Topic 11

Question 1

Describe the structure of a chromatid and the possible positions of the centromere

Answer 1

p arm and q arm
p arm tends to be shorter and the one at the top
metacentric
p arms and q arms same length 
submetacentric
p arms shorter than q arms
acrocentric
v short p arm
telocentric
no p arm (none in human body)

Question 2

What are telomeres?

Answer 2

repeated sequences at each end of the chromatids
the repeats that make up a telomere are eaten away slowly over many division cycles, providing a buffer that protects the internal chromosome regions bearing the genes

Question 3

How can chromosomes be analysed?

Answer 3

metaphase spreads
requires actively dividing cells
growth arrested in metaphase
staining of chromosomes
banding patterns 
dark patterns A T rich 
light patterns G C rich 

ideograms
diagram of banding patterns of chromosomes

karyotyping

Question 4

What are alleles?

Answer 4

alleles are different variants of genes
homologous chromosomes have the same genes but different variants (alleles)
sister chromatids have identical alleles
this is because the maternal and paternal copy of the gene can be different

Question 5

How many chromosomes are there in mitosis?

Answer 5

chromosome number stays the same, you have one round of replication and one round of division

Question 6

Describe what happens in prophase

Answer 6

chromosome condensation
nuclear envelope breakdown
centrosome separation
spindle fibres appear

Question 7

Describe what happens in prometaphase

Answer 7

spindle fibres attach to chromosomes
chromosomes condense
centrosomes move to poles of cell

Question 8

Describe what happens in metaphase

Answer 8

chromosomes align at the metaphase plate

Question 9

Describe what happens in anaphase

Answer 9

centromeres divide

sister chromatids move to opposite poles

Question 10

Describe what happens in telophase

Answer 10

nuclear envelope reforms
chromosome decondensation
spindle disassembly

Question 11

Describe what happens in cytokinesis

Answer 11

cytoplasm divides

parent cell becomes two daughter cells with identical genetic information

Question 12

What is nondisjunction?

Answer 12

missegregation of chromosomes
leads to aneuploidy (an unusual amount of chromosomes)
sister chromatids don’t separate and the whole of the chromosome goes to one end of the cell

Question 13

What happens in first post-zygotic division nondisjunction?

Answer 13

leads to non mosaic phenotype
the cell with the extra chromosome will continue to go through mitosis
the cell missing the chromosome will die off

Question 14

What happens in later post-zygotic nondisjunction?

Answer 14

leads to mosaicism, the presence of two or more cell lines in an individual

Question 15

How many chromosomes are present in meiosis?

Answer 15

number of chromosomes reduced
one round of replication
two rounds of division

Question 16

Describe what happens in meiosis 1

Answer 16

prophase 1
centrosomes separate
sister chromatids of a chromosome (eg maternal copy) and the sister chromatids of its homologue (eg paternal copy) have crossing over of genes to produce daughter chromosomes

metaphase 1
homologous pairs align on the metaphase plate and form a tetrad
random assortment of chromosomes

anaphase 1
homologous pairs separate to each pole

telophase 1
nuclear membrane appears
the daughter cells form as the cytoplasm divides and they are ready for another division

Question 17

What are the consequences of meiosis?

Answer 17

generation of genetic diversity, from:
crossing over of genetic material in prophase 1
random assortment of chromosomes in metaphase 1

maintaining constant chromosome number from generation to generation

Question 18

Describe what happens in meiosis 2

Answer 18

prophase 2
chromosomes condense
the nuclear envelope breaks down
centrosomes move apart

metaphase 2
the chromosomes line up individually along the metaphase plate

anaphase 2
the sister chromatids separate and are pulled towards opposite poles of the cell

telophase 2
nuclear membranes form around each set of chromosomes
chromosomes decondense

Question 19

What is the difference in spermatogenesis and oogenesis?

Answer 19

from one spermatocyte to 4 sperm

from one oocyte to 1 egg and 3 polar bodies

Question 20

What are the consequences of faulty meiosis?

Answer 20

nondisjunction 
leads to aneuploidy in gametes and monosomy and trisomy in the zygote 
1/3 of identified miscarriages
infertility
leading cause of mental retardation

Question 21

What happens in meiotic nondisjunction?

Answer 21

in meiosis 1
ends up with gamete having extra copy of that chromosome
after meiosis 2, form either 50% trisomy or 50% monosomy

in meiosis 2
ends up with gamete having one cell with extra copy and one missing it
forms 25% trisomy, 25% monosomy or 50% normal zygote