Welcome to Genetics:Welcome to Genetics:Unit 8 Seminar!Unit 8 Seminar!

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Agenda

• Brief Review of Unit Material• Self Assessment Questions• Question

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12Molecular Mechanisms of Mutation and DNA Repair

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Mutations• A mutation is any heritable change in the genetic

material

• Mutations are classified in a variety of ways

• Most mutations are spontaneous: they are random, unpredictable events

• Each gene has a characteristic rate of spontaneous mutation, measured as the probability of a change in DNA sequence in the time span of a single generation

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Table 12.1

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Mutations• Rates of mutation can be increased by treatment with

a chemical mutagen or radiation, in which case the mutations are said to be induced

• Mutations in cells that form gametes are germ-line mutations; all others are somatic mutations

• Germ-line mutations are inherited; somatic mutations are not

• A somatic mutation yields an organism that is genotypically a mixture (mosaic) of normal and mutant tissue

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Mutations• Among the mutations that are most useful for

genetic analysis are those whose effects can be turned on or off by the researcher

• These are conditional mutations: they produce phenotypic changes under specific (permissive conditions) conditions but not others (restrictive conditions)

• Temperature-sensitive mutations: conditional

mutation whose expression depends on temperature

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Mutations• Mutations can also be classified according to their

effects on gene function: A loss-of-function mutation (a knockout or null) results in complete

gene inactivation or in a completely nonfunctional gene product

A hypomorphic mutation reduces the level of expression of a gene or activity of a product

A hypermorphic mutation produces a greater-than-normal level of gene expression because it changes the regulation of the gene so that the gene product is overproduced

A gain-of-function mutation qualitatively alters the action of a gene. For example, a gain-of-function mutation may cause a gene to become active in a type of cell or tissue in which the gene is not normally active.

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Mutations• Mutations result from changes in DNA

• A base substitution replaces one nucleotide pair with another

• Transition mutations replace one pyrimidine base with the other or one purine base with the other. There are four possible transition mutations

Discussion Question 1

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• Please give an example of a transition mutation?

Discussion Question 1

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• Please give an example of a transition mutation?

• G -> A; purine -> purine• C -> T; pyrimidine -> pyrimidine

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Mutations• Transversion mutations replace a pyrimidine with

a purine or the other way around. There are eight possible transversion mutations

• Spontaneous base substitutions are biased in favor of transitions:

• Among spontaneous base substitutions, the ratio of transitions to transversions is approximately 2:1

Discussion Question 2

• Please give an example of a transversion mutation?

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Discussion Question 2

• Please give an example of a transversion mutation?

• G -> T; purine -> pyrimidine• C -> A; pyrimidine -> purine

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Mutations• Mutations in protein-coding regions can change an

amino acid, truncate the protein, or shift the reading frame:

• Missense or nonsynonymous substitutions result in one amino acid being replaced with another

• Synonymous or silent substitutions in DNA do not change the amino acid sequence

• Silent mutations are possible because the genetic code is redundant

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Mutations• A nonsense mutation creates a new stop codon

• Frameshift mutations shift the reading frame of the codons in the mRNA

• Any addition or deletion that is not a multiple of

three nucleotides will produce a frameshift

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Sickle-cell anemia The molecular basis of sickle-cell anemia is a mutant gene

for -globin

The sickle-cell mutation changes the sixth codon in the coding sequence from the normal GAG, which codes for glutamic acid, into the codon GUG, which codes for valine

Sickle-cell anemia is a severe genetic disease that often results in premature death

The disease is very common in regions where malaria is widespread because it confers resistance to malaria

What is sickle cell anemia? (cont’d)

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Transposable Elements• In a 1940s study of the genetics of kernel mottling in

maize, Barbara McClintock discovered a genetic element that could move (transpose) within the genome and also caused modification in the expression of genes at or near its insertion site

• Since then, many transposable elements (TEs) have been discovered in prokaryotes and eukaryotes

