injected this mixture into mice.

Before neither heat-killed S strain or live R strain bacteria made the mice sick, but the mixture of the two caused the mice to develop pneumonia and die.

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Griffith: Part 2

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What was in the mice lungs?

Griffith examined the lungs of mice that had been infected with the mixture of dead S strain and live R strain bacteria and found them filled with disease-causing bacteria.

This indicated that a chemical factor was transferred from the dead S strain bacteria to the live R strain bacteria that transformed them into disease-causing bacteria.

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What was the chemical factor?

He also noted this factor was passed on as the bacteria reproduced.

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1 What is bacterial transformation?

A The inheritance of genetic material

B The exchange of genetic material between strains of bacteria

C The interaction between strains of bacteria

D The passage of genetic material from parent to offpsring

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2 Why was Griffith's experiment significant?

A It showed that a chemical factor transformed R strain bacteria into S strain bacteria

BIt proved dead bacteria could still transmit disease directly to mice

C It indicated proteins were the source of genetic material

D None of the above

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Oswald Avery, Colin MacLeod, and Maclyn McCarty were the first to demonstrate that DNA was the substance that caused bacterial transformation. Avery's group built on Griffith's work to determine which chemical was responsible for transforming the R strain bacteria.

In the 1930s and 1940s, at the Rockefeller Institute for Medical Research in New York City, Avery and his colleagues suggested that DNA, rather than protein as was believed at the time, was the hereditary material in bacteria.

Avery, Macleod, MacCarty

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Avery's Experiment

They mixed the S lysate with R strain bacteria and determined that the contents of cell parts in the S lysate still allowed transformation to occur.

Lysate

First they repeated Griffith's experiment by mixing heat-killed S strain and R strain bacteria and verifying transformation occurred.Then they lysed the S cells by adding detergent. Detergent disrupts the cell membrane and cell wall, causing the DNA, RNA, proteins and other molecules to spill out.

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Their Three Mixtures

Next they mixed heat-killed S strain lysis containing DNA, RNA, and Protein with R strain bacteria and allocated the mixture into three test tubes:

To tube A they added DNase - an enzyme that destroys DNA molecules.

To tube B they added RNase.

To tube C they added Protease.

Finally, they injected each mixture into the mice and waited for results.

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3 What does the enzyme RNase do?

A Breaks down RNA molecules

B Synthesizes RNA molecules

C Breaks down proteins

D Synthesizes proteins

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4 If DNA were the molecule being transferred from dead S strain bacteria to live R strain bacteria, then the mice injected with DNase treated bacteria would most likely

A Survive

B Die

C Remain unaffected

D Pass on pneumonia to their offspring

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Avery's Results

The results of the experiment showed that the mice injected with both the RNase and Protease treated bacterial cells died.

However, the mice injected with the DNase treated bacterial cells survived.

*Destroying the DNA prevented transformation of R strain bacteria.

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5 What did Avery's experiment prove?

A Bacteria can exchange genetic information

BDNA is the molecule that causes bacterial transformation

C RNA and Proteins are the molecules responsible for transferring genetic information

D DNase breaks down DNA molecules

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Hershey and Chase

Alfred Hershey and Martha Chase conducted a series of experiments helping to confirm that DNA was the genetic material in cells.

Hershey and Chase showed that when viruses (made of proteins and DNA) infect bacteria, their DNA enters the host cell but most of their proteins do not.

Hershey shared the 1969 Nobel Prize in Physiology for his work involving the genetic nature of viruses.

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The Hershey Chase Experment

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6 What would you expect to see if protein had been injected into the cell instead?

A Red-labeled cells

B Green-labeled cells

C Cells containing phosphorous

D Cells containing oxygen

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DNA Structure & Semi-Conservative Replication

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Structure of DNAIn 1962, the Nobel Prize in Physiology and Medicine was awarded to James Watson, Francis Crick, and Maurice Wilkins for their determination of the structure of DNA in 1953.

Unfortunately, the rules of the prize award state it can only go to the living. This meant Wilkin's colleague Rosalind Franklin who collected all the data they used could not receive honor.

