Chapter 16: The Molecular

Basis of Inheritance

Essential Knowledge

3.a.1 – DNA, and in some cases RNA, is the primary source of heritable information (16.1 & 16.2).

3.c.1 – Changes in genotype can result in changes in phenotype (16.2).

Question? Traits are inherited on chromosomes,

but what in the chromosomes is the genetic material?

Two possibilities: Protein DNA

Qualifications Protein:

Until 1940s, evidence for protein was STRONG!

Very complex structureHigh specificity of function

DNA:Simple structureNot much known about it (early 1900’s)

Griffith - 1928 Pneumonia in mice Two strains:

S – pathogenic (caused pneumonia) R - harmless

Griffith’s Experiment

Result Something turned the R cells into S cells (in

4th experiment) Transformation - the assimilation of

external genetic material by a cell And…the pathogenic trait was inherited by all

new offspring!

Problem Griffith used heat

Heat denatures proteins DNA – heat stable

Then, could proteins still be the genetic material?

Griffith’s results were 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

Avery, cont. Experiment not believed

Why? Scientists thought bacteria make-up was

considerably different from humans/other living organisms

Hershey- Chase 1952 Genetic information of a virus or phage Phage

Virus that attacks bacteria and reprograms host to produce more viruses (by injecting its own DNA)

Virus Intro DNA and/or RNA core Enclosed by envelope

Made of protein To reproduce, a virus must attach to a cell

and inject its genetic info (either RNA/DNA) INTO the cell

Bacteria with Phages

Phage Components Hershey/Chase knew viruses reproduced, but

didn’t know what was injected… Two main chemicals:

Protein DNA

Hershey/Chase used tracers Radioactive isotope 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 infected bacteria cells

Hershey - Chase movie

Result DNA enters the host cell, but the protein did

not Therefore, DNA is the genetic material that is

passed down

Picture Proof

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

Rosalind Franklin Also used x-ray crystallography Determined DNA had two strands Died in 1958 Her colleague got Nobel Prize (because

Franklin published under his name!)

DNA Composition Made of nucleotides: (3 parts)

1. Deoxyribose Sugar (5-C ring)2. Phosphate (PO4-)

3. Nitrogen Bases: A,T,C,G• Purines: A,G• Pyrimidines: C,T

DNA Backbone Polymer of sugar - phosphate 2 backbones present Phosphate of one nucleotide is attached to

sugar of the next Alternates sugar-phosphate

Nitrogen Bases Bridge the backbones together Purine + Pyrimidine = 3 rings

Keeps a constant distance between the 2 backbones

Nucleotide held together by H-bonds

Chargaff’s Rule Studied chemical composition of DNA Found:

the nucleotides were found in certain ratios % composition differed between species

Chargaff’s Rule A = T G = C Example: in humans

A = 30.9%T = 29.4%G = 19.9%C = 19.8%

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

Purin

esPyrim

idines

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

existing DNA) Completed during S-phase of Interphase

Problem: When cells replicate, the genome must be copied exactly How is this done?

Models for DNA Replication

1. Conservative – one old strand, one new strand

2. Semiconservative – each strand is 1/2 old, 1/2 new

3. 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

Parent strand serves as a template

Replication - Preview DNA splits by breaking the H-bonds

between the backbones. Then DNA builds the missing

backbone using the old backbone as a template.

DNA is replicated in only a few hours.

Origins of Replication Specific sites on the DNA molecule that start

replication. Recognized by a specific DNA base sequence. Proteins/enzymes initiate replication

Prokaryotic replication Ex: bacteria (E. coli) Circular DNA 1 origin site Replication runs in both directions from the

origin site

Eukaryotic replication Many origin sites.

100s/1000s Replication bubbles fuse to form new DNA

strands. Faster replication (usually) Replication also runs in both directions from

origin site

DNA Elongation Done so by DNA Polymerases Adds DNA triphosphate monomers to the

growing replication strand These triphosphate contain the complementary

nucleotides Matches A to T and G to C

Energy for Replication Exergonic rxn Comes from the triphosphate

monomers. Loses two phos as each

monomer/nucleotide is added. Similar to ATP cycle

ATP contains ribose sugar DNA = deoxyribose

Problem of Antiparallel DNA

The two DNA strands run antiparallel to each other

Two “ends” of strand 3` - sugar/OH end 5` = phosphate end

New DNA strand can only elongate in the 5` 3` direction Old DNA strand 3’ 5’

Leading Strand Continuous replication toward the

replication fork in the 5`3` direction Leading strand is a NEW strand that’s being

added

Lagging Strand Discontinuous synthesis away from the

replication fork Replicated in short segments as more

template becomes opened up Lagging strand is also NEW!

Replication fork animation

Priming DNA Polymerase cannot initiate DNA

synthesis (by itself) Nucleotides can be added (only to an existing

chain). This nucleotide chain is called a primer

Primer Made of RNA 10 nucleotides long Added to DNA by an enzyme called primase DNA is then added to the RNA primer (to

finish replication) A primer is needed for each DNA elongation

site This is called “Priming”

Enzymes DNA Ligase - joins all DNA fragments

together Helicase - unwinds the DNA double helix DNA polymerase – elongation, replacement

of RNA primer with DNA

Other Proteins in Replication

Single-Strand Binding Proteins - help hold the DNA strands apart

Primase – priming (adds RNA section to existing chain)

Enzyme Summary

DNA Replication Error Rate

1 in 10 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 Replication overview

DNA Repair 50+ different DNA repair enzymes known Failure to repair may lead to cancer or other

health problems Ex:

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 nucleotides in

the 5` 3` direction Therefore, 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/sex cells

Limited activity in active cells (such as skin cells)

Control of telomerase may stop cancer, or extend the life span

Summary Recognize scientists and the experiments that lead to

the understanding of the molecular basis of inheritance.

Identify the double helix composition and structure of DNA.

Identify the process and steps of DNA replication. Recognize the problems in replicating the ends of the

DNA molecules. Give an example of DNA proofreading and repair. Gain familiarity with the packing of DNA into a

Eukaryotic chromosome.

Exclusion Statement

You do NOT need to memorize the names of the steps and particular enzymes involved in DNA replication EXCEPT for the following: DNA polymerase Ligase RNA polymerase Helicase Topoisomerase