DNA Replication

Structure of DNA• Watson & Crick

– developed double helix model of DNA• other leading scientists working on question:

– Rosalind Franklin

– Maurice Wilkins

– Linus Pauling

1953 | 1962

Franklin Wilkins Pauling

Watson and Crick1953 article in Nature

CrickWatson

Rosalind Franklin (1920-1958)

Double helix structure of DNA

“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”Watson & Crick

Semiconservative DNA Replication

Base pairing in DNA• Purines

– adenine (A)– guanine (G)

• Pyrimidines– thymine (T)– cytosine (C)

• Pairing– A : T

• 2 bonds

– C : G• 3 bonds

Copying DNA• Replication of DNA

– base pairing allows each strand to serve as a template for a new strand

– new strand is 1/2 parent template & 1/2 new DNA

• semi-conservative copy process

Replication: 1st step• Unwind DNA

– helicase enzyme unwinds part of DNA helix

replication fork

DNAPolymerase III

Replication: 2nd step

Build daughter DNA strand add new complementary

bases Wih the help of DNA

polymerase III

Fast & accurate!• It takes E. coli <1 hour to copy

5 million base pairs in its single chromosome – divide to form 2 identical daughter cells

• Human cell copies its 6 billion bases & divide into daughter cells in only few hours– remarkably accurate– only ~1 error per 100 million bases– ~30 errors per cell cycle

1

2

3

4

What does it really look like?

Transcription

fromDNA nucleic acid language

toRNA nucleic acid language

How Are Different Types of Cells Created and Maintained?

By differential gene expression.

The same genetic information is in all 100 trillion cells of any one person. Different cells use the same blueprint in different ways.

How?

by regulating the flow of information from DNA to protein.

2005-2006

RNA• ribose sugar

• N-bases– uracil instead of

thymine– U : A– C : G

• single stranded

RNADNAtranscription

RNA

RNA is a nucleic acid that uses a different sugar than DNA and the base uracil (U) in place of thymine (T).

RNA Is Single-Stranded which allows it to travel in and out of the nucleus

There are Different RNAs with Distinct Functions

Matching bases of DNA & RNA

• Match RNA bases to DNA bases on one of the DNA strands

U

A G GGGGGT T A C A C T T T T TC C C CA A

U

UU

U

U

G

G

A

A

A C CRNA polymerase

C

C

C

C

C

G

G

G

G

A

A

A

AA

5' 3'

Transcription

The enzyme RNA polymerase opens the DNA strands and synthesizes an RNA complementary to only one of the DNA strands.

RNA polymerase acts here

RNA polymerase

Transcription

The decision to transcribe a gene is the most important step in the control of gene expression.

Transcription starts and stops at distinct sites at the ends of a gene.

A gene

The “Central Dogma”• Flow of genetic information in a cell

– How do we move information from DNA to proteins?

transcription

translation

replication

proteinRNADNA trait

DNA gets all the glory, but proteins do all the work!

mRNA

From gene to protein

DNAtranscription

nucleus cytoplasm

aa

aa

aa

aa

aa

aa

aa

aa

aa

aa

aa

proteintranslation

ribosome

trait

2007-2008

Any Questions??