Franklin died at the age of 37 in 1958 from ovarian cancer which is thought to be the result of her work with X-ray radiation incurred while doing the research.

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X-ray Crystallography

Franklin and Wilkins used a technique called X-ray crystallography to discover more information about the structure of DNA.

X-ray crystallography is a method of determining the arrangements of atoms within a crystal. When X-rays (a type of electromagnetic wave) strike a crystal, they diffract around electrons. The angles and intensities of diffracted beams can be used to determine the position of atoms and chemical bonds. This diffraction pattern

indicated the double helical shape of DNA

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Watson and Crick's Double Helix Using information from the work of Wilkins and Franklin, James Watson and Francis Crick were the first to propose the double helical nature of DNA.

Francis Crick is also well known for coining the term "central dogma" regarding the flow of genetic information from DNA to RNA to protein.

The day they discovered the helix in 1953, they are said to have left their lab, walked into a pub in Cambridge, England and interrupted the patrons' lunchtime shouting "we have discovered the secret to life!"

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Double Helix

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7 The scientists associated with the discovery of the structure of DNA were:

A Hershey and Chase

B Watson, Crick, Wilkins, and Franklin

C Avery, MacLeod, and McCarty

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8 These scientists showed that DNA was at the root of bacterial transformation.

A Hershey and Chase

B Watson, Crick, Wilkins, and Franklin

C Avery, MacLeod, and McCarty

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9 Four of the following terms all involve the experiment of Hershey and Chase. Choose the one which does not belong.

A helix structure

B DNA

C virus

D host

E bacteriophage

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DNA DNA is made up of two chains of repeating nucleotides.

1 Nucleotide

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DNA

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Deoxyribonucleic Acid

DNA is a good archive for genetic information since the bases are protected on the inside of the helix.

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10 If one strand of DNA is CGGTAC, the complementary strand would be:

A GCCTAG

B CGGTAC

C TAACGT

D GCCATG

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11 Four of the following are associated with DNA. Choose the one which is not.

A uracil

B thyamine

C adenine

D guanine

E cytosine

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12 If one strand of DNA is AGCTGA, the complementary strand would be:

A TCGACU

B TCGACT

C AGCTGA

D AGTCGA

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Replication

The functions of a cell are determined by its DNA.

Cells have to reproduce many times. In complex organisms, trillions of copies are made from one original cell.

But when cells reproduce, their DNA has to reproduce as well.

The structure of DNA reveals how trillions of copies of the DNA in one of your cells can be made, and be nearly identical each time.

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Consider these three facts:

· Each individual strand of DNA is held together by strong covalent bonds.

· The two strands are held to one another by weaker hydrogen bonds.

· Each base (ACGT) attracts only its complementary base (TGCA)

Replication

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DNA Molecule as Template

Each molecule of DNA is made of a template strand and a new strand.

The template is used to make the new strand.

The template strand is also known as the parent strand since it came from the original DNA molecule.

The new strand is also known as the daughter strand.

templatestrand

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DNA nucleotide monomers are made ahead of time and stored in the cell.

When it is time for the DNA to replicate itself, the nucleotides are ready to be added to the new growing strand of DNA.

Replication

DNA polymerase is the enzyme responsible for adding each new nucleotide to the growing strand.

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ReplicationDNA is anti-parallel. Each strand has two ends: a 5' end and a 3' end. Nucleotides can only be added to the -OH end (3`), not the 5` so all strands grow from the 5' end to the 3' end.

Each molecule of DNA is made of one "old" and one "new" strand.The "old" strand is used as a template to make the "new" strand.

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The template strands of the DNA molecule separate and the new strands are made on the inside.

Semi-Conservative DNA Replication

The new strands are made in the 5' - 3' direction.

The result of this process is 2 new DNA molecules each having an old template strand and new strand.

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Two parent strands

One parent and one daughter strand

One parent and one daughter strand

Semi-Conservative DNA Replication

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DNA replication is said to be a Semi-Conservative process. This means that the template DNA strand is partially saved and reused throughout the process.

The base sequence on the template strand will allow for the creation of the base sequence on the new strand.

3' ATCGGGTTAACGCGTAAA 5' template strand

5' ______________________ 3' new strand

Semi-Conservative Replication

What is the sequence of the new strand?

