Chromosomes And Gametes Flashcards
What is the defining feature of all evolving living organisms?
The ability to reproduce + pass on DNA
A second principle fundamental to evolution is
Variation - the replicating system must undergo changes (adapt to environ)
Karyotype
the number and visual appearance of the chromosomes in the cell nuclei of an organism or species
P arm
Short
Centromere
Constriction point of chromosome
Q arm
Long
What are gene locations based on?
p and q arm
For genes to be functional, DNA must be able to: (3)
➢ replicate
➢separate its 2 copies at mitosis
➢maintain itself between generations
Sexual reproduction requirements (3)
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.
Gene Transcription & Translation (4)
*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
How can the same gene be tissue specific?
By having alternative promotors
How can one gene give rise to several products?
Spliced into different ways (inclusion and exclusion of certain exons)
Products formed by one gene are called…
Isoforms
How can the protein be modified once made? (3)
- 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
Prohormones
How hormones are secreted and as a result need to be enzymatically processed to form the active hormone e.g. pre-proGnRH
Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones…
Asexual reproduction
What is the name given to the duplication of the DNA in this process?
Mitosis
What are the DNA requirements for sexual reproduction?
Fusion of haploid cells (gametes) to create unique diploid progeny
Somatic or diploid cells give rise to (2)
– give identical progeny, usually have limited number of divisions,
–eg hepatocytes, pancreas, skin cells
What are the advantages of sexual reproduction? (3)
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
X and Y chromosomes - history (5)
*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
What is a gamete?
Sex cell - unlike other cells, gametes go through cycles of diploidy and haploidy
What is different about haploid cells?
Half the amount of chromosomes
What is the origin of gametes? (4)
-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
Duplication of chromatids (4)
*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
Synapsis (4)
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.
What is the main difference between mitosis and meiosis?
Mitosis produces 2 new identical diploid daughter cells but meiosis produces non-identical haploid daughter cells.
Why is meiosis advantageous? (2)
–random distribution of male and female homologous chromosomes
–chromosomal crossing over occurs
Comparison of meiosis and mitosis
Table
How is genetic variability achieved? (2)
*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
what is crossing Over of Sex Chromosomes and why does it occur?
*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
Chromosomal Abnormalities: Aneuploidies (6)
*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
What is non-disjunction?
failure of homologous chromosome to separate during MI or sister chromatids to separate during MII, resulting in extra or missing chromosomes
Aneuploidy commonality (6)
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
What is the effect of Maternal Age and Risk of Trisomy? (4)
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