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Transposable Elements• The genomes of most organisms contain multiple copies

of each of several distinct families of TEs

• Once situated in the genome, TEs can persist for long periods and undergo multiple mutational changes

• Approximately 50 % of the human genome consists of TEs; most of them are evolutionary

remnants no longer able to transpose

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Transposable Elements• TEs can cause mutations by insertion or by

recombination

• In Drosophila, about half of all spontaneous mutations that have visible phenotypic effects result from insertions of TEs

• Genetic aberrations can also be caused by recombination between different (nonallelic) copies of a TE

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Spontaneous Mutations• Mutations are statistically random events—there

is no way of predicting when, or in which cell, a mutation will take place

• The mutational process is also random in the sense that whether a particular mutation happens is unrelated to any adaptive advantage it may confer on the organism in its environment

• A potentially favorable mutation does not arise because the organism has a need for it

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Mutation Hot Spots

• Mutations are nonrandom with respect to position in a gene or genome

• Certain DNA sequences are called mutational hotspots because they are more likely to undergo mutation than others

• For instance, sites of cytosine methylation are usually highly mutable

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Mutagenes• Almost any kind of mutation that can be induced by a mutagen can

also occur spontaneously, but mutagens bias the types of mutations that occur according to the type of damage to the DNA that they produce

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DNA Repair Mechanisms• Many types of DNA damage can be repaired• Mismatch repair fixes incorrectly matched base

pairs• The AP endonuclease system repairs nucleotide

sites at which the base has been lost• Special enzymes repair damage caused to DNA by

ultraviolet light• Excision repair works on a wide variety of

damaged DNA• Postreplication repair skips over damaged bases

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Ames test• In view of the increased number of chemicals used

and present as environmental contaminants, tests for the mutagenicity of these substances has become important

• Furthermore, most agents that cause cancer (carcinogens) are also mutagens, and so mutagenicity provides an initial screening for potential hazardous agents

• A genetic test for mutations in bacteria that is widely used for the detection of chemical mutagens is the Ames test

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Ames test• In the Ames test for mutation, histidine-requiring

(His-) mutants of the bacterium Salmonella typhimurium, containing either a base substitution or a frameshift mutation, are tested for backmutation reversion to His+

• In addition, the bacterial strains have been made more sensitive to mutagenesis by the incorporation of several mutant alleles that inactivate the excision-repair system and that make the cells more permeable to foreign molecules

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13Molecular Genetics of Cell

Cycle and Cancer

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• There are two major parts in the cell cycle:Interphase: G1 = gap1 S = DNA synthesis G2 = gap2Mitosis: M

• There are two essential functions of the cell cycle: To ensure that each chromosomal DNA molecule is

replicated only once per cycle To ensure that the identical replicas of each

chromosome are distributed equally to the two daughter cells

The Cell Cycle

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The Cell Cycle• The cell cycle is under genetic control

• A fundamental feature of the cell cycle is that it is a true cycle: it is not reversible

• Many genes are transcribed during the cell cycle just before their products are needed

• Mutations affecting the cell cycle have helped to identified the key regulatory pathways

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Key Mutational Targets• Many cancers are the result of alterations in cell

cycle control, particularly in control of the G1-to-S transition

• These alterations also affect apoptosis through their interactions with p53

• The major mutational targets for the multistep cancer progression are of two types: Proto-oncogenesTumor-suppressor genes

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Key Mutational Targets• The normal function of proto-oncogenes is to

promote cell division or to prevent apoptosis • The normal function of tumor-suppressor genes is

to prevent cell division or to promote apoptosis

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Oncogenes• Oncogenes are derived from normal cellular genes

called proto-oncogenes

• Oncogenes are gain-of-function mutations associated with cancer progression

• Oncogenes are gain-of-function mutations because

they improperly enhance the expression of genes that promote cell proliferation or inhibit apoptosis

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Tumor-Suppressor Genes• Tumor-suppressor genes normally negatively

control cell proliferation or activate the apoptotic pathway

• Loss-of-function mutations in tumor-suppressor genes contribute to cancer progression.

Questions?

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