7, 8 Flashcards
(73 cards)
1
Q
what did Walter Sutton discover?
A
studied great lubber grasshopper:
- body cells contained 22 chromosomes and X & Y chromosomes
- gametes contained 11 chromosomes and X or Y in equal numbers
- after fertilisation, cells with XX were females and cells with XY were males
1
Q
anatomy of a chromosome
A
- metaphase chromosomes are classified by the position of the centromere: metacentric (if it is in the middle of the chromosome) or acrocentric (if located close to an end)
- sister chromatids are held together by centromeres
- homologous chromosomes are a pair of chromosomes—one from each parent—that have the same size, shape, and genes at the same locations, but may carry different versions (alleles) of those genes.
2
Q
draw metacentric and acrocentric chromosomes
A
slide 5
3
Q
key stages of mitosis
A
Replication:
Interphase - cell grows and DNA is copied
Prophase – Chromosomes condense, spindle forms
Segregation:
Metaphase – Chromosomes line up in the middle
Anaphase – Chromatids are pulled apart
Telophase – New nuclei form
Cytokinesis – Cell splits in two genetically identical 2n daughter cells
4
Q
key stages of meiosis
A
Interphase – DNA is copied
Prophase I – Homologous chromosomes pair up and cross over
Metaphase I – Pairs line up in the middle
Anaphase I – Homologous chromosomes separate
Telophase I – Two nuclei form
Prophase II – New spindles form in each cell
Metaphase II – Chromosomes line up in the middle
Anaphase II – Chromatids are pulled apart
Telophase II – Four nuclei form
Cytokinesis – Four haploid cells result
5
Q
key differences between mitosis and meiosis
A
Number of divisions:
Mitosis = 1
Meiosis = 2
Number of daughter cells:
Mitosis = 2 (diploid)
Meiosis = 4 (haploid)
Genetic similarity:
Mitosis = Identical to parent
Meiosis = Genetically different (variation)
Purpose:
Mitosis = Growth, repair, asexual reproduction
Meiosis = Sexual reproduction (gametes)
Homologous chromosomes:
Mitosis = Do not pair
Meiosis = Pair and crossover in Prophase I
6
Q
how was movement of chromosomes during mitosis and meiosis understood ?
A
microscopy provided a means to follow movement of chromosomes during cell division
7
Q
gamete
A
contains one-half the number of chromosomes as the zygote
8
Q
haploid
A
cells that carry only a single chromosome set
9
Q
diploid
A
cells that carry two matching chromosome sets
10
Q
n
A
the number of chromosomes in a haploid cell
11
Q
2n
A
the number of chromosomes in a diploid cell
12
Q
is chromosome number constant across species?
A
no, it varies from species to species but does not correlate with the size or complexity of the animal
13
Q
somatic cells
A
divide mitotically and make up the vast majority of an organism’s tissues
14
Q
germ cells
A
specialised role in the production of gametes:
15
Q
process of creation of a diploid offspring
A
- germ cells arise during embryonic development in animals and floral development in plants
- undergo meiosis to produce haploid gametes
- gametes unite with gamete from opposite sex to produce diploid offspring
16
Q
karyotype
A
- produced by cutting micrograph images of stained chromosomes and arranging them in matched pairs
- sex chromosomes and autosomes are arranged in homologous pairs
17
Q
autosomes
A
pairs of non-sex chromosomes
18
Q
sex chromosome
A
- provide basis for sex determination
- one sex has matching pair, other sex has non-matching sex chromosomes
- variation in sex determination between species
19
Q
mammal and drosophila sex chromosomes
A
female: XX
male: XY
20
Q
some grasshoppers sex chromosomes
A
female: XX
male: XO
21
Q
fish, birds, moths sex chromosomes
A
female: ZW
male: ZZ
22
Q
sex determination in humans
A
children receive only one X chromosome from mother but either X or Y from father
23
Q
X chromosomes in females
A
females typically have two X chromosomes that are genetically identical
24
how are chromosome behaviour and inheritance related?