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13 The 3' end of a DNA strand has a phosphate at the end.

True

False

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14 Why does a DNA strand only "grow" in the 5' to 3' direction?

A because DNA can only add nucleotides to the 3' end of the molecule

B because DNA can only add nucleotides to the 5' end of the molecule

C because mRNA can only read a DNA molecule from 5' to 3'

D because mRNA can only read a DNA molecule from 3' to 5'

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15 If the parent DNA strand is 5' ATCGATACTAC 3', what will the daughter stand be

A 5' TAGCTATGATG 3'

B 3' ATCGATACTAC 5'

C 5' UAGCUAUGAUG 3'

D 3' TAGCTATGATG 5'

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16 "Semi-conservative" means:

A the DNA is used slowly

B the DNA is sometimes reused

C the DNA is partially saved and reused in the process

D only part of the DNA is used

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17 Which of the following is a nucleotide unit found in DNA?

A ribose + phosphate + thymine

B deoxyribose + phosphate + uracil

C deoxyribose + phosphate + cytosine

D ribose + phosphate + uracil

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DNA Replication

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In order for DNA replication to occur, DNA strands, which are naturally twisted in the shape of a double helix, must be relaxed, unwound, and opened-up to allow each strand to be copied. Then nucleotides must be added to each strand.

Many enzymes are involved in this process.

DNA Replication In-Depth

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Topoisomerase binds to the DNA strand and cuts the double helix, causing the molecule to untwist and relax.

DNA Replication In-Depth

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Helicase breaks hydrogen bonds between nucleotide base pairs causing the two strands to separate and form areplication fork.

Small proteins called single-stranded binding proteins stabilize each strand.

DNA Replication In-Depth

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18 Which enzyme causes the double helix to unwind by breaking hydrogen bonds?

A Topoisomerase

B Helicase

C Polymerase

D RNAse

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Recall in a DNA molecule, DNA strands are antiparallel, meaning that one strand runs from the 3' OH group at one end of the molecule to 5' phosphate group at the other end of the molecule. The other strand runs in the opposite direction from 5' to 3'.

At the replication fork, the new strand added in the 5' to 3' direction is referred to as the leading strand. The new strand added in the 3' to 5' direction is called the lagging strand.

DNA Replication In-Depth

5'

3'

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On the leading strand, new complementary nucleotides are added continuously by the enzyme DNA polymerase.

*DNA polymerase can only add nucleotides to the 3' end of a parent strand, so the leading strand elongates toward the replication fork in the 5' to 3' direction.

DNA Replication In-Depth

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19 DNA Polymerase adds nucleotides in which direction?

A 3' to 5'

B 5' to 3'

C 3' to 5' and 5' to 3'

D 5' to 5'

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The lagging strand elongates away from the replication fork.

DNA polymerase cannot add nucleotides to the end of this parent strand continuously, and the process of adding nucleotides discontinuously becomes a bit more complicated.

DNA Replication In-Depth

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First an enzyme called DNA primase synthesizes a short RNA primer - a complimentary sequence of RNA that binds to the DNA on the lagging strand.

DNA Replication In-Depth

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The RNA primer allows DNA polymerase to bind and add short segments of nucleotides called Okazaki fragments. Okazaki fragments are between 100-200 nucleotides long in eukaryotes and about 1000-2000 nucleotides long in prokaryotes.

DNA Replication In-Depth

5' 3'

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The short Okazaki fragments are joined together by the enzyme DNA ligase to produce a continuous strand.

DNA Replication In-Depth

5' 3'

Click Here forAnimation

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20 In this diagram, the highlighted arrows are pointing to

A The leading strand

B DNA Polymerase

C Okazaki Fragments

D DNA Ligase

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21 The green dot represents which enzyme?

A Helicase

B Ligase

C DNA Primase

D DNA Polymerase

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22 The highlighted arrow is pointing to the

A Parent strand

B Leading strand

C Lagging strand

D Okazaki fragment

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23 The highlighted arrow is pointing to the

A Lagging strand

B Leading strand

C Replication fork

D Replication bubble

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Lagging and leading strands grow simultaneously resulting in the formation of two new DNA molecules.

