1
Q
Sources of genetic variation
A
- independent assortment at metaphase 1
- crossing over at prophase 1
- fusion with 2 gametes
2
Q
Human chromosome number
A
46 - 22 pairs and a pair of sex chroomosomes
3
Q
Telocentric
A
centromere at one end (<1.7 ratio)
4
Q
Acrocentric
A
centromere off centre (>1.7 ratio)
5
Q
Metacentric
A
centromere in middle (>7 ratio)
6
Q
Telomere role
A
stabilise the chromosomeW
7
Q
What does nucleoli contain
A
rRna and components of ribosomes
8
Q
Nucleolar organiser
A
- secondary constriction
- located in different positions in different species
- contains a cluster of genes that code for rRNA
9
Q
Primary constriction
A
centromere
10
Q
Why can horse and donkeys form a mule zygote
A
homologous pairs dont interact during mitosis but are similar enough to make zygote
11
Q
Euchromatin
A
- loosely packed
- rich in genes
- increased recombination frequency
12
Q
Heterochromatin
A
- tightly packed
- less genes
- decreased recombination frequency
13
Q
Common chromosome stain
A
Giemsa
14
Q
Chromatin
A
- DNA associated with histones
- not uniformly distributed
15
Q
Nucleosomes
A
DNA packed around histones
16
Q
Solenoid
A
nucleosomes organised into coils
17
Q
Proteinaceous scaffold
A
final arrangement of solenoid coils
18
Q
Prokaryotic genome size and density
A
- smaller
- more dense
19
Q
Why are eukaryotic genomes less dense
A
- genes with multiple exons are spliced in multiple different ways
20
Q
Major components of eukaryotic genome
A
- 1.5 % exons
- Unique = introns, non coding DNA, regulatory sequences, exons
- Repetitive DNA (L1 and Alu) -includes gene families, telomeric repeats, satellite repeats and transposable elements
21
Q
Dispersed gene families
A
- DNA sequence of genes within family that have diverged to different functions
- all proteins coded by family of homologous genes
- some have become pseudogenes
- dispersed throughout genome
- Hemoglobin = alpha and beta globin gene families have multiple different genes at different stages of life
22
Q
Tandem gene families
A
- multiple repeats of the same gene (duplication)
- organised as tandem repeats
- often share similar functions
- examples = histones and rRNA gene in nucleolar organiser
23
Q
Satellite repeats
A
- highly repeated tandem sequences
- heterochromatic short AT rich tandem repeats in the centromere
- can be very abundant
- short DNA sequence repeats
- vary between individuals so can be used for DNA fingerprinting and paternity tests
24
Q
Microsatellites
A
shorter repeated DNA sequence (2-6bp) at a particular locus on the chromosome
25
Minisatellites
longer repeats which doesnt code for proteins (15-100bp)
26
Transposed sequences
- mobile DNA sequences
- can insert themselves into many different locations in the genome
27
Telomeric repeats
- composed of tandem arrays
- sequence = TTAGGG
28
How can telomeres be visualised
probe chromosome with short sequence of fluorescent ssDNA that binds via complementary base pairing, ssDNA finds homologous chromosomal region during hybridisation
29
Genes
physical and functional unit of heredity
30
Locus
specific place where a gene is located
31
Alleles
different versions of the same gene
32
Homologous
paired chromosomes with same gene sequence and loci but may differ in alleles
33
Characteristics of mitosis
- in somatic cells
- one cell division results in 2 daughter cells
- chromosome number maintained
- one S phase per cell division
- no pairing of homologs
- no crossover
- centromeres divide at anaphase
- conservative
- can be diploid or haploid
34
Characteristics of meiosis
- cells in sexual cycle
- 2 cell divisions resulting in 4 cells
- chromosome number is halved
- one S phase for both divisions
- full interaction of homologs at prophase 1
- atleast 1 crossover per homolog during prophase 1
- centromeres divide at anaphase 1
- promotes variation
- cells
35
Ratio that occurs when genes are found on different chromosomes
9:3:3:1 = independent assortment/unlinked
36
Linked genes
found close together so they have a decreased chance of crossover/recombination
37
Test cross
- involves crossing a heterozygous individual with homozygous recssive
38
Trans test cross
dominant allele on different homolog, has decreased recombination
39
Cis test cross
dominant allele on the same homolog, increased recombination
40
Inter chromosomal recombination
genes on different chromosome so recombination occurs via independent assortment during anaphase
41
Intra chromosomal recombination
recombination of genes on the same chromosome
42
How does the recombination frequency change the farther apart the genes are
it gets closer to 50% but cannot become more than 50% as only 2 sister chromatids are involved
43
Advantages of 3 point test cross
- includes double crossovers
- allows the order of genes to be determined
- produces 2 parental and 6 recombinant progeny
44
Interference
- occurs when expected frequency is not equal to the observed
- one crossover is inhibiting the other crossover
- when I = 1 there is complete interference
45
Neurospora
- haploid fungi (n=7)
- asexually and sexually
- has 2 identical looking mating types which can be regarded as simple sexes
- one of the first model organisms
46
Neurospora life cycle
- when 2 different mating types come into contact their cell walls and nuclei fuse to form a diploid nuclei
- perithecia forms which contain asci
- diploid nuclei undergoes meiosis to produce 8 ascospores in one long asci
- each meiocyte produces a linear array of 8 ascospores
47
M1 pattern of segregation
crossover hasnt occured - 4:4 pattern
48
M2 pattern of segregation
crossover between gene and centromere - 2:2:2:2 pattern