chromosomes determine characteristics of organism (eg its sex); therefore, basis of inheritance must reside there
25
mitosis ensures that
every cell in an organism carries the same set of chromosomes
26
meiosis ensures that
one member of each chromosome pair is distributed to gamete cells
27
gametogenesis
the process by which germ cells differentiate into gametes
28
6 links between chromosome and gene behaviour
- each cell contains 2 copies of each chromosome; each cell contains 2 copies of each gene
- chromosome complements appear unchanged during transmission from parent to offspring; genes appear unchanged during transmission from parent to offspring
- homologous chromosomes pair and then separate to different gametes; alternative alleles segregate to different gametes
- maternal and paternal copies of chromosome pairs separate without regard to the assortment of other homologous chromosome pairs; alternative alleles of unrelated genes assort independently
- at fertilisation an egg's set of chromosomes unite with randomly encountered sperm's chromosomes; alleles obtained from one parent unite at random with those of another parent ; alleles obtained from one parent unite at random with those of another parent
- in all cells derived from a fertilised egg, one half of chromosomes are of maternal origin, and half are paternal; in all cells derived from a fertilised gamete, one half of genes are of maternal origin, and half are paternal
29
Thomas hunt morgan
- started using a new genetic model: drosophila melanogaster, the common fruitfully
- in 1910, morgan discovered a white-eyed male among his true-breeding stocks of red-eyed flies (first drosophila mutation identified)
30
advantages of drosophila as a model organism
- small size
- short generation time of 10 days at room temp
- each female lays 400-500 eggs
- easy to culture in laboratory
- small genome
- large chromosomes
- many mutations available
31
draw a table describing how sex determination in humans differs from drosophila
32
nomenclature for drosophila genetics
wild type allele: allele that is found in high frequency in a population (greater than or equal to 1%) is denoted with a +
mutant allele: allele that is found in low frequency (less than 1%): denoted with no symbol
recessive mutation: gene symbol is in lower case
dominant mutation: gene symbol is in upper case
33
examples of notations for drosophila
- Cy, Sb, D are dominant mutations
- vg, v, y, e are recessive mutations
- vg+ - wild type allele for vestigial gene locus
- Cy+ - wild type allele for curly gene locus
34
eye colour in drosophila
- flies with red eyes carry the wild type allele, w+
- any variation in eye colour carry a mutant allele (eg white eye colour), w
35
sex linkage
genes for specific traits are carried on sex chromosomes
36
hemizygous
means having only one copy of a gene instead of the usual two (eg in males with X-linked genes)
37
what was Bridges' hypothesis and how did it arise?
X chromosomes were not segregating properly in meiosis, leading to 1/2000 females being white-eyed and 1/2000 males being red-eyed during a w/w x w+/Y cross
38
how was Bridges' hypothesis proved?
he did large enough experiments to observe rare meiotic events he called non-disjunction.
39
draw 3 diagrams:
- normal X chromosome segregation
- nondisjunction in meiosis I
- nondisjunction in meiosis II
slide 34
40
how did nondisjunction lead to unexpected phenotypes in drosophila?
males receive one X chromosome from father: red eyes
females receive two X chromosomes from mother: white eyes
41
aneuploidy
abnormal number of chromosomes
42
X-linked recessive traits exhibit 6 characteristics seen in pedigrees
- Trait appears in more males than females.
- Mutation and trait never pass from father to son.
- Affected male does pass X-linked mutation to all daughters, who are then unaffected carriers.
- Trait often skips a generation.
- Trait only appears in successive generations if sister of an affected male is a carrier. If so, her sons have a 50% chance of showing the trait.
- All sons of affected female show trait and all daughters of affected female are carriers.
43
example of X-linked recessive trait in humans
haemophilia A
44
X-linked dominant traits exhibit 4 characteristics seen in pedigrees
- Trait appears in more females than males.
- Sons and daughters of an affected heterozygous female have a 50% chance of showing the trait.
- Trait is seen every generation.
- All daughters but no sons of affected male show trait.
45
example of X-linked dominant trait in humans
hypophosphatemia
46
Y-linked traits exhibit 3 characteristics seen in pedigrees
- Trait appears only in males.
- All sons but no daughters of affected male show trait.
- Females do not show and cannot transmit trait.
47
what unusual observations did Bateson and Punnett make?
when crossing purple flowers, long pollen (PPLL) with red flowers, round pollen (ppll)
they found more parental-type combinations and fewer recombinants than expected.
however, according to Mendel’s law of independent assortment, they expected a 9:3:3:1 ratio in the F2 generation.
48
what unusual observations did Morgan make?
crossed red eyed, full size wing (pr+vg+/pr+vg+) with purple eye, vestigial wing (pr vg/pr vg)
again, parental combinations appeared more often than recombinant ones and recombinant types were less frequent than expected by chance.