DNA Replication In-Depth

Click Here forAnimation

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DNA Replication In-Depth

Cuts double helix to prepare for replication

Breaks H bonds, unwinding DNA strands

Adds nucleotides to parent strand

Synthesizes RNAPrimer

Brings together Okazaki fragments

Short pieces of nucleotidesadded to lagging strand

Write the name of the enzyme that matches the description.

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DNA Replication In-Depth

DNA replication begins at specific sites on a chromosome called origins of replication and can occur at many different places on a chromosome simultaneously.

Click Here forAnimation

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24 DNA replication is initiated at

A The origin of replication

B One site at a time

C The edges of a chromosome

D The lagging strand

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RNA Transcription

Return toTable ofContents

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DNA to RNA

We stated earlier in this chapter that the functions of a cell are determined by DNA, and this is true.

But DNA cannot function by itself...it needs the help of RNA.

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RNA

Recall that RNA is made up of a sugar molecule and phosphate group "backbone" and a sequence of nitrogen bases:

RNA is essential for bringing the genetic information stored in the DNA to where it can be used in the cell.

These bases hydrogen bond in pairs: A bonds to U and G bonds to C.

Adenine (A)Uracil (U) Guanine (G)Citosine (C)

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25 RNA is more stable than DNA.

True

False

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Transcription

Transcription is the process by which RNA strands are synthesized from DNA strands. This is the first step in the transport of the genetic information contained in DNA.

The process of making RNA from DNA is called transcription because the DNA sequence of nucleotides is being rewritten into the RNA sequence of nucleotides, which differ only slightly. The process of transcription is very similar to that of DNA replication.

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TranscriptionIn DNA replication both strands are used as templates.

Which DNA strand is used to make RNA?

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One of the DNA strands has the code for the protein that will be made from RNA; that code is called the gene.

The strand with the genes is called the "non-template strand." This IS NOT the strand that is transcribed.

The other strand is the mirror image of the first, it carries the mirror image of the gene, not the gene itself. It is called the "template strand." This IS the strand that gets transcribed into RNA.

Transcription: DNA Strands

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non-template strand of DNAtemplate strand of DNA

transcription of template strand

RNA

Note: the non-template strand of DNA (the gene)

matches the new RNA strand

This makes sense in that the RNA will be the mirror image of the DNA it is transcribed from. And the non-coding strand is the mirror image of the gene.

Transcription: DNA Strands

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template strand

non-template strand

DNA Strands

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Transcription proceeds along the DNA molecule, coding an RNA molecule. The RNA molecule is always made from the end with a phosphate group towards the end with a hydroxyl group.

Transcription

Just like in DNA replication, RNA is made from the 5' end to the 3' end.

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RNA DNA

A bonds with T

U bonds with A

G bonds with C

C bonds with G

Transcription

Transcription is made possible by the fact that the different bases are attracted to one another in pairs.

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DNA Replication Transcription

Two new double-stranded DNA are produced

One new single-stranded RNA is produced

Adenine from the parent strand bonds with thymine on the new daughter strand of DNA

Adenine on the DNA strand bonds with uracil on the new RNA strand.

The whole DNA molecule is replicated

Only the strand with the code for the gene is transcribed.

Synthesis of both occur in the 5' to 3' direction

Replication vs Transcription

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26 What was the first genetic storage molecule?

A DNA

B protein

C RNA

D amino acid

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27 What molecule is now used to store genetic information?

A DNA

B protein

C RNA

D amino acid

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28 The strand that is transcribed into RNA is called the

A Template Strand

B Non Template Strand

C RNA Strand

D Amino Acid Strand

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29 The strand that is NOT transcribed into RNA is called the

A Template Strand

B Non Template Strand

C RNA Strand

D Amino Acid Strand

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30 Genes are located on the

A Template Strand

B Non Template Strand

C RNA Strand

D Amino Acid Strand

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Initiation Elongation Termination

Steps of Transcription

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olymerase

Non-Template

Promotor Region

An enzyme called RNA Polymerase attaches to the promoter sequence on the DNA. The Promoter is a specific sequence of bases that the RNA polymerase recognizes.

