Replication of DNA DNA is a unique molecule because it is able to direct its own replication and...

Replication of DNADNA is a unique molecule because it is able to direct its own

replication and reproduce itself EXACTLY.

What I need to know:-

1. DNA can make an exact copy of itself2. The stages of DNA replication3. The requirements for DNA replication4. The use of a primer and DNA polymerase5. DNA replication of the lagging strand

DNA replicationDNA can reproduce itself exactly and

this is called REPLICATION.

Full replication is complete in 6 stages:

1. DNA double helix unwinds2. Weak hydrogen bonds break causing the 2 strands

to separate3. Free DNA nucleotide joins complimentary pair on

open strand4. Weak hydrogen bonds reform between base pairs5. Strong chemical bond forms between both

nucleotides (controlled by enzyme called DNA POLYMERASE)6. Newly formed daughter DNA (identical to original)

begins to wind into double helix

Stage 1 – DNA double helix unwinds

Stage 2 – Weak hydrogen bonds break causing the

2 strands to separate

Stage 3 - Free DNA nucleotide joins complimentary pair on open strand

Stage 4 – Weak hydrogen bonds reform between base pairs

Stage 5 – Strong chemical bond forms between both nucleotides

(controlled by enzyme called DNA POLYMERASE)

Stage 6 – Newly formed daughter DNA (identical to original) begins to wind into

double helix

This process can only take place if the nucleus contains:

• DNA (acts as template for new molecule)

• Supply of DNA nucleotides

• Appropriate enzymes

• Supply of ATP

Semi-conservative replication

Replication results in two new DNA molecules. It is said to be

SEMI-CONSERVATIVE because each new DNA

molecules receives one strand of the original parent

molecule.

Enzyme control

DNA is a complex process that involves many enzymes

The section of DNA that is to be replicated unwinds and unzips creating a y-shaped replication fork.

On the leading strand

The enzyme DNA polymerase controls the sugar-phosphate bonding between nucleotides on the new DNA strand. This will only work if it has two nucleotides to join.

Therefore a primer is needed to initiate DNA replication.

A primer is a short strand of complementary nucleotides formed at the 3’ end of the parental DNA strand.

DNA replication on this strand is continuous

On the lagging strand

DNA polymerase is only able to add nucleotides to the free 3’ end of a growing strand.

Therefore the DNA on the parental strand that has the 5’ end exposed is replicated in fragments starting at the 3’ end of a primer.

As the replicated strand is discontinuous, the enzyme LIGASE joins the fragments together

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter14/dna_replication.html

What is required for DNA replication?• DNA• Primers• Free DNA nucleotides• Enzymes• ATP

DNA and the bigger picture

Once the two identical daughter DNA strands form, they coil up and become IDENTICAL

CHROMATIDS (held together by a centromere).

Following DNA replication (and before cell division) the cells DNA content is at its

maximum.DNA replication ensures that an exact copy of a species genetic information is passed on.

If DNA failed to replicate itself, mitosis and meiosis would be unable to take place.

DNA is essential for life to exist.

Meselson and Stahl Experiment• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cg

i?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experiment

Now attempt the following questions

1) Decide whether the following statements are true or false. When the statement is false, give the word that should have been used in place of the word in bold print

A – during DNA replication , each DNA parental strand acts as a template.

B – A guanine base can only pair up with a thymine baseC – An adenine base can only pair with a cytosine base.D – Complementary base pairs are held together be weak anti-

parallel bondsE – each new DNA molecule formed by replication contains one

parental and one new strand.2) Describe the stage of DNA replication on the leading strand3a) name four requirements needed for DNA replication to occur.3b) briefly explain why DNA replication is important

Glossary• ANTIPARALLEL – describes the orientation of the strands in a DNA duplex; these run in

opposite directions with respect to their 5’-3’ polarity

• CENTROMERE – region of a replicated chromosome where 2 chromatids are held together

• CHROMATID – refers to the copy of a chromosome after DNA replication and prior to cell division. Each replicated chromosome is composed of 2 chromatids, joined at the centromere

• DNA POLYMERASE – an enzyme that creates DNA molecules by assembling neucleotides

• LAGGING STRAND – the DNA strand that is replicated discontinuously from the 5' to the 3' direction

• LEADING STRAND – the DNA strand that is synthesised continuously during replication

• PRIMER – short fragment of DNA that serve as a starting point for DNA replication

• REPLICATION FORK – The point at which the two strands of DNA are separated to allow replication of each strand

• SEMICONSERVATIVE REPLICATION - the normal process of DNA synthesis, in which the two original strands of the molecule separate, and each acts as a template on which a new complementary strand is laid down