What you need to know:

How is DNA replication preserve the sequence of bases?

What is semi-conservative replication?

How is DNA different from RNA?

DNA Replication

Theories of DNA replication

Conservative Original DNA double helix acts as a template for producing a new templateThe original double helix

is conserved

Destructive

Nucleotides from the original strands end up in all daughter strands

The original double helix is destroyed

Semi Conservative

... the strands are separated and each daughter double helix inherits one original strand

The original DNA strands are conserved, but...

adenine guanine cytosine

tyrosine

How do we know which theory is correct?

Note: all bases contain nitrogen

The Meselson-Stahl Experiment

Bacteria grown on a source of heavy nitrogen (15N) produces heavy DNA

Bacteria grown on a source of light nitrogen (14N) produces light DNA

We can measure the weight of the DNA in a cell by extracting it and then centrifuging it in a caesium chloride gradient

DNA sample

CsCl gradient

Low CsCl conc.

High CsCl conc.

Centrifuge: spin very fast

When centrifuged...... molecules migrate until they reach a layer of the same density

Bacteria fed 14N Bacteria fed 15N Bacteria fed 15N then one generation on 14N

Intermediate band implies that DNA replication is

not conservative

Light band

Heavy band

Bacteria fed 15N then two generations on 14N Further generations on 14N

Light band gets bigger Intermediate band remains the same size Heavy band does not reappear

DNA replication is semi conservative

The two original strands act as templates for the synthesis of two new strands

AC TT AG

AC TT AG

AC TT AG

AC TT AG

CG

Free nucleotides pair with complementary bases

Template strandPairing of complementary bases preserves the sequence of bases when DNA replicates

AC T

T AG

AC T

T AG

AC T

T AG

AC T

T AG

C

G

The enzyme Helicase:

- unwinds the double helix

- breaks the hydrogen bonds between complementary bases

- separates the two strands

The Details:

AC T

T AG

AC T

T AG

AC

TA

CT

C

TA

G

TA

G

G

Free nucleotides pair up with complementary bases, and are held in place, by hydrogen bonding

Helicase

AC T

T AG

AC

TA

CT

AC

T

TA

G

TA

G

TA

G

DNA Polymerase creates two new strands of DNA by creating covalent bond between adjacent nucleotides

TG

TA

GT

A

G

AC

TC

AC T

T AG

AC

TA

CT

AC

T

TA

G

TA

G

TA

G

DNA Polymerase

TG

TA

GT

A

G

AC

TC

New DNA strand New covalent bonds

RNA – Ribose Nucleic Acid

Phosphate

Ribose (Pentose sugar)

Base

• cytosine (C)• guanine (G)• adenine (A)• uracil (U)

RNA is single stranded

Function of RNA – to form a copy of a gene which can be used by ribosomes as a template for synthesising a polypeptide

Each gene is a template for the production of one polypeptide

The copying of a gene from DNA to RNA is called transcription

The ‘reading’ of a RNA copy of a gene and synthesis of a polypeptide chain by the ribosome is called

translation

Homework:

Outline the process of DNA transcription

Explain what a codon is