Lecture 2: Mutation, Variation and genome organisation Flashcards
What are mutation?
- the failure to store genetic information faithfully.
- affect whole chromosome or single genes
- spontaneous or induced
- harmful, neutral or beneficial
- the source of ALL genetic variation
- necessary for natural selection and evolution
Causes of mutation.:
induced mutations- radiation, UV, X-rays. Anything that increases chemical reactivity in the cells - MUTAGEN
e.g. Chernobyl 1986
Fukushima 2011
Karyotype..
is simply a picture of a person’s chromosomes
Humans chromosomes are present in..
pairs they’re diploid.
23 pairs, 2n = 46
Gametic cells are
(egg and sperm)
haploid!
Chromosomal variation:
- Polyploidy
- Aneuploidy
- Translocations
- Deletions
- Inversions
Polyploidy:
cells and organisms are those containing more than two paired (homologous) sets of chromosomes.
Bacteria are usually…Diploid, monoploid or dispermy?
MONOPLOID
If greater than 2 sets of chromosomes i.e. dispermy
results in 3 complete sets of chromosomes; 3n = 69.
Happens in 1-3% of conceptions - ALWAYS LETHAL .
Dispermy-
fertilisation of one egg by two sperm
Polyploidy in higher plants is
common. Related species often vary in ploidy.
Aneuploidy is when
one set of chromosomes is incomplete. e.g. a chromosome is missing, an extra chromosome is present
Autosome..
any chromosome that is not a sex chromosome
Nullisomy:
Autosomes when both members of a pair are missing - lethal
Monosomy:
Autosomes when one member of a pair is missing - lethal
Trisomy:
Autosomes when one extra chromosome; usually lethal
Trisomy 21 -
Down syndrome.
Longevity = 40+ not considered lethal
Aneuploidy in sex chromomses..
Lacking a chromosome:
45X - Turner’s syndrome
45Y - Inviable
Extra chromosomes: 47XYY, 47XXY, 47XXX Minor effects
Translocations:
Exchange of parts between NON-HOMOLOGOUS chromosomes
Homologous chromosomes..
=chromosomes that contain alleles for the same gene
Translocation;
carriers and offspring
carriers are normal
Offspring can have the wrong number of copies of each chromosome - usually lethal
Deletions:and example
- Part of a chromosome is missing
- Patient only has one copy of each gene in that region
- severity depends on size of missing region
- chromosome 5, Cri-du-chat syndrome
Inversions:
Breakpoints in chromosome appear. Middle section is inverted.
say: ABCDEFGHI
ABC|DEF|GHI
ABCFEDGHI
Often no effect on phenotype- a balanced rearrangement.
Possible problem in meiosis
Meiosis…
type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells
Structure of DNA:
-sugar phosphate backbone
-polymer made up of units called nucleotides
-bases held together by H-bonds
-Adenine
-Thymine
-Guanine
-Cytosine
-AT & CG
(in RNA T swaps to U)
DNA is a
Double helix
Size of the human genome…
3X10^9 (3 billion) base pairs long
Cricks second great discovery :
The genetic code.
Where 3 nucleotides are called a codon
Every codon specifies one amino acid
An amino acid can be encoded by more than one codon - the code is DEGENERATE
Organisation of the human genome:
- 20-300 genes
- only 1.5% encodes for proteins
- ~3% regulates protein expression
- ~ 45% caused by transposons
- ~6.6% not transcribed
- ~44% Contains many useful variations
DNA mutations: coding region mutations..
- substitutions
- insertions and deletions
DNA mutations: Non-coding region mutations..
- repeat length variation
- Useful genetic markers
Substitution Mutation…
CAT<> CAC
Can be silent (synonymous mutation) or replacement mutation (non-synonymous) depending on where the substitution occurs
Insertion/ deletion (indels) mutation…
Result is different protein encoded due to a shift in the open reading frame
Non- coding region mutation:
% of genome not involved in protein coding/expressio?..
Variation in these parts is..
95%
Very useful!
Single Nucleotide Polymorphisms (SNPs)
How many SNPs are found in man?
12,000,000 - 0.3% of genome
What is an SNP?
a variation in a single nucleotide that occurs at a specific position in the genome, where each variation is present to some appreciable degree within a population
Satellite DNA..
Micro satellites (or simple tandem repeats). Nearly always harmless. Widely used in finding diseases, conservation genetics, evolutionary genetics, agricultural improvement
Gametic mutations:
Heritable?
Effect?
Affect all cells?
Heritable? YES
Effect? Can be severe
Affects all cells? YES
Somatic mutations:
Heritable?
Effect?
Affect all cells?
Heritable? NO
Effect? Usually mild
Affects all cells? NO
Genetic variation is caused by…
MUTATION
there would be no evolution without
MUTATIONS
Mutations can affect
Chromosomes or nucleotides
Chromosomal mutations are often..
Lethal
The genetic code is…
Degenerate
Nucleotide mutation scan occur in ..
Coding or non-coding regions
Gametic mutations are,,
inherited
