Evolution 2

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central dogma of molecular biology

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'once information gets into protein, it can't flow back to DNA'

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exon

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In eukaryotes, the transcribed sequences of a gene=coding regions

After a gene is transcribed, portions transcribed are processed into messenger RNA (mRNA)

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intron

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noncoding regions separating genes

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codon

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triplet on mRNA that specifies an amino acid

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source of completely new genetic variation

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All arises via mutation

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mutation

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= change in the genetic material carried by individual organisms

= an error in the replication of a nucleotide sequence, or any other alteration of the genome that is not manifested as reciprocal recombination
=independent from whether or not it has a phenotypic effect

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is mutation adaptation?

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No. Considered by most biologists to be errors. Thus, it is generally thought to be not an adaptation, but a consequence of un-repaired DNA damage

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3 classes of mutations (list)

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1) Gene/point mutations (we will focus on these)

2) Sequence changes arising from recombination (intragenic recombination results in new DNA sequences much like crossing over between genes)

3) Changes caused by transposable elements (jumping genes)

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base-pair substitutions

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replace one nucleotide with another

Transition--change to same class, eg purine to purine
Transversion--change to opposite class, eg. purine to pyrdamine

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Frameshifts

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insert or remove base pair. Changes every amino acid downstream.
-insertion
-deletion

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Synonymous mutations

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– have no effect on the amino acid sequence of the protein, therefore no effect on phenotype and the mutation is ‘hidden from selection’

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Non-synonymous mutations

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– result in an amino acid change, this change may have little or no effect on the function of the protein, and thus little phenotypic effect, or there may be substantial functional and phenotypic effects

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Estimating mutation rates

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a) indirectly: comparisons among species
b) directly: experiments

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what's "dubious" about estimating mutation indirectly?

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it assumes mutations are not affected by natural selection (i.e. are ‘neutral’)

For example, if many mutations reduce the fitness of organisms, then many mutations will be removed from the population via natural selection, leading us to underestimate the rate of mutation = fewer mutations remain and are observed than actually arose

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“Mutation-Accumulation experiments”

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Done in lab, usually with yeast.

Mutation rates are low, therefore experiments require many generations and many offspring
Must be careful that natural selection does not act, because selection can remove new mutations from the population, resulting in an underestimate of the mutation rate

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What findings emerged from mutation accumulation experiments?

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1) Mutation rates vary among species
2) Mutation rates vary among genes.

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Evolutionary consequence of mutation rates

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Mutation rates *per gene* or *per nucleotide* site are indeed very low

However, summed over all genes in the genome, the input of new variation via mutation can be considerable

e.g. mutation rate in humans of 2.5 x 10-8 mutations per site per generation = 175 new mutations in each person

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Effect of mutations on phenotype

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* Mutations can alter phenotypic characters such as size, color or enzyme activity

* The effects can vary from none to drastic

* Many mutation are pleiotropic

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pleiotropy

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= multiple phenotypic effects of a single nucleotide or gene (
meaning they affect more
than one character)

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effects of mutation on fitness

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Can range from highly advantageous (increase fitness) to neutral (no effect on fitness) to highly disadvantageous (decrease fitness)

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fitness

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Fitness = survival and reproductive success

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what's meant by "mutations occur at random"?

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understand exactly what this means

The chance that a particular mutation will occur is NOT influenced by environment in which that mutation would be dis/advantageous

NOTE: Does not mean that all genes and genomic regions are equally mutable!

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What can we learn from lab cultures of the bacterium Pseudomonas fluorescens?

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ALot of mutations are neutral or have not effect

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Karyotype

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= The description of an organisms complement of chromosomes, including their number, size, shape, and internal arrangement

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Polyploidization

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– is alteration in the number of whole sets of chromosomes (e.g. chromosomal doubling) in karyotype

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Chromosomal rearrangements

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– are changes in the structure of chromosomes, caused by breaks in chromosomes, followed by rejoining of the pieces in new configurations (consequences of such rearrangements discussed later in the course, for now simply know what they are and roughly how they occur)

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Two types of karyotype changes

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1. polyploidization
2. chromosomal rearrangements

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Chromosome inversions

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-one of chromosome pair "rolls"
-can jumble sequences