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F : Genetic control

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Objectives

Todays session, well learn:

.. What are nucleotides

.. Structure & function of DNA & RNA .. DNA replication

.. transcription (DNA RNA)

.. translation (RNA Proteins)

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Recap

Remember Mitosis

What happens before the M phase

Why chromosome must divide before mitosisoccur

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DNA Replication

What is DNA replication/DNA synthesis

is the process of copying a double-stranded DNA strand

doubling DNA

during interphase (S phase of cell cycle)

There were three theories

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Possible mechanisms of DNA replication

i) The conservative model

The parental DNA remains intact

Separate daughter DNA copies are built from newmolecules of deoxyribose, phosphate & organic bases

Outcome: 1 will be an entirely new molecule

1 will be entirely the original material

Conservative Replication

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The semi-conservative model

DNA molecule splits into 2 separate strands & each of the

strands are replicated

Each of the 2 strands will have 1 strand of new material & 1strand of the original material

Semi-conservative Replication

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iii) The dispersive model

parental DNA would be broken down

nucleotides replicated before being dispersed of randomly

throughout the new molecule The new molecules will contain both the new & original

material but are randomly distributed & not necessarily in

the same amount in each molecules

Dispersive Replication

The experiments of Meselsohn & Stahl

http://highered.mcgraw-

hill.com/olc/dl/120076/bio22.swf

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Proven: DNA replicates semi-

conservatively

Each new double helix has 1 originalstrand and 1 newly synthesised strand

DNA replication semi conservative

Original Gen #1 Gen #2

Conservative15N15N 15N15N

14N14N15N15N14N14N

Semiconservative

15N15N15N14N

15N14N14N14N

Dispersive15N15N

VARIED VARIED

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DNA Replication

DNA strands are antiparallel

(like divided highway) & complementary

One strand is 5 to 3 & the complimentary strand is

3 to 5 So each strand can serve as a template for the

reproduction of the opposite strand

The template strand is preserved as a whole piece

The new strand is assembled from nucleotides This process is called semi-conservative replication

Ideally, the two resulting strands are identical

http://www.uam.es/personal_pdi/ciencias/jhermoso/documents/meselsonystahl_000.swf

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3 enzymes

Gyrase (unwinds DNA)

Helicase (breaks H-bonds) DNA Polymerase (binds free nucleotide)

DNA Ligase (joins okazaki fragments of

nucleotide)

DNA endonuclease (proof-reads new DNA double

helix)

DNA Replication

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DNA Replication (i) Unwinding of DNA double strand

Gyrase

Breaking hydrogen bonds between

complementary base pairs

Helicase

Replication forkcreated

Each DNA acts as template

DNAReplication Fork

http://highered.mcgraw-

hill.com/olc/dl/120076/micro04.swf

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DNA polymerase then binds to the single

stranded DNA & moves along the strand

free nucleotides with correct complement base

will bind to template strand

How Nucleotides are Added in DNA Replicationhttp://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf

http://www.susanahalpine.com/anim/Grolier_Anims/DNA.htm

The new strand of DNA is synthesised in

the 5 3 direction

Therefore only 1 strandcan be copied continuously

(continuous/forward

replication)

leading strand

DNA Replication (ii)

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The copying of the other strand has to keep

being started again because the DNA

polymerase moves away from the

unwinding enzyme in the 5 3 direction

This results in small gaps

(okazaki fragments)because the

DNA polymerase cannot bind the 3 endof the newly synthesized strand to the 5 end

This is called (discontinuous/backward replication)

Lagging strand

DNA Replication (ii)

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DNA Replication (iii) DNA ligase joins the okazaki fragments

together to form continuous strand

DNA endonucleaseproof-reads new DNAdouble helix

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DNA Replication (summary)

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DNA Replication

Unwinding of DNA double helixBinds to the single stranded

DNA & moves along the strand

Joins the okazaki fragments

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DNA Replication

http://trc.ucdavis.edu/biosci10v/bis10v/media/ch09/

replicating_dna.html

Interactive DNA Rep with quiz

http://www.bioteach.ubc.ca/TeachingResources/M

olecularBiology/DNAReplication.swf

Animations

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1. Which statement about the strands of a newly replicated DNAmolecule is correct?

A Both strands are made up of newly assembled nucleotides.

B Both strands contain some nucleotides from the original molecule.

C One strand is new and the other is part of the original molecule.D The sugar-phosphate chains are conserved and new bases are

inserted between them.

2. Which statement correctly describes the transcription ofDNA?

AIt is a semi-conservative process.B It occurs at the surface of the ribosome.

C It produces messenger RNA.

DIt produces polypeptides.

Practice Questions

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3. Which structural feature of the DNA molecule varies?

A the arrangement of the sugar-phosphate groups

B the double helical arrangement

C the order of bases on a single nucleotide chain

Dthe pairing of purines with pyrimidines

4. Explain how the structure ofDNA enables it to replicate semi

conservatively.

Practice Questions

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Objectives

Next session, well learn:

.. What are nucleotides

.. Structure & function of DNA & RNA .. DNA replication

.. transcription (DNA RNA)

.. translation (RNA Proteins)

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Homework

Reading

what is gene?

what is transcription?

process of transcription

References

Jones, Fosbery, Taylor & Gregory pg 66,68-70

Taylor Green & Stout pg

Clegg & MacKean pg 205-206