Ch. 10

DNA, RNA and Protein Synthesis

10.1 DNA Structure

Discovery of DNA

Frederick Griffith: 1928

Discovered that harmless bacteria could change & become virulent

Griffith called this process transformation

Avery & McCarty: 1944 □ DNA is the transforming factor.

Hershey & Chase: 1952

Studied bacteriophages

Proved conclusively: DNA carries genetic code.

DNA Structure

polymer of nucleotides

nucleotide has 3 parts:

- deoxyribose (5-C sugar)

- phosphate group

- a nitrogenous base

Nitrogenous Bases

DNA contains 4 nitrogenous bases:

Purines (double ring)

1. adenine (A)

2. guanine (G)

Pyrimidines (single ring)

3. cytosine (C)

4. thymine (T)

nucleotides link together - form long chains

backbone of DNA: sugars and phosphates bond covalently

bases stick out from chain:

DNA is double stranded P

S B

P

S B

P

S B

X-ray Evidence

Franklin & Wilkins (1950)

took x-ray of DNA molecule

Indicates DNA molecule is twisted

Watson & Crick (1950’s)

built 3-D model of DNA

Franklin’s x-ray helped w/ structure

Watson & Crick twisted model like a spiral:

“double helix”

Chargraff

• research showed:

% adenine = % thymine

% cytosine = % guanine

• So…

adenine pairs w/ thymine

(A - T)

guanine pairs w/ cytosine

(G - C)

Base Pairing (A-T & C-G):

hydrogen bonds holds 2

strands of DNA together

1962 Watson, Crick & Wilkins receive Nobel Prize

for discovery of DNA structure

**Franklin did not receive prize because she had died (prize not awarded post-humously)

10.2 DNA & RNA

DNA Replication

Each strand is a pattern for a new strand.

Before a cell can divide:

DNA must replicate so each new cell has a complete copy of DNA

Watson & Crick hypothesized that DNA UNZIPS:

- Enzyme (DNA helicase) breaks bonds between base pairs

- New base pairs match up w/ freed bases (DNA polymerase catalyzes reaction)

T

A

G

C

C

T

A

G

C

The DNA Code

DNA has set of instructions coded in sequence of nucleotides.

To decode message: part of the DNA sequence is copied into RNA.

RNA: nucleic acid that is a messenger between DNA and ribosomes.

RNA carries out protein synthesis

Structure of RNA

Long polymer of nucleotides

Differences from DNA:

RNA sugar is ribose (DNA is deoxyribose)

RNA is single stranded (DNA is double)

RNA has Uracil instead of Thymine

U bonds with A

3 Forms of RNA

Each form has a different job:

1. messenger RNA (mRNA)- carries DNA code to ribosome

2. transfer RNA (tRNA)- brings amino acids to ribosome

3. ribosomal RNA (rRNA)- forms structure of ribosome

RNA Transcription

DNA’s code is copied into RNA

Transcription transfers info from DNA to RNA

RNA polymerase (enzyme) attaches to DNA

It separates stands, & builds complimentary strand of RNA

ex:

DNA: A C T A T A C A T G

RNA: U G A U A U G U A C

10.3 Protein Synthesis

Protein Synthesis

Nucleotides arranged in groups of 3 bases called a Codon

Each codon codes for 1 amino acid

Remember: proteins are polymers of amino acids

All proteins are made of different combinations of 20 amino acids

One codon (AUG) is the start codon

Codes for the a.a. methionine (met)

3 codons are STOP codons: UAA, UAG, UGA.

Do not code for an a.a.

Stops protein synthesis

Translation

Translation is the decoding of mRNA message into protein.

mRNA does not work alone:

tRNA brings a.a. to ribosomes

rRNA forms ribosome (site of protein synthesis)

Anticodon - 3 bases on tRNA molecule

Anticodon base pairs w/ codon on mRNA

matching tRNA anticodon to mRNA codon, puts correct a.a. place

Steps in protein synthesis

1.Ribosome binds to molecule of mRNA

2.START codon (AUG) binds to 1st tRNA anticodon

3.2nd tRNA anticodon binds to 2nd mRNA codon

4. Peptide bond forms between 2 a.a.

5. protein grows until ribosome reaches STOP codon

6. no anticodon matches STOP codon: protein synthesis stops

7. ribosome releases new protein