Chromosomes And Gametes

Question 1

What is the defining feature of all evolving living organisms?

Answer 1

The ability to reproduce + pass on DNA

Question 2

A second principle fundamental to evolution is

Answer 2

Variation - the replicating system must undergo changes (adapt to environ)

Question 3

Karyotype

Answer 3

the number and visual appearance of the chromosomes in the cell nuclei of an organism or species

Question 4

P arm

Answer 4

Short

Question 5

Centromere

Answer 5

Constriction point of chromosome

Question 6

Q arm

Answer 6

Long

Question 7

What are gene locations based on?

Answer 7

p and q arm

Question 8

For genes to be functional, DNA must be able to: (3)

Answer 8

➢ replicate
➢separate its 2 copies at mitosis
➢maintain itself between generations

Question 9

Sexual reproduction requirements (3)

Answer 9

Each parent passes on one allele (i.e. one version of a particular gene) to each offspring

–Copy number variants (CNV) occur if there are one, three or more copies of alleles

If Alleles are heterozygous = the phenotype of the trait can be dominant or recessive.

Question 10

Gene Transcription & Translation (4)

Answer 10

*Promotor and coding sequence transcribed into a gene product
*Introns are removed from exon by splicing
*mRNA exported out of nucleus
*Translated into proteins in ribosomes i.e. complexes of tRNA and proteins
*Proteins then folded into unique 3D structure that determines function

Question 11

How can the same gene be tissue specific?

Answer 11

By having alternative promotors

Question 12

How can one gene give rise to several products?

Answer 12

Spliced into different ways (inclusion and exclusion of certain exons)

Question 13

Products formed by one gene are called…

Answer 13

Isoforms

Question 14

How can the protein be modified once made? (3)

Answer 14

• Post-translational modification eg phosphorylation,
  –eg. how are LH and FSH modified?
  –Glycosylation i.e. adding on carbohydrates to protein, making protein more stable and soluble

Question 15

Prohormones

Answer 15

How hormones are secreted and as a result need to be enzymatically processed to form the active hormone e.g. pre-proGnRH

Question 16

Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones…

Answer 16

Asexual reproduction

Question 17

What is the name given to the duplication of the DNA in this process?

Answer 17

Mitosis

Question 18

What are the DNA requirements for sexual reproduction?

Answer 18

Fusion of haploid cells (gametes) to create unique diploid progeny

Question 19

Somatic or diploid cells give rise to (2)

Answer 19

– give identical progeny, usually have limited number of divisions,
–eg hepatocytes, pancreas, skin cells

Question 20

What are the advantages of sexual reproduction? (3)

Answer 20

Prevents the accumulation of genetic mutations
–Increase in genetic diversity
–Maintenance occurs because of the advantage of genetic variability

–Variation in off-spring → survival of the fittest? Better able to evolve and adapt to changing environment

Question 21

X and Y chromosomes - history (5)

Answer 21

*Thought to have differentiated from a pair of identical chromosomes (autosomes)..300 million years ago

*Ancestral mammal developed a variation which made it male….gradually this chromosome became the Y and the other the X.

*With evolution, genes advantageous to either sex became focussed on X or Y and those for ‘maleness’ close to SRY gene.

*X chromosome → 1000 working genes
*Y chromosome → 86 working genes

–Recent comparisons of human and chimpanzee Y chromosomes shown that human Y chromosome has not lost any genes since divergence of human and chimpanzees 6-7 million years ago

Question 22

What is a gamete?

Answer 22

Sex cell - unlike other cells, gametes go through cycles of diploidy and haploidy

Question 23

What is different about haploid cells?

