Agenda

1. Review homework

2. DNA replication

3. Begin HW (DNA Replication worksheet)

4. Quiz will be Wednesday instead and will include DNA replication

#1

A, T, G or C

#2

#3

They match up perfectly to form hydrogen bonds (2 for AT, 3 for GC)

#4

The two bases can’t H-bond, so nothing holds them together. It will stick out

#5

H-bonds are weak!

DNA must become separated to be copied or expressed

#6

GATTACA

CTAATGT

#7

If 15% is Adenine, 15% is also Thymine

That leaves 70% split between Guanine and Cytosine.

So 35% each

#8

X-ray crystallography, helped determine shape

#9

Structure

How it’s copied

How it leads to proteins

Sequence

#10

Why 99% of it is non-coding

What that 99% does

How the environment affects gene expression

Where it came from/how it originated

DNA Replication

DNA Replication

• Must occur every time a new cell is to be made

• E.g. prior to mitosis or meiosis

DNA Replication

Watson and Crick noticed the huge benefit of double strands

Each strand can serve as a template for making the other

Semiconservative Model Each strand serves

as a template for the creation of a new strand of DNA

2 DNA molecules are created, each containing 1 strand of the original DNA

Semiconservative Replication

DNA Replication is Remarkably Fast and Accurate!

Humans have 46 chromosomes, and thus 46 DNA molecules

About 6 billion base pairs

DNA replication takes just a few hours, even in humans

Only 1 error per 1 billion nucleotides

Step 1 – Separate the Strands

Helicase breaks the hydrogen bonds

Step 2-

•Primase adds a short RNA primer to begin new strands

Step 3

• DNA polymerase builds a new DNA strand

DNA is Antiparallel

• Strands run opposite directions

• DNA polymerase only works in one direction

• So the two strands are built in opposite directions

Leading vs. Lagging Strands

Leading• Built in one piece

• Built in the overall direction of replication

• Only one primer needed

Lagging• Built in pieces

called Okazaki fragments

• Built in the opposite direction of replication

• Many primers needed

Step 4

• DNA ligase seals together Okazaki fragments

Step 5 - Proofreading

Nucleases cut out (incise) the incorrect nucleotide

DNA polymerase adds the correct nucleotide

Ligase connects the new nucleotide to the strand

Topoisomerase

Helicase Single Stranded Binding Proteins

Primase

3’

5’

DNA Polymerase

Leading Strand

Lagging Strand

Okazaki Fragments

DNA Ligase

Mutations are Rare… But Happen

• Most errors are fixed in proofreading, so DNA replication is incredibly accurate

• 1 error every billion nucleotides. We have 6 billion. Every time DNA is copied there are ___ errors

• We’ll talk about the significance of these mutations later