Chapter 16

Molecular Basis of Inheritance

DNA genetic material

• Chromosomes composed of DNA + protein

DNA base composition

Nucleotide base

GuanineCytosineThymineAdenine

Guanine, C5H5N5O

DNA is a polymer of nucleotides

Chargaff’s rules (1950)

[T] = [A] [G] = [C]

A certain chromosome is 19% A. What is the % of C?

DNA structural model

Watson, Crick, Franklin 1953

X-ray crystallographyDNA is helicalSpacing of basesWidth of helix suggested 2 strands

DNA double helix

Sugar-phosphate “backbone”

Anti parallel strands

• Bases face inward

• Hydrogen bonds connect bases

A - T (2 bonds)G - C ( 3 bonds)

Original DNAcopied to new DNAhelix

Original DNA broken up and combined in new DNA

1 strand original DNA maintained in new DNA

Meselson and Stahl 1950s

1. Label DNA (E. coli) with 15N in growth media

2. Transfer E. coli to 14N media for 1 generation (20 min)

Results:The density of the DNA is

intermediate

Cells grown longer 14N, make lighter DNA

What would the DNA density be after 20 more minutes of cell group?

15N DNA1.724 gm/cm3

14N DNA 1.710 gm/cm3

DNA replication is semi-conservative

DNA replication: mechanism (E. coli)

E. coli genome = 4 X 10 6 bp DNA1 circular chromosome1 origin of replication (ori)

• Ori nucleotides– Replication proteins attach to ori– Forms a replication bubble

• Two strands of DNA open

Replication fork in both directions

Proteins in DNA replication Table 16.1

1. DNA polymerase (enzyme)Adds nucleotides 5’ 3’ direction only

2. Helicase (enzyme) – unwinds double helix3. Single stranded binding protein (SSB) binds to

DNA strands to stabilize them

4. Topoisomerase (enzyme) – breaks, rejoins DNA to relieve physical stress

5. Primase – synthesizes a primer

Each strand is a template for new DNA

Leading strand is

Lagging strand is

DNA replication leading strand: steps

1. Primase (enzyme) – synthesizes primer complementary to leading strand– primer is ~10 bases

2. DNA polymerase (pol III) synthesizes new strand 5’ 3’

G, A, T, C nucleotides complementary to template strand500 nuc/secContinuous elongation until end of chromosome

DNA Synthesis steps: lagging strand1. Primase makes RNA primer2. DNA adds nucleotides to primer in 5’ 3’

direction only

3. DNA pol III detaches

Okazaki fragment• ~ 1, 000 nucleotides

long

4. Another primer added, another Okazaki fragment formed

Many primers needed

5. Gaps between primers filled in

6. Ligase enzyme bonds fragments

DNA replication Fig. 16.17

Telomeres, the protective ends

Linear DNA has telomeres• No genes• Repetitive DNA TTAGGG up to 1000 times

Human chromosomes capped by telomeres

5'...TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG TTAGGG..3‘ 3'...AATCCC AATCCC AATCCC AATCCC AATCCC AATCCC..5'

• Chromosomes shorten with each cell division

• When telomeres are too short cell senescence(irreversible)

~ 125 cell divisions (humans)……life span?Telomeres shorten ~100 bp each time cell divides

Mouse fibroblasts in culture

• Cells that do not divide often– Example: heart muscle

Telomeres do not shorten with age

• Lagging strand problem

• Animation garland

• Embryonic cells, some wbc, stem cells, cancer cells express telomerase

White blood cell cervical cancer cell embryo