DNA

DNA is the molecule of life. All living things contain DNA.

DNA is found in the nucleus of cells.

DNA contains genetic codes that determine physical features.

DNADNA stands for deoxyribonucleic acid. It may sound

gibberish, but the name actually tells us two things:

- DNA contains deoxyribose (a 5-carbon sugar)- DNA is a nucleic acid (molecule made up of

nucleotides)

Nucleotide

Nucleotides consist of a sugar molecule attached to a nitrogen base and a phosphate group.

Sugar

Phosphate

Nitrogen Base

Nucleotides have 3 parts:1) Sugar2) Nitrogen Base3) Phosphate

The Nitrogen Bases

There are 4 possible different nitrogen bases:

-adenine

-guanine

-cytosine

-thymine

These 4 different bases allow for genetic

diversity

Base-pairing rules

DNA is a double stranded molecule – the two strands are connected by the nitrogen bases.

Adenine can only pair with thymine (and vice versa).

Guanine can only pair with cytosine (and vice

versa). C G

T A

Purines vs. Pyrimidines

The molecular structure of the

4 bases fall under two categories:

1) Purines- double ring structures

- adenine and guanine are purines

2) Pyrimidines- single ring structures

- thymine and cytosine are pyrimidines

Purines always bond with pyrimidines (as per the base pairing rules).

HINT:Think OPPOSITES- The BIGGER word is the

smaller molecule; the smaller word is the BIGGER molecule.

C = GA = T Chargaff’s rules

Erwin ChargaffIn 1947, an American scientist named Erwin Chargaff discovered that:the amount of guanine and cytosine bases are equal in any sample of DNA.

The same is true for the other two nitrogen bases: The amount of adenine and thymine are equal in any sample of DNA.

The observation that ______and that ______ became known as _______________.

At the time this observation was made, it was not clear why this fact was so important.

X-Ray Evidence by Rosalind Franklin

In the early 1950’s, a British scientist, Rosalind Franklin began to study DNA.

She used a process called _____________.

She took a large, purified sample of DNA, aimed a powerful x-ray beam at the sample, then recorded the scattering pattern of x-rays on film.

X-ray diffraction

These x-rays suggested that there were two strands, twisted in a helix and the nitrogen bases were in the center of the moleclule.

At the same time Franklin was doing her research with a colleague Maurice Wilkins, two scientists named __________________ and _________________, were trying to understand the structure of DNA by building models of it. They were getting nowhere.

The Players

JamesWatson

FrancisCrick

MauriceWilkins

Rosalind Franklin

Early in 1953, Watson was shown a copy of Franklin’s x-ray patterns, and he immediately realized how the DNA molecule was arranged. Within weeks, Watson and Crick built a model that showed: 1) The _________ of DNA. 2) And explained how DNA could ________ information and be ________.

Francis CrickJames Watson

structurecarry copied

Watson and Crick described the DNA

molecule as a __________ or

spiral consisting of __________ wound around each other.

double helix

two strands

Timeline

1953 – Watson and Crick solve the structure of DNA

1958 – Rosalind Franklin dies of ovarian cancer at age 37

1962 – Watson, Crick and Wilkins win the Nobel Prize

Rosalind Franklin did not receive a Nobel Prize as they must be given

to a living person

DNA- The Double HelixWatson and Crick discovered that the

shape of DNA was a ___________.

_______ = 2 strands of nucleotides

_______ = twisted

Picture a twisted ladder.

double helix double

helix

The sides of the ladder are composed of sugar (deoxyribose) molecules and phosphates. This is called the “sugar-phosphate backbone”.

The nitrogen bases make up the rungs (steps) of the ladder.

The two strands of DNA are connected to each other at the bases.

The bases bond together using hydrogen bonds.

Adenine and thymine have two hydrogen bonds.

Guanine and cytosine have three hydrogen bonds.

T A

GC

Hydrogen bonds are the weakest type of bond.

You might think that DNA should be strongly held together- but it does need to unzip- and quite often!

DNA comes apart during DNA Replication (the copying of DNA).

How can all of this DNA fit inside a cell?

The structure of the chromosome allows the DNA to be packed very tightly inside the cell.

