AP Biology 2007-2008

Synthesis Of DNA Presentation By G.H Ngubane

07 March 2014@

University of Johannesburg

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DNA synthesis occurs by the process of replication.

During replication, each of the two parental strands of DNA serves as a template for the synthesis of aComplementary strand.

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Each molecule generated by the replication process contains

one intact parental strand and one newly synthesized strand.

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In eukaryotes, DNA replication occurs during the S phase of the cell cycle

The cell divides during the next phase (M), and each daughter cell receives an exact copy of the DNA of the parent cells.

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Watson and Crick1953 article in Nature

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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

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Directionality of DNA You need to

number the carbons! it matters!

OH

CH2

O

4

5

3 2

1

PO4

N base

ribose

nucleotide

This will beIMPORTANT!!

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The DNA backbone Putting the DNA

backbone together refer to the 3 and 5

ends of the DNA the last trailing carbon

OH

O

3

PO4

base

CH2

O

base

OPO

C

O–O

CH2

1

2

4

5

1

2

3

3

4

5

5

Sounds trivial, but…this will be

IMPORTANT!!

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Anti-parallel strands Nucleotides in DNA

backbone are bonded from phosphate to sugar between 3 & 5 carbons DNA molecule has

“direction” complementary strand runs

in opposite direction

3

5

5

3

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Bonding in DNA

….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?

3

5 3

5

covalentphosphodiester

bonds

hydrogenbonds

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Base pairing in DNA Purines

adenine (A) guanine (G)

Pyrimidines thymine (T) cytosine (C)

Pairing A : T

2 bonds C : G

3 bonds

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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

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DNA Replication Large team of enzymes coordinates replication

Let’s meetthe team…

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Replication: 1st step Unwind DNA

helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins

single-stranded binding proteins replication fork

helicase

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DNAPolymerase III

Replication: 2nd step

But…We’re missing

something!What?

Where’s theENERGY

for the bonding!

Build daughter DNA strand add new

complementary bases DNA polymerase III

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Energy of Replication The nucleotides arrive as nucleosides

DNA bases with P–P–P P-P-P = energy for bonding

DNA bases arrive with their own energy source for bonding

bonded by enzyme: DNA polymerase III

ATP GTP TTP CTP

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Adding bases can only add

nucleotides to 3 end of a growing DNA strand need a “starter”

nucleotide to bond to

strand only grows 53

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

energy

energy

energy

Replication energy

3

3

5B.Y.O. ENERGY!

The energy rulesthe process

5

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Limits of DNA polymerase III can only build onto 3 end of

an existing DNA strand

Leading & Lagging strands

5

5

5

5

3

3

3

53

53 3

Leading strand

Lagging strand

Okazaki fragments

ligase

Okazaki

Leading strand continuous synthesis

Lagging strand Okazaki fragments joined by ligase

“spot welder” enzyme

DNA polymerase III

3

5

growing replication fork

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DNA polymerase III

Replication fork / Replication bubble

5

3 5

3

leading strand

lagging strand

leading strand

lagging strandleading strand

5

3

3

5

5

3

5

3

5

3 5

3

growing replication fork

growing replication fork

5

5

5

5

53

3

5

5lagging strand

5 3

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DNA polymerase III

RNA primer built by primase serves as starter sequence

for DNA polymerase III

Limits of DNA polymerase III can only build onto 3 end of

an existing DNA strand

Starting DNA synthesis: RNA primers

5

5

5

3

3

3

5

3 53 5 3

growing replication fork

primase

RNA

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DNA polymerase I removes sections of RNA

primer and replaces with DNA nucleotides

But DNA polymerase I still can only build onto 3 end of an existing DNA strand

Replacing RNA primers with DNA

5

5

5

5

3

3

3

3

growing replication fork

DNA polymerase I

RNA

ligase

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DNA polymerases DNA polymerase III

1000 bases/second! main DNA builder

DNA polymerase I 20 bases/second editing, repair & primer removal

DNA polymerase III enzyme

Arthur Kornberg1959

Roger Kornberg2006

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Editing & proofreading DNA 1000 bases/second =

lots of typos!

DNA polymerase I proofreads & corrects

typos repairs mismatched bases removes abnormal bases

repairs damage throughout life

reduces error rate from 1 in 10,000 to 1 in 100 million bases

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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

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The importance of nucleotide sequence

The sequence of nucleotides form the unique genetic information of an organism.

The more closely related two organisms are, the more alike the sequence of nucleotides in the DNA will be.

Ex: Gel Electrophoresis

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Gel Electrophoresis

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REACTION:oThe DNA occurs simultaneously

forming sister chromatids.oNucleotides should always be in pair.oNucleotides are held together with

loose hydrogen bonds.oEvery cell in our body has the same

copy of DNA, and the DNA will copy itself trillion of times in our lifetime.

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Structure of a nucleotideA nucleotide is made

of 3 components: A Pentose sugar This is a 5 carbon

sugar The sugar in RNA

is ribose. The sugar in DNA

is deoxyribose.

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Structure of a nucleotide A Phosphate group Phosphate groups

are important because they link the sugar on one nucleotide onto the phosphate of the next nucleotide to make a polynucleotide.

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Structure of a nucleotide A Nitogenous base

In DNA the four bases are: Thymine Adenine Cytosine Guanine

In RNA the four bases are: Uracil Adenine Cytosine Guanine

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Which came first … The language ?

The storage medium ? The translation machinery ?

The transcription machinery ?

The transfer medium ?

The letters ?

The ATP power plant ?

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Every component used in RNA transcription is made by RNA transcription.

ATP Synthase

tRNA SynthaseRNA PolymeraseRibosomes

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Even if every component is present and in it’s place. Once a cell dies it will not spontaneously revive. It’s order begins to decay.

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The chemicals, components and arrangements necessary for life spontaneously disassemble if the processes of a living cell which maintain order cease to function.

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Acknowledgements 1. Ramesh Mahindrakar

2. RAD DAD GONE MAD Doug Hove

3. MBBS IMS MSU

4. Claudeaa 

5. Valerie Evans

To view their work click on their names or see links in the

reference list.

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Reference List1.DNA code of life by  Ramesh Mahindrakar.

Working at G.E. SOCIETY'S J.S.S.COLLEGE GOKAK 

On Nov 08, 2013

http://www.slideshare.net/RameshMahindrakar/ii-puc-dna?v=qf1&b=&from_search=2

2.The DNA Code by RAD DAD GONE MAD Doug Hove. Mechanic Electrian / Fundamental Independent Baptist at Pleasant Valley Baptist Church 

On Jan 24, 2008

http://www.slideshare.net/CreationCD/the-dna-code?v=qf1&b=&from_search=7

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Reference List3.Bio-Chem synthesis of DNA by MBBS IMS MSU 

On Jun 21, 2010

http://www.slideshare.net/ananthatiger/biochem-synthesis-of-dna?v=qf1&b=&from_search=9

4. DNA replication by Claudeaa 

On Mar 12, 2012

http://www.slideshare.net/claudeaa/dna-replication-11970385?v=default&b=&from_search=2

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Reference List

5. Chapter 12.2 DNA replication by Valerie Evans 

On Dec 01, 2011

http://www.slideshare.net/evansv1/chapter-122-dna-replication

To view my Slide Share PowerPoint presentation click here slide share PowerPoint

Or click on this link http://www.slideshare.net/Gordon-3