Step 1 - Initiation

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RNA Polymerase synthesizes the new RNA by moving down the DNA template strand reading the bases and bringing in the new RNA nucleotides with the proper complementary bases.

As the RNA Polymerase runs down the DNA, it actually unwinds the DNA!

Step 2- Elongation

Non-Template

new mRNA

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RNA Polymerase gets to a sequence on the DNA called a Termination Sequence. This sequence signals the RNA Polymerase to STOP transcription.

Step 3 - Termination

Non-Template

Termination Sequence

olymerase

The RNA Polymerase falls off the DNA. The new RNA strand separates from the DNA and the DNA recoils into a helix.

Click Here to see an animationof Transcription

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31 The transfer of genetic material from DNA to RNA is called:

A translation

B transcription

C elongation

D promotion

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32 What is the function of the promoter sequence on the DNA?

A it is where the RNA polymerase recognizes and binds to initiate transcription

B it is where the RNA gets copied

C it is where the RNA polymerase binds to on the 5' end of the DNA initiating transcription

D it is where the RNA polymerase binds to on the 3' end of the DNA initiating transcription

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33 If the template strand of DNA is 5' ATAGATACCATG 3', which is the RNA strand produced from transcription

A 5' UAUCUAUGGUAC 3'

B 5' TATCTATGGTAC 3'

C 3' UAUCUAUGGUAC 5'

D 3' TATCTATGGTAC 5'

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34 If the non-template strand of DNA is 3' ACGATTACT 5', which is the RNA strand produced through transcription

A 3' TGCTAATGA 5'

B 3' UGCUAAUGA 5'

C 5' UGCUAAUGA 3'

D 5' ACGAUUAGU 3'

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Gene Expression,Central Dogma

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Evolution

Remember that back in time, the functions performed directly by RNA were taken over by proteins.

The shapes of proteins are determined by the sequence of their amino acids. Proteins must be coded with the correct sequence of amino acids to have the right shape.

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The first step in specifying the amino acids to create a protein is to transcribe the DNA code into RNA code.

Each DNA molecule is very long, and contains codes for a very large number of proteins. The code for a single protein is transcribed into a single strand of mRNA or messenger RNA.The DNA code necessary to specify a single protein is called a gene.

Transcription and Genes

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The gene is coded, on the DNA, with the bases A,T,C and G.

Transcribing that code into a strand of mRNA requires converting that DNA code into RNA code with the bases A,U,C and G. It also requires knowing where each gene starts and stops on those very long DNA molecules.

This process is the beginning of gene expression.

Transcription and Genes

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Gene ExpressionGene expression is the process of taking the code in the nucleic acid and making into the protein.

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35 What is a gene?

A segment on the amino acid

B segment on the protein

C segment on the DNA that codes for a protein or RNA

D segment on the RNA that codes for codons

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36 What is meant by "gene expression"?

A making the protein or RNA coded in the nucleic acid

B making amino acids so they can be made into protein

C making tRNA only

D folding of the protein

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Expressing the information stored on a gene into a protein requires translating:

· First from the 4 letter language of DNA to RNA · Then from the 4 letter language of RNA to the 20 letter language of proteins.

DNA to RNA to Protein

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If one letter in the DNA codes one amino acid, there'd be 16 amino acids that couldn't be specified, since DNA only uses 4 letters.

If a 2 letter code were used, we could specify up to 16 different amino acids (4 x 4); we'd still be short.

So, each amino acid is specified by a 3 letter DNA code; this 3 letter code is called a codon.

Codons

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A codon is a 3 base sequence on either DNA or mRNA that "codes" for an amino acid.

The Universal Genetic Code

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There are two main roles for the additional codons:punctuation and protection.

Codons specify instructions for transcribing from DNA to RNA.

For example, the beginning and end of each gene on a strand of DNA are specified by codons. Since there are hundreds of genes on each DNA strand, punctuation is essential.

Codons

All 4 codons above code for the same amino acid - Leucine.

Redundant Genetic Code

While a codon can only specify a single amino acid, there is more than one codon that can specify that amino acid.