A chromosome is composed of:

DNA and proteins. The DNA is wrapped tightly around proteins called _______.histones

Together, the DNA and histone molecules form a beadlike structure called a ___________. nucleosome

DNA double helix

histones

coils

nucleosomes

supercoils

Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils.

Nucleosomes seem to be able to _______ enormous _______ of DNA into the tiny space available in the cell nucleus.

lengthsfold

Hershey-Chase Experiment

Heredity Information in DNA or the Histone Proteins?

For a long time it was not certain whether the hereditary information being passed along was found in the DNA or in the histone proteins which the DNA wraps around to make chromatin. In 1952, just before the discovery of DNA’s structure by Watson and Crick, an experiment performed by Alfred Hershey and Martha Chase finally gave the definitive answer.

DNA Replication

Recall that DNA is found in the nucleus of all cells.

In order to make more cells (which you are constantly doing), you must make a copy of DNA first!

DNA Replication occurs during the synthesis stage of Interphase in the cell cycle (before the cell actually divides).

DNA Replication- Step 1

The first step required in order for DNA to make a copy of itself is to break those hydrogen bonds between the bases.

An enzyme called DNA helicase breaks the hydrogen bonds and unzips the original parent DNA molecule.

DNA Replication- Step 2

Once the DNA strands are unzipped, the nucleotides are exposed.

The second step involves another enzyme called DNA Polymerase. This enzyme reads the DNA and determines which NEW nucleotides to add to the parent strand.

1

2

1

3

Replication Forks

DNA is a very long molecule that must be tightly coiled and packed into our cells.

If the enzymes had to go from one end of DNA all the way to the other, it would take too long!

Replication forks form at multiple points in the DNA to speed up replication.

2

Replication fork

3

Two replication forks make replication “bubbles”.

5’ and 3’Since DNA is a 3-Dimensional molecule made of linked nucleotides,

it really doesn’t have a “left” or “right”; “up” or “down”.

If we have to refer to DNA’s direction we use 5’ and 3’ (5 prime and 3 prime).

Recall that deoxyribose is a 5-carbon sugar. These numbers (5,3) are in respect to the position on the 5-carbon sugar.

During DNA replication, DNA polymerase READS the parent molecule in the 3’ 5’ direction.

New DNA is synthesized in the 5’ 3’ direction (opposite).

(How to Remember? When you READ a book you would read chapters 3 to 5)

Final Product- DNA Replication

The final product of DNA replication is two molecules of DNA (4 strands total since each molecule is double stranded).

However, it would not

be appropriate to call

the molecules “new”.

DNA Replication is semi-conservative (semi= half; conserve= to save)

Each time DNA is copied, the original DNA molecule is saved. DNA is never destroyed during replication!

Each new molecule consists of one parental strand, and one (new) daughter strand.

The Mechanisms of Replication – A Closer View

This replication of an enormous amount of genetic information is achieved with

very few errors - only one error per 10 billion

nucleotides. The replication is a speedy and accurate

process.

More than a dozen enzymes and proteins participate in DNA

replication.

Accidental changes can occur in existing DNA after replication. The DNA can become ________ from exposure to chemicals, radioactivity, X-rays, ultraviolet light, and molecules in cigarette smoke. Each cell continuously___________________________________. About 130 DNA repair enzymes have been identified so far.

damaged

monitors and repairs its genetic material

Repairing the damage:

1. The damaged segment of DNA is cut out by enzymes called nucleases.

 2. The resulting gap is filled in with

new nucleotides by DNA polymerases.

 3. Other enzymes (ligases) seal the

free ends of the new DNA to the old DNA, making the strand complete.

Damaged DNA

Nucleases cut out the damaged section.

DNA polymerases replace the gaps with new nucleotides.

Ligases seal the new section in place.