Answer 23

Half the amount of chromosomes

Question 24

What is the origin of gametes? (4)

Answer 24

-Gametes are highly specialised cells originating from which kind of cells?
–Gametes are formed from germ line cells: primordial germ cells that migrate into the gonad and then differentiate to either male or female gametes
–The process producing oocytes – oogenesis (incorporated as part of folliculogenesis)
–The process producing sperm - spermatogenesis

Question 25

Duplication of chromatids (4)

Answer 25

*Chromosomes replicate during S-phase of cell cycle
*Remain attached at the centromere
*Each copy known as a chromatid → the 2 copies are identical to each other → “sister” chromatids
*Exact copy of original chromosomes

Question 26

Synapsis (4)

Answer 26

It’s the pairing of homologous chromosomes to form a Tetrad in Prophase I

• Genetic material from the homologous chromosomes is randomly swapped
  *This creates 4 unique chromatids hence increasing overall genetic diversity of the gametes.

Question 27

What is the main difference between mitosis and meiosis?

Answer 27

Mitosis produces 2 new identical diploid daughter cells but meiosis produces non-identical haploid daughter cells.

Question 28

Why is meiosis advantageous? (2)

Answer 28

–random distribution of male and female homologous chromosomes
–chromosomal crossing over occurs

Question 29

Comparison of meiosis and mitosis

Answer 29

Table

Question 30

How is genetic variability achieved? (2)

Answer 30

*Independent Assortment
–homologous pairs of chromosomes line up at the equator (midpoint) of the spindle during metaphase I.
–However, the orientation of the members of the pair is random with respect to which member is closer to which pole.

*Crossing Over (Recombination)
–Corresponding pieces of chromatids of maternal and paternal homologues (non-sister chromatids) are exchanged during synapsis when the homologues are aligned side by side.
–Each of the affected chromatids has a mixture of maternal and paternal genetic information

Question 31

what is crossing Over of Sex Chromosomes and why does it occur?

Answer 31

*Sex chromosomes align but crossing over does not usually occur in X and Y chromosomes apart from at the pseudoautsomal regions (PAR)

Why?
–They are hemizygous to each other & so recombination proved harmful
–PAR allows the X & Y chromosomes to pair and properly segregate during

Question 32

Chromosomal Abnormalities: Aneuploidies (6)

Answer 32

*A gain or loss of chromosomes from the normal 46 is called aneuploidy, affecting normal development and functioning.

*Since each chromosome contains hundreds of genes, the addition or loss of a single chromosome disrupts the existing equilibrium of the cell leading to profound phenotypes.

*Using cytogenetics (including karyotyping) - aneuploid gametes produced at surprisingly high rates in humans

–Majority occur from an error in maternal meiosis I because human oocytes are arrested in prophase I for decades

–Aneupolidy is present in 6% of sperm from ostensibly normal men and in 20% of oocytes

–Aneuploidy in eggs is leading cause of infertility, miscarriage and congenital syndromes

Question 33

What is non-disjunction?

Answer 33

failure of homologous chromosome to separate during MI or sister chromatids to separate during MII, resulting in extra or missing chromosomes

Question 34

Aneuploidy commonality (6)

Answer 34

50% of recognized pregnancy loss result from chromosomal abnormality.
*Most common aneuploidies in humans are trisomies (0.3% of live births).
*Viable ones are:
–Trisomy 21 (aka Down’s syndrome, 1:750 births)
–Trisomy 18 (Edwards syndrome)
–Trisomy 13 (Patau syndrome)

*50% of patients with primary amenorrhea as a result of premature ovarian insufficiency (POI) have an abnormal karyotype

*Sex chromosome aneuploidy more viable, usually random event (not inherited):
–Turner syndrome (45, X monosomy) » caused by complete or partial absence of 2nd sex chromosome (occurrence 1:2 000 female births) → phenotype=short stature, primary amenorrhea (classic Turners)

–Klinefelter syndrome (47,XXY trisomy) » caused by presence of two X and one Y chromosome (occurrence 1:500 male births) → variable phenotype=taller than average, small testes producing reduced testosterone, infertility

Question 35

What is the effect of Maternal Age and Risk of Trisomy? (4)

Answer 35

Multiple mechanism contribute to the maternal age effect –
*Recombination failure
*Premature homologue separation
*Premature sister chromatid separation due to loss of cohesion between sister centromeres