Chapter 18 Facts - Genetics, A Conceptual Approach, V.3 By Benjamin A. Pierce

Front 1

What is a mutation?

Back 1

DNA is very stable, and it is very accurate when it replicates. A mutation is when there is an inherited change in genetic information. The descendants can be cells or organisms.

Front 2

Describe somatic mutations.

Back 2

Somatic mutations are in somatic tissues, which don’t make gametes. When a mutated somatic cell divides, the mutation is passed on to daughter cells.

Front 3

Describe germ-line mutations.

Back 3

Germ-line mutations are in cells that make gametes. A germ-line mutation can be passed on to future generations.

Front 4

Describe gene mutations versus chromosome mutations.

Back 4

Gene mutations only affect a single gene, while chromosome mutations affect the number or structure of chromosomes.

Front 5

What is a base substitution, and what are the two types?

Back 5

A base substitution is the simplest type of gene mutation, where a single nucleotide is altered in DNA. There are two types: transitions and transversions.

Front 6

Describe a transition base substitution.

Back 6

In a transition base substitution, a purine is replaced by a different purine, or when a pyrimidine is replaced by a different pyrimidine.

Front 7

Describe a transversion base substitution.

Back 7

In a transversion base substitution, a purine is replaced by a pyrimidine, or a pyrimidine is replaced by a purine.

Front 8

What is the relation of the possible numbers of transition base substitutions versus transversions?

Back 8

The number of possible transversions is twice the number of possible transitions, but transitions are more common.

Front 9

What are insertions and deletions, and what do they cause?

Back 9

Insertions and deletions are the addition or removal of one or more nucleotide pairs. These are more common that base substitutions, and they can cause frameshift mutations.

Front 10

What are frameshift mutations, and what is their effect?

Back 10

Frameshift mutations are changes in the reading frame as a result of an insertion or a deletion. They normally alter all the amino acids that are coded by the nucleotides, so they change the phenotype.

Front 11

What are in-frame mutations and deletions?

Back 11

In-frame mutations and deletions are ones where the deletion or addition is of three nucleotides in a row, so they don’t change the reading frame, but they can still affect the phenotype.

Front 12

Describe expanding trinucleotide repeats, and where they are found.

Back 12

An expanding trinucleotide repeat is a mutation where the number of copies of a trinucleotide (a set of three nucleotides) increases in number. Is is seen in the gene FMR-1, which causes fragile-X syndrome.

Front 13

Describe forward mutations versus reverse mutations.

Back 13

A forward mutation alters the wild-type phenotype, while a reverse mutation changes a mutant back to the wild-type phenotype.

Front 14

What is a missense mutation?

Back 14

A missense mutation is where a base substitution results in a different amino acid being put into the protein.

Front 15

What is a nonsense mutation?

Back 15

A nonsense mutation is where a sense codon (codes for an amino acid) is changed into a nonsense codon (a codon that ends translation). If this mutation happens early in the mRNA sequence, the protein will be shortened and not functional.

Front 16

What does a silent mutation do?

Back 16

A silent mutation creates a different DNA sequence, which specifies the same amino acid as the wild-type sequence because the genetic code is redundant.

Front 17

What is a neutral mutation?

Back 17

A neutral mutation is a missense mutation that changes the amino acid sequence of the protein, but doesn’t change its function.

Front 18

When do neutral mutations happen?

Back 18

Neutral mutations occur when one amino acid is replaced by another amino acid that is chemically similar, or when the affected amino acid doesn’t have a big influence on the protein.

Front 19

What is a loss of function mutation?

Back 19

A loss of function mutation causes a complete or partial absence of normal protein function. It changes the protein structure so that the protein doesn’t work right anymore.

Front 20

What are loss of function mutations normally, and what does the organism have to be to show them?

Back 20

Loss of function mutations are normally recessive, and a diploid organism has to be homozygous for a loss of function mutation to show the effects.

Front 21

What is a gain of function mutation?

Back 21

A gain of function mutation produces a totally new trait, or else causes a trait to appear in the wrong tissue or at the wrong time in development; they are normally dominant.

Front 22

What are conditional and lethal mutations?

Back 22

Conditional mutations are only expressed under certain conditions, and lethal mutations cause premature death.

Front 23

What is a suppressor mutation, and what are the two types?

Back 23

A suppressor mutation is a genetic change that hides or suppresses the effect of another mutation. It is different from reverse mutations, where the genes actually change back to normal. There are two types, intragenic and intergenic.

Front 24

Describe intragenic suppressor mutations.

Back 24

Intragenic suppressor mutations are in the same gene as the one that contains the mutation that is being suppressed and can work in several ways.

Front 25

What does an intragenic suppressor mutation do?

Back 25

The suppressor can change a second nucleotide in the same codon changed by the original mutation which makes the same amino acid as it would normally, or it could suppress a frameshift mutation, or it can add a nucleotide to the third codon and restore a reading frame, or it can make compensatory changes in the protein.

Front 26

Describe intergenic suppressor mutations.

Back 26

Intergenic suppressor mutations happen in a gene other than the one with the original mutation. They can work by changing the way that mRNA is translated.

Front 27

What happens in an intergenic suppressor mutation?