49
How do different ascus arrangements occur
centromeres attaching to the spindle at random during the 2nd meiotic division
50
To calculate the map distance between a gene and centromere
(recombo/total x100) / 2
- HAVE TO DIVIDE BY 2 AS ONLY HALF OF THE PRODUCTS OF ANY MEIOSIS WITH A SINGLE CROSSOVER WILL BE RECOMBINANT
51
physical basis of recombination
- crossing over involves breakage and reunion of non-sister chromatids at the 4 chromatid stage
- chiasmata = sites of crossing over
52
Physical map of human chromosomes
- shows actual location of genes
- good resolution and accurate
- sequence based
- gene loci determined
- sources = somatic cell hybrid mapping and genome sequencing
53
Genetic map of human chromosomes
- created from recombination frequency data
- limited accuracy
- gene order and relative position determined
- source = recombination analysis of phenotypic and molecular markers
54
Why is mapping human genes difficult
inability to perform a testcross and humans have a small number of progeny
55
X chromosome mapping
- first human chromosome mapped
- suitable for mapping by recombination analysis because males are hemizygous
56
Techniques to construct human genome maps
- human and mouse somatic cell hybrids = different human chromosomes will be lost in different cell lines
- FISH = fluorescent hybridisation, tags chromosomes
- molecular markers = map gene of interest to a molecular marker
57
Structural chromosomal changes
- deletion
- duplication
- inversion
- translocaion
58
Deletions
- usually requires 2 chromosomal breaks
- results in the absence of a centromere
- terminal = at end
- interstitial = within chromosome arm
- intragenic = small deletion within a gene that inactivates the gene
- multigenic = involve several genes, creates dosage problem
- in meiosis chromosomes with deletion will form a loop to account for the loss
59
Duplications
- tandem = adjacent, inserted next to duplicate
- insertional = located elsewhere in genome
- a diploid cell with a duplication with have 3 copies which causes a dosage issue
- effective for increasing the number of copies and size
- to detect a duplication look at banding patterns and presence of loops at meisosi
60
Inversions
- involve 2 chromosome breaks in same chromosome, region is flipped then reinserted
- paracentric = centromere is outside the inversion
- pericentric = centromere is inside the inversion
- dont change the amount of genetic material
- has to have 1 centromere and 2 telomeres to survive
- acentric chromosome = lacks a centromere so it will not segregate in anaphase
- inversion loop forms during meiosis to pair with homolog
61
Translocations
- involves movement of genetical material between non-homologous chromosomes or within the same chromosome
- reciprocal = part of one chromosome is exchanged with one another
- non reciprocal = part of one chromosome moves to another without exchange
- can result in hybrid genes
- chimps have more chromosomes due to translocation and loss of chromosome
62
chimp and human chromosome difference - translocation
- human chromosome 2 is metacentric with G-banding patterns that match 2 acrocentric chromosomes in chimps
- human chromosome 2 has end to end fusion of 2 chromosomes
63
Numerical chromosomal changes
- euploidy
- aneuploidy
64
Euploidy
- change in number of sets
- common in plants = polyploidy
- major mechanism by which new plant species have evolved
- increased chromosome number = increased cell size
- autopolyploid = sets from one species
- allopolyploid = sets from 2 or more species that are closely related and partly homologous
65
Aneuploidy
- change in number of individual chromosomes
- can arise in chromosome is lost if centromere is deleted, small chromosomes generated by translocations are lost, gametes arise from nondisjunction due to failure of seperation
66
Meiotic nondisjunction
- happens in humans but most die in utero
- monosomy = 2n-1, non viable except turners chromosomes
- trisomy = 2n + 1, viable in 21 (down syndrome),13,18 and sex chromosomes
67
Mosaic
genetically distinct cells within an organism that are derived from a single zygote
- at birth = nondisjunction
- later in life = alterations
68
Chimera
genetically distinct cells within an organism that are derived from multiple zygotes
- birth = fusion of 2 zygotes
- later in life = transfusion of donor cells
69
Types of sex determination
- chromosomal = sex determined by genes located on the sex chromosomes
- genes
- environment
70
Gene dosage
number of copies in a gene present in a genome
71
Turners syndrome
- XO = sterile female
- aneuploidy
72
Klinefelters syndrome
XXY = sterile male
- anueploidy
73
SRY gene
- responsible for maleness
- encodes for a transcription factor
- protein binds to DNA and stimulates transcription of other genes which promotes development of testes
- turns on SOX 9
- SOX 9 cooperates with other genes to activate expression of AMH
- determined genetically and hormones
74
Androgen insensitivity syndrome
mutation on gene for androgen receptor on the X chromosome, embryo develops as female
75
Barr body
- inactivated X chromosome
- stable through mitosis but reactivated for meiosis in those that give rise to ova
- inactivation involves modification of DNA and histones
- multiple copies of RNA product of XIST attach and inactivate X
- females arent that affected by X linked diseases due to the 2 copies of X
76
Tortoiseshell cats
- mosaic of 2 type of cells = active X derived from male and active X derived from female
- occurs early in development
Biosci 202
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