The Molecular Basis of Heredity

Chapter 16

Learning Target 1

I can explain why researchers originally thought protein was the genetic material.

Protein as the genetic material?

T.H. Morgan – fruit flies Discovered genes as part of

chromosomes Chromosomes made of protein and

DNA Protein?

More known Diverse structures Specificity of function

DNA? Little known Seemed too uniform to be the

genetic code of all life

Learning Target 2

I can summarize the experiments performed by the following scientists and identify the evidence that

DNA is the genetic material.

Learning Target 3

I can describe the structure of DNA, explain the base-pairing rule, and describe its significance.

DNA Structure

Anti-parallel Structure

Sugar carbons are numbered 1’-5’

One side of DNA runs in the 3’ direction

The other side runs in the 5’ direction

This is important to replication

Learning Target 4

I can describe the semiconservative model of replication and the significance of the experiments

by Meselson and Stahl.

DNA Replication

Making DNA from existing DNA

Semi-conservative At the end of DNA replication,

each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication)

Model proposed by Meselson and Stahl

Meselson & Stahl

Learning Target 5

I can describe the process of DNA replication, including the role of the origins of replication and

replication forks.

DNA Replication Step 1:

Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases

Replication bubble forms as two parental strands separate

Replication fork forms at end of each replication bubble

DNA Replication Step 2:

Single-strand binding proteins hold the unpaired DNA strands apart while new DNA strands are being synthesized

Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork

DNA Replication

Step 3: Primase creates a short RNA primer that binds to the

parent DNA to signal DNA polymerase III where to begin adding nucleotides

RNA primer will later be replaced with DNA nucleotides

DNA Replication

Step 4: DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA

primer Leading strand

Produced continuously in 5’-3’ direction Elongation moves towards replication fork

Lagging strand Produced in pieces

Okazaki fragments Elongation moves in opposite direction of replication fork (5’-3’)

DNA Replication

Step 5: Lagging strand is completed as DNA ligase seals

Okazaki fragments

Proofreading & Repair

Initial error rate in replication is 1 in 100,000 nucleotides

DNA polymerases proofread and correct errors Error rate in

completed replication is 1 in 10 billion bases

Mismatch Repair

For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations

Many enzymes involved Cut out damaged section

(nuclease) Replace with new

nucleotides (DNA polymerase)

Seal in place (DNA ligase)

Learning Target 6

I can describe the structure and function of telomeres.

Telomeres As cells divide, chromosomes erode after

multiple DNA replications Telomeres

End caps of non-coding DNA that protect the coding genes on the chromosomes Think of the plastic pieces on the ends of your

shoelaces These sequences shorten after each round of

replication so the actual genes do not Part of aging process?

Telomerase

Enzyme that lengthens the ends of telomeres in germ cells (give rise to gametes)

Prevents loss of critical genes in zygote

Cancer cells have mechanisms that activate telomerase to allow unlimited division without loss of DNA