49
what did Frans Janssens observe in 1909? what did Morgan suggest relating to this?
chiasmata, regions in which nonsister chromatids of homologous chromosomes cross over each other. Morgan suggested these were sites of chromosome breakage and exchange resulting in genetic recombination
50
cytogenetics
The study of chromosomes inside cells, especially during cell division.
51
crossovers
- physical exchanges among nonsister chromatids; visualised cytologically as a chiasma (plural = chiasmata)
- typically, several crossing-over events occur within each bivalent or tetrad in each meiosis
52
function of chiasmata
physically hold homologous chromosomes together and assure proper segregation at anaphase I
53
Morgan's first prediction and second prediction relating to crossing over
First:
- Crossing over occurs during meiosis between homologous chromosomes, allowing genes to be exchanged.
- This explains how linked genes can sometimes produce recombinant offspring
Second:
- The frequency of crossing over between two genes is proportional to the distance between them on the chromosome.
- Genes closer together have less chance of crossover (more likely inherited together).
54
Mendel's fruit fly test cross progeny ratios
>50% of progeny have parental type (1/4 one, 1/4 other)
<50% have recombinant phenotype
55
genetic recombination depends on
the reciprocal exchange of parts between maternal and paternal chromosomes
56
what did McClintock and Creighton observe?
- They studied corn (maize) chromosomes with visible physical markers (like a knob on one chromosome end and a translocation on the other).
- They tracked the inheritance of these physical chromosome markers alongside genetic traits.
- this provided visual confirmation that chromosomes cross over
- correlation between genetic crossover and chromosomal crossover
- this was verified in drosophila by Stern
57
Meiosis contributes to genetic diversity in 2 ways
- independent assortment of non homologous chromosomes creates different combinations of alleles among chromosomes
- crossing over between non sister homologous chromatids creates different combinations of alleles within each locus
58
when does recombination take place?
at the four-chromatid (four-strand) stage of Meiosis I (prophase I)
59
5 stages of crossing over
1. leptotene: thread-like chromosomes begin to condense, becoming visible as discrete structures, although the sister chromatids cannot yet be distinguished
2. zygotene: chromosomes are clearly visible and begin pairing with homologous chromosomes along the synaptonemal complex to form a bivalent, or tetrad
3. pachytene: the homologs synapse fully, recombination nodules appear along the synaptonemal complex
4. diplotene: the bivalent pulls apart slightly, but homologous chromosomes remain connected due to recombination at crossover sites (chiasmata)
5. diakinesis: the bivalent condenses further
60
synaptonemal complex
zipper-like elaborate protein structure that aligns chromosomes base pair by base pair
61
terminalization
process where the chiasmata move from the middle of the homologous chromosomes toward the ends (telomeres) as meiosis progresses, especially during diplotene and diakinesis stages
62
what happens after terminalization?
anaphase I
63
Holliday's model for mechanism of recombination
1. Homologues physically break, exchange parts, and rejoin
2. Breakage and repair create reciprocal products of recombination
3. Recombination events can occur anywhere along the length of a DNA molecule, but at some locations with a higher frequency than others
4. The exchange is precise - no gain or loss of nucleotide pairs occurs– thus preventing mutation from occurring
5. Gene conversion - where small segments of information from one homologous chromosome transfers to the next - may result in unequal yield of the two alleles
64
draw a table showing the meiotic products based on crossover between genes
slide 27
65
genetic recombination
mixing of genes during gametogenesis produces gametes with combinations of genes that are different from the combinations received from parents
66
unlinked vs linked genes
- genes on non homologous chromosomes (unlinked genes) assort independently
- genes on the same chromosome co-segregate
67
crossovers result in recombination between
linked genes
68
crossover frequency is a function of
the distance between two loci
69
can crossover ever occur when genes are completely linked?
no; the genes are too close
70
recombination frequency can never exceed
50% (ie more recombinant types than parental types)
71
single crossover vs double crossover
single: parental and recombinant
double: all parental
72
how do you test for linked vs unlinked genes?
test cross to distinguish between linked and unlinked genes, you'd typically cross a heterozygous individual with a homozygous recessive individual for the traits being studied.
2 genes unliked = 1:1 between parental genotypes and recombinant genotypes
2 genes linked = more parental genotypes than recombinant genotypes