Back 27

In an intergenic suppressor mutation, the original DNA sequence is AAC for leucine. The sequence mutates to ATC, a stop codon. The ATC nonsense mutation can be suppressed by a different gene that codes for tRNA, and this second mutation can result in a codon that can pair with the UAG stop codon.

Front 28

What is the effect of intergenic suppressor mutations?

Back 28

The effect of the intergenic suppressor mutation would depend on the role of the amino acid in the structure of the protein, but the effect will probably be less detrimental than the effect of a nonsense mutation, which stop translation prematurely.

Front 29

When can intergenic suppressors also work?

Back 29

Intergenic suppressors can also work through genic interactions when polypeptide chains produced by two genes can interact to make a functional protein. A mutation in one gene can change the encoded polypeptide, so the interaction between the polypeptides is ruined, so a good protein is not made.

Front 30

What is a mutation rate?

Back 30

A mutation rate is the frequency at which a wild-type allele changes into a mutant allele, and it is shown as the number of mutations per biological unit.

Front 31

What are the three things mutation rates are affected by?

Back 31

Mutation rate calculations are affected by three things: frequency of change in DNA, the probability that the change will be repaired, and the probability that a mutation will be recognized and recorded.

Front 32

What are adaptative mutations?

Back 32

Adaptative mutations are when stressful environments can induce more mutations in bacteria, since change is necessary for survival.

Front 33

What are spontaneous mutations, and what is their primary cause?

Back 33

Spontaneous mutations come from natural changes in the DNA structure. The primary cause is wobbling codons, where normal, protonated, and other base forms can pair with DNA.

Front 34

What are induced mutations?

Back 34

Induced mutations come from changes caused by environmental chemicals or radiation.

Front 35

What is an incorporated error, and when do they occur? What do they produce?

Back 35

An incorporated error occurs when a mismatched base is incorporated into a new nucleotide chain. The original incorporated error then leads to a replication error, which makes a permanent mutation.

Front 36

What is strand slippage? When does it happen?

Back 36

Strand slippage happens when one nucleotide strand makes a small loop, and if the loop is on a new strand, an insertion happens.

Front 37

What causes unequal crossing over, and what is the result?

Back 37

Unequal crossing over is caused by misaligned strands, and results in one DNA molecule with an insertion and one with a deletion.

Front 38

What is a mutagen?

Back 38

A mutagen is any environmental agent that significantly increases the mutation rate above the spontaneous mutation rate.

Front 39

Describe base logs.

Back 39

Base logs are one class of chemical mutagens, and they are chemicals with structures like the four DNA bases. DNA polymerases can’t tell them from the regular bases, so if they are present during replication, they can get put into the new DNA.

Front 40

Describe the 5BU base log example. What is its structure? What does it cause?

Back 40

5BU is base log analog of thymine. It has the same structure as thymine, except it has a bromine (Br) atom on the 5-carbon atom instead of a methyl group. Normally it pairs with adenine like thymine does, but sometimes it causes a transition by pairing with guanine.

Front 41

Describe pyrimidine dimers.

Back 41

Pyrimidine dimers are formed when purine and pyrimidine absorb UV light and form chemical bonds between adjacent pyrimidine molecules on the same DNA strand. They are made of two thymine bases (thymine dimers).

Front 42

What do pyrimidine dimers do?

Back 42

Pyrimidine dimers distort the DNA configuration and block replication. Most pyrimidine dimers are repaired immediately, though. When they block replication, the cell dies, so UV light kills bacteria well.

Front 43

What is the SOS system in bacteria?

Back 43

The SOS system helps bacteria circumvent the replication blocks from pyrimidine dimers and other kinds of DNA damage, but it makes mistakes in the process and greatly increases DNA mutations.

Front 44

What does mismatch repair do?

Back 44

Mismatch repair corrects many incorrectly inserted nucleotides. The system detects them and corrects small unpaired DNA loops. The mismatch repair enzymes cut out the distorted fragment and fill the gap with new nucleotides using the old DNA strand as a template.

Front 45

What is direct repair?

Back 45

Direct repair changes altered nucleotides back to their original (correct) structures.

Front 46

Give and describe an example of direct repair.

Back 46

Photoreactivation is the best example of direct repair, and it uses white light and photolyase to do the repairs.

Front 47

What happens in base-excision repair?

Back 47

In base-excision repair, the mutated base is excised and then the entire nucleotide gets replaced. It is catalyzes by a set of enzymes called DNA glycolases, each of which recognizes and removes a specific type of modified base by cleaving the bond that links the base to the 1’-carbon atom of the deoxyribose sugar.

Front 48

What happens after the base is removed in base-excision repair?

Back 48

After the base is removed in base-excision repair, the enzyme AP endonuclease cuts the phosphodiester bond, and other enzymes remove the deoxyribose sugar. Then, DNA polymerase adds news nucleotides to the exposed 3’-OH group.

Front 49

What happens in nucleotide excision repair?

Back 49

In nucleotide-excision repair, bulky DNA lesions (like pyrimidine dimers) are removed by a complex of enzymes that scan the DNA, separate the nucleotide strands at the mutation region, cleave the sugar-phosphate DNA backbone, peel away part of the damaged strand, fill in the gap with DNA polymerase, and seal the nicks with DNA ligase.