Summary Video

DNA Replication

DNA Replication Fork

Self Check Quiz

1. The letters D.N.A. stand for ________________________.

2. DNA is shaped like a _______ _______.

3. The four nitrogen bases are: adenine, ___________, _________, ____________.

4. Adenine always bonds with ____________.

5. Cytosine always bonds with ____________.

6. DNA is important because it determines your physical _______________.

7. DNA replication is _______- _________________.

8. DNA replicates (circle one) [before | after] cell division.

9. DNA replicates using specific [enzymes | carbohydrates].

10. Thymine and cytosine are [purines | pyrimidines].

11. Nitrogen bases are paired together using [hydrogen | covalent] bonds.

Deoxyribonucleic acid

Double helix

thyminecytosine guanine

thymine

guanine

traits

semi conservative

Why DNA is important:

DNA is important because it holds the “recipe” for making proteins.

Your entire body is made out of proteins!

DNA is your personalized instruction manual and yours is unique to you (though everyone in this room shares about 99% of the same DNA, that’s what makes us human!)

The Genetic CodeThe DNA molecule, with its four

nitrogenous bases, is the ____ for all _________ that are made in a cell.

code proteins

Genes are made of _____. A gene is the _________________that controls the production of specific ________, such as enzymes, structural proteins,

oxygen-carrying proteins, etc.

DNA coded DNA instructions

proteins

The DNA inherited by an organism dictates the synthesis of certain

proteins. Proteins are the link between

genotype and phenotype.The proteins that are made will determine what traits show up in the offspring.

Gene expression:

The process by which DNA directs the

synthesis of proteins.

The expression of genes includes two stages:

transcription and translation

DNA is very important; it controls the workings of the cell.

However, it is trapped inside the _______.

In order to get all of its instructions to the rest of the cell, DNA relies on its trusty sidekick....

nucleus

Ribonucleic Acid

R.N.A. is also a nucleic acid- it is made out of linked nucleotides (like DNA). Recall that nucleotides are made of a sugar, phosphate, and nitrogen base.

DNA vs. RNA

RNA and DNA are very similar, but there are some differences.

First of all, DNA is double stranded, and RNA is single stranded. This means that RNA is SMALLER than DNA.

RNA contains 4 nitrogen bases: adenine, guanine, cytosine and URACIL.

*Thymine is NOT present in RNA.

Uracil is complementary to adenine in DNA. It essentially takes the place of thymine.

The last major difference between DNA and RNA is that RNA contains the 5-carbon sugar ribose. (Recall DNA contains deoxyribose).

Ribose has one more oxygen atom than deoxyribose.

Ribose Deoxyribose

RecapRNA is single stranded, so it is smaller than

DNA. This means it can leave the nucleus (which DNA cannot).

RNA contains the sugar ribose.

RNA has 4 bases: A, G, C, and U. The base pairing rules are as follows:

C pairs with G

G pairs with C

A pairs with U

U pairs with ANO thymine in RNA

3 Types of RNA

RNA’s job is to help DNA make proteins.

DNA must deliver its code to the remainder of the cell - it relies on 3 molecules:

1) Messenger RNA (mRNA)

2) Transfer RNA (tRNA)

3) Ribosomal RNA (rRNA)

Messenger RNA

mRNA is complementary to the original strand of DNA. mRNA is first created in the nucleus and then travels to the ribosomes out in the cytoplasm. mRNA uses the DNA’s code (or message) to make proteins!

Example: DNA Strand: G G C T T A

mRNA strand: C C G A A U

Proteins

Recall that proteins are made up of smaller parts called amino acids. Another word for protein is “peptide”.

Individual DNA codes are called “codons”. The codons correspond to specific amino acids. mRNA also has codons, which are complementary to DNA codons.

Codons

Codons consist of groups of 3 nucleotides called triplets.

(Example) DNA codon:cytosine-cytosine-adenine

(CCA for short)

Each codon codes for one amino acid. This is where we need RNA’s help.

CC

A

DNA Template Strand:

A C G T T A G C C

mRNA strand (which leaves nucleus to build proteins):

U G C A A U C G G

mRNA is always complementary to the template DNA strand.

How many codons are there in mRNA above?

What does the other DNA strand look like?

DNA: A C G T T A G C C

mRNA: U G C A A U C G G

cysteine asparagine arginine

Three DNA codons are transcribed into three mRNA codons. mRNA codons are specific to amino acids.

This is the beginning step of PROTEIN SYNTHESIS.

