DNA replication 2

DNA replication is

semiconservative

what is Semiconservative?

DNA copies itself

1 new and 1 old

Three Models of DNA replication

Conservation

Semiconservative

Dispersive

Conservation

Both parents strands stay together, don’t seperate after replication

Semiconservative 

Each has one parent and one daughter AFTER replication 

Dispersive 

Parent and daughter DNA is interspred AFTER replication 

Meleson-stahl experiment wanted to prove?

How DNA replicates

Steps of Meselon-stahl experiement steps:

1. Grew bacteria in medium, containing heavy isopes of nitrogen, so it became heavy 

2. Switched bacteria to normal nitrogen and allowed DNA to replicate 

3. Used “Density Gradient Centrifugation” seperate DNA molecules based on density. 

Meselon-stahl conclussion:

DNA is semiconservative

had a intermediate density 

Intermediate density 

Each DNA has 1 heavy and 1 light after one round of replication

Bacterial DNA replication requires:

Coordination with cell division

Origin of replication

DNA polymerases

Accessory Enzymes

Coordination with Cell division:

Carefully timed

Genetic material is copied before cell divides

makes sure each new cell gets complete copy

Origin of Replication:

Replication STARTS on this bacterial chromosome

DNA first seperates

DNA polymerases:

Build new DNA strands

add nucleotides

Accessory Enzymes:

Helicase

Gyrases

Ligases

Helicase

Unwind and seperate two DNA strands

Gyrases

Relieve tension caused by unwinding

Ligases

Join DNA fragments together especially Okazaki fragments on lagging strand 

Three stages of Bacterial DNA replication:

1. Intiation

2. Elongation 

3. Termination

Replication fork:

When a strand seperates and becomes a “Y” shape

one origin of replication in bacteria:

Origin of Chromosomal replication

Bidirectional:

DNA synthesis occurs in
both directions.

DNA helicase:

BREAKS hydrogen bonds between DNA strands

Topoisomerase 2 (DNA gyrase)

Prevents DNA getting tightly coiled

Single strand:

Keeps parental strands APART

Primase

makes a short RNA PRIMER

DNA Polymerase 3:

Does most of the copying

DNA polymerase 1:

Removes the RNA primer and replaces it with DNA

DNA ligase

Link Okazaki fragments together

DNA polymerase only works:

5-3 and requires a primer

5 DNA polymerase:

1 and 3—→ Normal replication

2,4, and 5 ——> Repair and replicate DAMANGE RNA

DNA polymerase is

processive

Processive

Performs many repeated actions without letting go 

DnaA boxes

Specific DNA sequences found at the OriC

DnaA binding

Bends DNA t the orgin of replication

making it stretch of AT-rich DNA to seperate

DnaB

The helicase enzyme (unwind f

Continus DNA Synthesis

One long DNA strand

Leading strand

Discontinus DNA SYnthesis

Lagging strand

Okazaki Fragment

Short pieces of DNA made on lagging strand

eventually connected by DNA Ligase

Fidelity

How accurate something is 

Why is DNA replication so accurate:

1. Mismatch base pairs: Doesn’t form property 

2. Shape Recognition- Site changes shapes to fit only correct base pair

3. Proofreading- uses 3-5, reverses and replaces it with correct nucleotide

Eukaryotic DNA replication more complex:

1. Have multiple, long, linear chromosome 

2. Tightly wrapped around proteins (histones). Forms nucleotides

3. Complex cell cycle regulation: dont during S phase

Multiple Orgin of replication why?

Takes long time to replicate DNA

DNA replication

Bidirectionally

Telomeres

Special DNA repeat sequences at end of chromosome 

Telomeric sequences

Replaced DNA base pair

Telomeres are made up of:

telomeric sequences

TerC

Template and guide for new DNA repeat

TerT

Enzyme that synthesizes DNA wing TERC template 

Synthesizes

Combine to create something new