cmb lect 6 2011 colour 2 slides per page

8/14/2019 CMB Lect 6 2011 Colour 2 Slides Per Page

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Lecture 6

BIOSCI 101: Cellular and Molecular Biology

Structure and function of

nucleic acids

Nucleic acids structure/function

The assumption that genes were protein surviveduntil the 1940's

- e a son r c mo e or

- sugar /phosphate backbone and attachedpurine and pyrimidine bases

- two antiparallel strands wound together in ahelix

- held together by hydrogen bonds A=T, G=C


- s a se y y rop o c n erac ons e weenthe bases

- the complementary pairing of the basesprovides a mechanism for the transmission ofgenetic information through many generations


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Sugarphosphatebackbone

Nitrogenous bases

Thymine (T)

5 end

The structure ofa DNA strand

Adeni ne (A)

Cytosine (C)

BIOSCI 101: Cellular and Molecular BiologyCampbell 16.5

Guanine (G)

Nitrogenous base

o sp at e

DNAnucleotide

Sugar(deoxyribose)

3 end

3.4 nm

C

C

C

C

G

G

G

G

T A

The doublehelix

1 nm

C

C

C

C

G

G

G

G

T

T

A

A

BIOSCI 101: Cellular and Molecular BiologyCampbell 16.7a

0.34 nm

C G

T

T

T

A

A

A


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The double helix (contd)5' end

3' end


3' end

5' end

Campbell 16.7b,c

Nucleic acids structure/function contd Types of nucleic acid in the cell

a

- DNA is the major hereditary material of thecell

- in prokaryotes DNA is free within the cell,probably associated with a site on the cell wall


- n eu aryo es s oca e n e nuc eusbut is also found in the mitochondria andchloroplasts


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Nucleic acids structure/function contd

Types of nucleic acid in the cell (contd)

- nuclear DNA is associated with histoneprotein in the form of chromatin fibres

- DNA binds to the five histone polypeptides asa string of beads along the DNA. The beadsare called nucleosomes

- the higher order of coiling of the


into nucleosomes reduces the DNA length 7-

fold

Levels of chromatin packing

DNA double helix(2 nm in diameter)

Nucleosome(10 nm in diameter)

HistonesHistonetail

H1

BIOSCI 101: Cellular and Molecular BiologyCampbell 16.22a

DNA, the doub le helix HistonesNucleosomes, or beads ona string (10-nm fiber)


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Levels of chromatin packing(contd)

Chromatid(700 nm)

30-nm fiber

30-nm fiber

Loops Scaffold

300-nm fiber


Replicated

chromosome(1,400 nm)

Looped domains(300-nm fiber) Metaphase

chromosome

Campbell 16.22b

Calculation of the amount of DNA

in the human body:diploid genome = 46 chromosomes

DNA in a typical human cell = 3 x 109 base pairs

so c romosome = x ase pa rs

ie = 6.5 x 107 base pairs

On a DNA helix, 10 base pairs take up 3.4 nm

So length of 1 chromosome = 6.5 x 107 x 0.34 nm

= 2.2 x 107 nm

= 2.2 cm


So total length of DNA in one cell = 2.2 x 46= 100 cm or 1 m

Human body has ~ 1012 cells so this means ~109 kmDNA in our bodies!!


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b) RNA

- in ormation is trans erre rom nuc eic aci intoprotein via RNA

- the sequence of the bases in DNA specify thesequence of bases in RNA

i) Ribosomal RNA (rRNA)

- stable RNA which comprises the bulk of the cellular


- synthesised in the nucleolus from highly repetitive

DNA- structural role in ribosomes

- thought to play a catalytic role in protein synthesis

A prokaryotic ribosome



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ii) Messenger RNA (mRNA)

- mRNA is the template for protein synthesisand is made using DNA as a template

- synthesised at a fast rate, degraded rapidly and


present in relatively small amounts

An RNA molecule



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iii) Transfer RNA (tRNA)

- small adapter molecules whose function is torecognise both an amino acid and its appropriatecodon on the mRNA in order to assemble amino acidsinto the right sequence during protein synthesis.

- at least one specific tRNA for each of the 20 aminoacids


- each tRNA contains about 80 nucleotides and is

folded into a cloverleaf shape having a series of loops- one of the most important of these loops is theanticodon loop

The structure of transfer RNA



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Replication of DNA

- process involving unzipping of the parentalmolecule and the use of each DNA strand as atemplate for the synthesis of a new strand

- semiconservative mechanism ie the twoprogeny molecules are composed of one oldstrand and one new strand (Messelson and Stahl)


-a multienzyme complex) which must be accurate

as well as fast (500 nuc/sec in bacteria and 50nuc/sec in mammals)

Replication of DNA the basicconcept

A

A

A

A

A

A

A

A

A

A

A

A

T

T

T

T

T

T

T

T

T

T

T

T

C

C

C

C

C

C

C

C

G

G

G

G

G

G

G

G


(a) Parent molecule (b) Separation o fstrands

(c) Daughter DNA molecules,each consisting o f oneparental strand and onenew strand


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(a) Conservativemodel

Parentcell

Firstreplication

Secondreplication

Three models ofDNA replication(b) Semiconservativemodel


Campbell 16.10

(c) Dispersive model


Replication of DNA (contd)

a) initiation begins at the origin of replication - aspecific nucleotide sequence

- prokaryotes have just one origin ofreplication while eukaryotes have many

- requires helicases and initiator proteins toopen up the helix by breaking apart the


y rogen on s n ng e ases a e or g n- single-strand binding proteins attach, holding

the separated strands apart and preventingkinking


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- as the strands of the helix separate, theadjacent portions become even more tightlycoiled. Topoisomerases break and reconnectone or both strands of the helix allowingswivelling to occur and thus relieving strain on


- primase synthesises short RNA primers

using the parental DNA as template

Origin of replication in prokaryotes

BIOSCI 101: Cellular and Molecular BiologyBIOSCI 101: Cellular and Molecular BiologyCampbell 16.12a


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Origins of replication in eukaryotes

BIOSCI 101: Cellular and Molecular BiologyCampbell 16.12b

Initiation of DNA replication

BIOSCI 101: Cellular and Molecular BiologyBIOSCI 101: Cellular and Molecular BiologyCampbell 16.13


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b) DNA polymerase "reads" the templatestrand via complementary base pairing

- synthesises new DNA at the replication forkin a 5'3' direction by adding nucleotides tothe 3' end of a primer or a growing DNA


s ran

Incorporation of a nucleotide



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- synthesis is bi-directional as a result ofcontinuous synthesis on the 5' 3' strand

(the leading strand) and discontinuoussynthesis on the 3' 5' strand (the lagging

strand) at both ends of the replication bubble

- the short fra ments on the la in strand


are known as Okazaki fragments and are

subsequently joined by DNA ligase

Continuous synthesis on the leading strand



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Discontinuous

synthesis on thelagging strand


Campbell 16.16


c) DNA polymerase proofreads the new strand as itis made

- enzyme backtracks if necessary to removeerrors


- energy required to power the replicationprocess comes from the nucleoside triphosphates- as they are added to the growing chain energyis released from their phosphate bonds


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A summary of DNA replication


And a question ..

What is meant b the descri tion "anti arallel" re ardin thestrands that make up DNA?

1) The twisting nature of DNA creates nonparallel strands.

2)The 5' to 3' direction of one strand runs counter to the 5'to 3' direction of the other strand.

3)Base pairings create unequal spacing between the two.

4) One strand contains only purines and the other containsonly pyrimidines.


cmb lect 6 2011 colour 2 slides per page

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