Protein = (well) protein synthesis = to make

DNA Template Strand: A G G C G T T A G

mRNA strand: U C C G C A A U C

1) UCC

2) GCA

3) AUC

Which three amino acids do these mRNA codons code for?

Codon Chart

Amino Acids

Though there are only 20 different amino acids, they are sequenced differently and come in different shapes to make for thousands of different proteins.

Protein Synthesis• A two part process in which DNA is

decoded into corresponding proteins• The first process is known as transcription• The second process is translation• Occurs in the nucleus and cytoplasm

Transcription

Transcription is the first part of protein synthesis.

During transcription, mRNA is created by transcribing the DNA’s code.

Transcription occurs in the nucleus.

(That’s where the DNA is!)

Transcription

During transcription, the enzyme RNA polymerase temporarily unzips DNA and adds complementary RNA nucleotides to the growing mRNA strand.

Transcription

Recall that mRNA is the messenger. It copies DNA’s code (or “message”; “instructions”) and it is now responsible for delivering this message to the rest of the cell.

Once the mRNA strand is completed, it leaves the nucleus (exits via nuclear pores).

Transcription is complete.

(No protein yet...next stop, the ribosomes!)

TranslationTranslation is the final step of protein synthesis-

it involves ALL THREE types of RNA (mRNA, tRNA and rRNA).

• Translation is a process in which the mRNA that was manufactured during transcription is translated into an amino acid sequence (proteins)

• occurs in the cytoplasm, on the ribosomes

Ribosomal RNA (rRNA)= a major component of ribosomes; also helps bond amino acids together to make polypeptides (proteins)!

 The goal of the 3 types of RNA is to work together to make proteins using the DNA’s instructions!

Translation

Transfer RNA (tRNA)= helps transfer amino acids to the corresponding mRNA codons (tRNA is always complementary to the mRNA strand)

mRNA codons: U G C A A U C G G

tRNA anticodons: A C G U U A G C C

tRNA bases are referred to as “anti-codons” because they are complementary to mRNA codons.  

(Example)

If the mRNA codon is CUU, that would translate to the amino acid leucine.

The tRNA molecule that will deliver leucine to the ribosome has the anticodon GAA.

Once the amino acid is delivered, the tRNA releases itself from the ribosome, and leaves to find another amino acid to add to the growing protein chain.

ANTICODON

LEUCINE

Translation tRNA transfers amino

acids to the ribosome. The amino acids are attached to the tRNA via a specialized enzyme called tRNA synthetase.

Analogy: tRNA is a librarian; mRNA codons are the book codes; amino acids are the books. The tRNA librarian reads the mRNA codons and fetches the appropriate amino acid book to add to the protein chain being created.

Label the following:mRNA, tRNA, ribosome, codons, anti-codons,

peptide (protein) chain, amino acid

Example: Putting It All together! If the sequence on the DNA molecule calls for a protein with the following DNA codons:(1) What would be the sequence of the mRNA?(2) What would be the sequence on the tRNA? (3) What would be the amino acid sequence of the protein being made? DNA TAC TTA CAA ACC ATA ATT mRNA  tRNA  

AUG AAU GUU UGG UAU UAACODONS

ANTICODONS UAC UUA CAA ACC AUA AUU

Methionine Asparagine Valine Tryptophan Tyrosine STOPAmino AcidSequence

Putting it all together

DNA codes for proteins

Proteins being made is called protein synthesis

Transcription occurs in the nucleus. The final product of transcription is an mRNA strand.

Translation occurs on the ribosomes. The final product of translation is a protein.

CodonsThere are 64 possible

codons:

43 = 64

64 codons for 20 amino acids.

Can more than one codon specify the same amino acid?

4 possible bases (A, T, C or G)

3 bases in a codon (triplets)

Start and Stop Signals

Proteins are made up of a very specific sequence of amino acids.

DNA contains “start” and “stop” codons so that the cell knows where to start decoding proteins and where to stop.

The start codon= AUG (codes for the amino acid methionine)

Stop codons= UAA, UAG, UGA

Summary Videos

Protein Synthesis

Prokaryotes vs Eukaryotes