Chapter 10DNA, RNA, & Protein Synthesis

Watson & CrickDuring the 1950’sJames Watson – American BiologistFrancis Crick – British Graduate Student• tried to determine the structure of DNA

1953: they came up with the structure- DNA is made up of 2 strands- double helix shape- they relied on other scientists to develop their

DNA model

Wilkins & Franklin

Maurice Wilkins – English physicist and molecular biologistRosalind Franklin - British biophysicist, physicist, chemist, biologist and X-ray crystallographer

- took X-ray diffraction photographers of DNA crystals

ChargaffIn 1949

Erwin Chargaff – American Chemist-discovered the key that lead to the understanding of DNA structure

- same amount of A as T- same amount of G as C

- was key because it means that there is base pairing

Pyrimidine = contains single ring (T & C)Purine = contains double ring (A & G)

Nobel Prize Winners

IN 19621. James Watson2. Francis Crick3. Maurice Wilkins

Why not Rosalind Franklin?

Franklin died in 1958 and could not receive the award, only the living can get a Nobel Prize

ReplicationComplementary base pairs allow for:

1. hydrogen bonds that help hold the 2 strands of DNA molecule together

2. helps explain how DNA replicates before a cell divides (one strand can serve as a template for making a new complementary strand)

What is DNA replication?DNA replication = process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission

What basically occurs?

1. The two nucleotide strands of the original double helix separate along the strands.

2. Each strand serves as a template to make new complementary strands.

3. After replication-2 identical double stranded DNA molecules separate and move to new cells formed during cell division.

Steps of Replication1. helicases = enzymes that separate the DNA strands - Helicases move along DNA molecule, breaking

hydrogen bonds, allowing the 2 strands of DNA helix to split

Steps of Replication2. DNA polymerase = enzymes that add complementary nucleotides to each of the original strands

- they are free floating in nucleus- hydrogen bonds are formed

Steps of Replication3. DNA polymerases finish replicating the DNA and fall off, resulting in 2 separate and identical DNA molecules that are ready to move to new cells during cell division

• Semi-Conserative Replication- 1 strand is new and 1 strand is the original- each (combination) kept (conserved) one of

the 2 original strands

• Replication occurs in many locations otherwise it would take 53 days to replicate

• DNA ligase = enzyme that joins the gaps

What about errors?o only about 1 error occurs for every billion of paired nucleotides addedo DNA polymerases have repair enzymes that “proof read” DNA o example: a friend might

check your term paper for spelling errors

o DNA example: if A pairs with a C instead of T, the repair enzymes would fix error by removing C

Mutation = a change in the nucleotide sequence of a DNA molecule

= can have serious effects on the function of an important gene and disrupt an important cell function

What happens if error is not corrected?

some DNA can be damaged from chemicals and UV radiation from the sun some mutations can lead to cancer explains how mutations can arise and lead to altered cells and organisms

changes allow individuals to survive and reproduce better, so these variations increase in the population over many generations

RNA vs DNA

RiboNucleic Acid• contains sugar ribose

• contains nitrogenous bases uracil (U), cytosine (C), guanine (G), and adenine (A)

• single stranded

• shorter in length

DeoxyriboNucleic Acid• contains deoxyribose

• contains nitrogenous bases thymine (T), cytosine (C), guanine (G), and adenine (A)

• double stranded

• longer in length

Types of RNA• each type plays a different role in protein synthesis

1. messenger RNA (mRNA) – single stranded RNA molecules that carries the instructions from a gene to make a protein

2. ribosomal RNA (rRNA) – part of the structure of ribosomes; ribosomes made of rRNA’s & many proteins

3. transfer RNA (tRNA) – transfers amino acids to the ribosome to make a protein

Flow of Genetic Information

1. TRANSCRIPTION

2. TRANSLATION

3. PROTEIN SYNTHESIS

Transcriptiontrans = “across” scribere = “to write”• process by which the genetic instructions in a

specific gene are transcribed or “rewritten” into an RNA molecule• DNA acts as a template for the synthesis of RNA