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This is called a "universal" code because ALL LIFE uses the same genetic code... If there were alternative codes that could work, they would have appeared in nature.

This tells us that this code goes back billions of years, beyond LUCA in the first cell... even before that.

While we speak of the translation of these codes from those used in DNA, mRNA and proteins; recognize that these translations occur due to very basic properties of the nucleotides and amino acids.

The Universal Genetic Code

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The 64 Codons61 of the codons code for an amino acid

3 of the remaining codons are STOP codons that do not code for an amino acid. They just signal that translation is over.

1 codon that codes for the amino acid methionine is also the START codon. This codon signals the beginning of the translation process.

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37 Each nucleotide triplet in mRNA that specifies an amino acid is called a(n)?

A mutagen

B codon

C anticodonD intron

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Second Position

A Adenine

B Glycine (Gly)

C STOP

D Arginine

E Valine

38 The codon UAG specifies:

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Second Position

A Adenine

B Glycine

C STOP

D Arginine

E Valine

39 The codon GUG specifies:

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A Adenine

B Glycine

C STOP

D Arginine

E Aspartic Acid

40 The codon GAG specifies:

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41 Which of the following amino acid sequences corresponds to this mRNA strand? CUCAAGUGCUUC

A ser-tyr-arg-gly

B val-asp-pro-his

C leu-lys-cys-phe

D pro-glu-leu-val

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The Central Dogma of Biology

To be used by the cell the DNA information must be transcribed into mRNA strands; then it can be translated into proteins.

DNA

RNA

Protein

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(Francis Crick)

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42 Which one of the following sequences best describes the flow of information when a gene directs synthesis of a cellular component?

A RNA to DNA to RNA to Protein

B DNA to RNA to Protein

C Protein to RNA to DNA

D DNA to Amino Acid to RNA to Protein

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43 The transfer of genetic material from DNA to RNA is called:

A translation

B transcription

C elongation

D promotion

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Three Types of RNA, Translation

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Three Types of RNA

mRNA or messenger RNA : carries the information for protein synthesis. This type of RNA is key to The Central Dogma.

rRNA or ribosomal RNA : a catalyst for protein synthesis

tRNA or transfer RNA : helps in the assembly of amino acids during protein synthesis

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DNA mRNA Proteintranscription translation

The specific RNA that transcribes information from DNA is called Messenger RNA (mRNA); it carries the genetic message to ribosomes, where it is translated.

Messenger RNA (mRNA)

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tRNAs transfer amino acids to the ribosome so that the ribosome can covalently bond them together to form the protein.

RNA, being single stranded, can fold in on itself. In tRNA, the RNA folds into a t-shape.

tRNA

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Notice the hydrogen bonds between the complementary bases.

This is an example of how the sequence of nucleotides in RNA results in a very specific shape.

tRNA

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The Anticodon Loop is a 3 base sequence on the tip that is complementary to the codon on the mRNA.

Active sites of tRNA

The Amino Acid Attachment Site is where the amino acid will attach to the tRNA.

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One side of tRNA binds to the correct amino acid.

The other side binds to the appropriate location on the mRNA strand, by binding to the complementary codon.

Since there are 61 mRNA codes for amino acids, this would seem to require 61 different types of tRNA, one to match each code at one end, and the appropriate amino acid at the other.There are actually 30 - 40 types of tRNA in bacteria and about 50 types in animals. This is possible because of the wobble position on tRNA's anticodon site.

tRNA

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This allows the last letter of the mRNA code to violate the complementary pairing rule. In general, the first two letters specify the amino acid, so this works.

Wobble Position

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44 Why does tRNA fold into its specific shape?

A The sequence and bonding of its amino acids

B The sequence of and bonding of nucleotides

C Its protein structure

D A and B

E A and C

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45 The result of tRNA not working would be:

A ribosomal cell death

B mRNA errors

C creation of faulty proteins

D the synthesis of DNA

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46 Which type of RNA functions as a blueprint for the genetic code?

A rRNA

B tRNA

C mRNA

D RNA polymerase

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tRNAs bond to the amino acid specified by their anti-codon.

The opposite side of each tRNA, the anti-codon, bonds to the matching codon on the mRNA, creating a string of amino acids in the proper sequence.

