Chapter 16 Molecular Basis of Inheritance. DNA genetic material Chromosomes composed of DNA + protein.

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Chapter 16

Chapter 16Molecular Basis of InheritanceDNA genetic materialChromosomes composed of DNA + protein

DNA base compositionNucleotide base


Guanine, C5H5N5ODNA is a polymer of nucleotides

Chargaffs rules (1950)[T] = [A] [G] = [C]

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

DNA structural modelWatson, Crick, Franklin 1953

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

DNA double helixSugar-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 DNA1 strand original DNA maintained in new DNAMeselson and Stahl 1950sLabel 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/cm314N DNA 1.710 gm/cm3DNA 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 nucleotidesReplication proteins attach to oriForms a replication bubbleTwo strands of DNA open

Replication fork in both directionsProteins in DNA replication Table 16.1DNA 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 stress5. Primase synthesizes a primer

Each strand is a template for new DNALeading strand isLagging strand is

DNA replication leading strand: stepsPrimase (enzyme) synthesizes primer complementary to leading strandprimer 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 chromosomeHumans 50 nuc/sec. Humans have 11 polymerases24DNA 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

O fragments are ~ 100 nuc long in nhumans264. 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 telomeresNo genesRepetitive DNA TTAGGG up to 1000 times

Human chromosomes capped by telomeres5'...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 cultureCells that do not divide oftenExample: 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 cellembryo


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