Chapter 16 The Molecular Basis of Inheritance

Question? Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? Two possibilities: Protein DNA

Qualifications Protein: DNA: very complex. high specificity of function. DNA: simple. not much known about it (early 1900’s).

Griffith - 1928 Pneumonia in mice. Two strains: S - pathogenic R - harmless

Griffith’s Experiment

Result Something turned the R cells into S cells. Transformation - the assimilation of external genetic material by a cell.

Problem Griffith used heat. Heat denatures proteins. So could proteins be the genetic material? DNA - heat stable. Griffith’s results contrary to accepted views.

Avery, McCarty and MacLeod - 1944 Repeated Griffith’s experiments, but added specific fractions of S cells. Result - only DNA transformed R cells into S cells. Result - not believed.

Hershey- Chase 1952 Genetic information of a virus or phage. Phage - virus that attacks bacteria and reprograms host to produce more viruses.

Bacteria with Phages

Phage Components Two main chemicals: Protein DNA Which material is transferred to the host?

Used Tracers Protein - CHONS, can trace with 35S. DNA - CHONP, can trace with 32P.

Experiment Used phages labeled with one tracer or the other and looked to see which tracer entered the bacteria cells.

Result DNA enters the host cell, but the protein did not. Therefore: DNA is the genetic material.

Picture Proof

Chargaff - 1947 Studied the chemical composition of DNA. Found that the nucleotides were in certain ratios.

Chargaff’s Rule A = T G = C Example: in humans, A = 30.9% T = 29.4%

Why? Not known until Watson and Crick worked out the structure of DNA.

Watson and Crick - 1953 Used X-ray crystallography data. Used model building. Result - Double Helix Model of DNA structure. (One page paper, 1953).

Rosalind Franklin

DNA Composition Deoxyribose Sugar (5-C) Phosphate Nitrogen Bases: Purines Pyrimidines

DNA Backbone Polymer of sugar-phosphate. 2 backbones present.

Nitrogen Bases Bridge the backbones together. Purine + Pyrimidine = 3 rings. Constant distance between the 2 backbones. Held together by H-bonds.

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Chargaff’s Rule Explained by double helix model. A = T, 3 ring distance. G = C, 3 ring distance.

Watson and Crick Published a second paper (1954) that speculated on the way DNA replicates. Proof of replication given by others.

Replication The process of making more DNA from DNA. Problem: when cells replicate, the genome must be copied exactly. How is this done?

Models for DNA Replication Conservative - one old strand, one new strand. Semiconservative - each strand is 1/2 old, 1/2 new. Dispersive - strands are mixtures of old and new.

Replication Models

Meselson - Stahl late 1950’s Grew bacteria on two isotopes of N. Started on 15N, switched to 14N. Looked at weight of DNA after one, then 2 rounds of replication.

Results Confirmed the Semiconservative Model of DNA replication.

Replication - Preview DNA splits by breaking the H-bonds between the backbones. Then DNA builds the missing backbone using the bases on the old backbone as a template.

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Origins of Replication Specific sites on the DNA molecule that starts replication. Recognized by a specific DNA base sequence.

Prokaryotic Circular DNA. 1 origin site. Replication runs in both directions from the origin site.

Eukaryotic Cells Many origin sites. Replication bubbles fuse to form new DNA strands.

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DNA Elongation By DNA Polymerases. Adds DNA triphosphate monomers to the growing replication strand. Matches A to T and G to C.

Energy for Replication From the triphosphate monomers. Loses two phosphates as each monomer is added.

Problem of Antiparallel DNA The two DNA strands run antiparallel to each other. DNA can only elongate in the 5’--> 3’ direction.

Leading Strand Continuous replication toward the replication fork in the 5’-->3’ direction.

Lagging Strand Discontinuous synthesis away from the replication fork. Replicated in short segments as more template becomes opened up.

Priming DNA Polymerase cannot initiate DNA synthesis. Nucleotides can be added only to an existing chain called a Primer.

Primer Make of RNA. 10 nucleotides long. Added to DNA by an enzyme called Primase. DNA is then added to the RNA primer.

Priming A primer is needed for each DNA elongation site.

Homework Reading – Chapters 16, 49 Chapter 48 ,15 – today Lab – Bacteria transformation, report due by Dec. 12 Chapter 16 – Mon. 12/9 Exam 4 – Tues. 12/10 3:00, TC 005 Chapter 49 – by Dec. 12

Movie – Leading Strand

Movie – Lagging Strand

Okazaki Fragments Short segments (100-200 bases) that are made on the lagging strand. All Okazaki fragments must be primed. RNA primer is removed after DNA is added.

Enzymes Replaces RNA primers with DNA nucleotides. DNA Ligase - joins all DNA fragments together.

Other Proteins in Replication Helicase - unwinds the DNA double helix. Single-Strand Binding Proteins - help hold the DNA strands apart.

Enzyme Summary

Movie - Review

DNA Replication Error Rate 1 in 1 billion base pairs. About 3 mistakes in our DNA each time it’s replicated.

Reasons for Accuracy DNA Polymerase self-checks and corrects mismatches. DNA Repair Enzymes - a family of enzymes that checks and corrects DNA.

DNA Repair 50+ different DNA repair enzymes known. Failure to repair may lead to Cancer or other health problems.

Example: Xeroderma Pigmentosum -Genetic condition where a DNA repair enzyme doesn’t work. UV light causes damage, which can lead to cancer.

Xeroderma Pigmentosum Cancer Protected from UV

Thymine Dimers T-T binding from side to side causing a bubble in DNA backbone. Often caused by UV light.

Excision Repair Cuts out the damaged DNA. DNA Polymerase fills in the excised area with new bases. DNA Ligase seals the backbone.

Problem - ends of DNA DNA Polymerase can only add nucleuotides in the 5’--->3’ direction. It can’t complete the ends of the DNA strand.

Result DNA gets shorter and shorter with each round of replication.

Telomeres Repeating units of TTAGGG (100- 1000 X) at the end of the DNA strand. Protects DNA from unwinding and sticking together. Telomeres shorten with each DNA replication.

Telomeres

Telomeres Serve as a “clock” to count how many times DNA has replicated. When the telomeres are too short, the cell dies by apoptosis.

Implication Telomeres are involved with the aging process. Limits how many times a cell line can divide.

Telomerase Enzyme that uses RNA to rebuild telomeres. Can make cells “immortal”. Found in cancer cells. Found in germ cells. Limited activity in active cells such as skin cells

Comment Control of Telomerase may stop cancer, or extend the life span.

Summary Know the Scientists and their experiments. Why DNA is an excellent genetic material. How DNA replicates. Problems in replication.