The ribosome makes covalent bonds between the amino acids. The result is a protein chain with the specified sequence of amino acids.

Translation - An Overview

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Translation Step 1- Initiation

5'

3'

The small subunit of the ribosome attaches to the mRNA at the bottom of the start codon (at the 5' end).

Then the large subunit of the ribosome comes in over the top.

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The ribosome goes to the 5' end of the mRNA because the 5' end is the beginning of where the gene on the DNA was transcribed into mRNA.

Also notice that there are 2 sites within the large subunit:

The P-site where the new protein will emerge

The A-site where the Amino Acids are delivered

5'

3'

Translation Step 1- Initiation

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As the leading edge of the mRNA, with the start code AUG, is exposed in the A site,

tRNA with the code UAC enters the site and hydrogen bonds with it, carrying methionine into the ribosome.

AUG

UAC

Met

The tRNAs, hydrogen bonded to their specific amino acids, surround the ribosome.

Translation Step 1- Initiation

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The methonine is removed from the tRNA and stays in the ribosome to be bonded with the next amino acid. The tRNA leaves the ribosome so another tRNA can enter.

Each tRNA will carry the appropriate amino acid into the ribosome to be bonded in the proper sequence, since each tRNA anticoding site matches the coding site on the mRNA, which is located at the A site of the ribosome.

Because each tRNA has an anticoding sequence it complimentary base pairs with the codon on the mRNA.

Translation Step 1- Initiation

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47 How does the anticodon on the tRNA and the codon on the mRNA match up?

A by hydrogen bonding/complimentary base pairing

B by ionic bonding

C by peptide bonds

D none of the above

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48 Why is Methionine the very first amino acid in all proteins?

A because it is coded by the stop codon

B because it is coded for by AUG which is the start codon

C Methionine is coded for by more than one codon

D none of te above

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The 2nd tRNA with its amino acid is delivered into the A-site in the ribosome.

The ribosome catalyzes a covalent bond between the amino acids.

Translation Step 2 - Elongation

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The tRNA that was in the A-site moves to the P-site and the tRNA that was in the P-site separates from its amino acid and the protein emerges from the P-site

The ribosome moves the mRNA using ATP

Translation Step 2 - Elongation

Elongation continues by adding one amino acid after another.

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UAA is 1 of the 3possible STOP codons.

The ribosome reaches a STOP codon. This signals the end of translation, the completion of the protein.The 2 subunits separate from each other.

Translation Step 3 - Termination

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The Result- A protein in its "primary sequence".

Translation Step 3- Termination

Click Here tosee animationof Translation

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49 What is the first event of translation?

A the tRNA comes in

B the small subunit of the ribosome and the 1st tRNA brings in Methionine to the start codon

C elongation happens

D the large subunit of the ribosome comes in

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50 During translation, the ribosome binds to a:

A DNA

B mRNA

C protein

D peptide bond

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51 What is the first step of translation called?

A transcription

B elongation

C termination

D initiation

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52 What is the P site of the ribosome?

A it is where the amino acids are delivered in

B it is where the protein or peptide will emerge

C it where the tRNA's will deliver in the next amino acid after each translocation

D it is where the proteins fold into their 3-d shape

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53 What is the function of the ribosome in translation?

A it makes a peptide/covalent bond using the energy from translocation

B it makes hydrogen bonds between the codons

C it makes covalent/peptide bonds between the codons

D none of the above

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54 What does termination in translation involve?

A translocation of the ribosome

B the ribosome gets to a stop codon and the small and large subunits of the ribosome separate

C RNA polymerase falls off the DNA

D a tRNA brings in an amino acid

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DNA RNA PROTEIN

replication

transcription translation

The Central Dogma

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55 What is transcription?

A the making of DNA from protein

B the making of RNA from amino acids

C the assembly of the protein

D the making of mRNA from the DNA code/gene

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56 What is translation?

A the assembly of the amino acids from the protein code

B assembly of amino acids coded for by the mRNA codons

C the making of mRNA

D assembly of codons from DNA template

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Article Discussion

Return toTable ofContents

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Intelligence Gene: How Your Parents Determine Half Your Fate

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