•occurs in the nucleus for eukaryotes

• occurs in the DNA containing region for prokaryotes

Steps of Transcription

1. RNA polymerase – enzyme that catalyzes the formation of RNA on a DNA template; binds to a promoter

• promoter = a specific nucleotide sequence of DNA where RNA polymerase binds & initiates transcription

- after RNA polymerase binds to the promoter, DNA strands unwind & separate

Steps of Transcription2. RNA polymerase adds free RNA nucleotides that are complementary to the nucleotides on one DNA strand

• the resulting chain is an RNA molecule• complementary base pairing determines the nucleotide sequence in the newly made RNA

FOR EXAMPLEDNA strand: ATCGACRNA strand: UAGCUG

• transcription uses only a specific region (a gene) on one of the 2 DNA strands to serve as a template

• as RNA polymerase moves past, the separated DNA strands unwind

Steps of Transcription3. RNA polymerase reaches a termination signal

termination signal = specific sequence of nucleotides that marks the end of a gene; a “stop” signal

• RNA polymerase releases both the DNA & the newly formed RNA• RNA made during transcription can be one of many types including mRNA, tRNA, or rRNA

• newly made RNA can now perform its job in the cell

• RNA polymerase can transcribe another gene

the next process…o genetic code = term used for the rules that relate how a sequence of nitrogenous bases in nucleotides corresponds to a particular amino acid

o amino acids are assembled based on instructions encoded in the sequence of nucleotides in the mRNAo 3 adjacent nucleotides (letters) in mRNA specify an amino acid (word) in a polypeptide

polypeptide = a chain of amino acids linked together by peptide bonds

codon = each 3-nucleotide sequence in mRNA that encodes an amino acid or signifies a start or stop signal; no codon encodes more than 1 amino acid

EX: GCU specifies amino acid ALANINE in genetic code

o genetic code is nearly universal to all life on Earth & supports the idea that all organisms share an ancient common ancestor

AUG – start codon – a specific sequence of nucleotides in mRNA that indicates where translation should begin

o certain sequences of nucleotides in mRNA do not code for amino acids, but instead signals for translation to end

stop codons = UAA, UAG, or UGA

Translationtranslation = the process of making of a protein

instruction for making a protein are copied from DNA to mRNA

Every protein is made of one or more polypeptides

20 different amino acids found in the proteins of living things

amino acid sequence determines how the polypeptides will twist & fold into 3-D structure of the protein

the shape of the protein id critical to its function

Steps of Translation

1. (Initiation) 2 ribosomal subunits, tRNA & an mRNA join together• enzymes attach a specific amino acid to

one end of each tRNA according to the genetic code

• other end of each tRNA contains an anticodon

translation or decoding of genetic instructions

anticodon = 3 nucleotides on RNA that are complementary to the sequence of a codon in mRNA

Steps of Translation2. (Elongation) polypeptide chain is put together

a tRNA, carrying the appropriate amino acid, pairs its anticodon with the second codon in mRNA

the ribosome then detaches methionine from the first tRNA & peptide bond forms between methionine & second amino acid

the first tRNA then exits the ribosome

the ribosome then moves a distance of one codon along the mRNA

Steps of Translation

3. (elongation continued) polypeptide chain continues to grow as the mRNA moves along the ribosome

new tRNA moves in, carrying an amino acid for the next mRNA codon

growing polypeptide chain moves from one tRNA to the amino acid attached to the next tRNA

Steps of Translation

4. (termination) polypeptide grows one amino acid at a time

the ribosome reaches the stop codon

newly made peptide falls off

Steps of Translation

5. (disassembly) the components of translation come apart

the last tRNA leaves the ribosome and the ribosome moves away from the mRNA

translation machinery is now free to translate the same or another mRNA

o several ribosomes may translate the same mRNA transcript at the same time

o In Eukaryotes – translation of mRNA occurs only after transcription is finished

o the new ribosome begins translating mRNA almost as soon as the preceding ribosome has moved aside

Human Genomegenome = the complete genetic content

o biologists have deciphered the order of the 3.2 billion base pairs in the 23 human chromosomes

o it would take 10 years to read the total sequence out loud

o the challenge now is to learn what information the DNA sequence actually encodes

o bioinformatics = new field that uses computers to compare different DNA sequences