chromosome and gametes Flashcards
(29 cards)
What must DNA be able to do for genes to be functional?
- replicate
- separate its 2 copies at mitosis
- maintain itself between generation
How does DNA requirements for sexual reproduction differ to somatic cell reproduction?
-each parent passes on one allele(one version of a particular gene) to each offspring.
-heterozygous alleles AA, Aa, Aa, aa so can be dominant or recessive trait.
Gene transcription and trasnlation
- promoter and coding sequence transcribed into 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 fold into unique 3D structure that determines function
How can the same gene be tissue specific?
- by having alternative promoters.
-same gene producing same protein but have different promoters depending on where it is makes it tissue specific. - eg. CYP19A1 uses different promoters in breast, ovary and brain.
How can same gene give rise to different proteins?
- alternative splicing of exons
-products are known as isoforms
-post translational modification eg. phosphorylation
=> eg. AKT used to phosphorylate FSH and LH to make it effective
-hormones are secreted as pro hormones and must be emzymatically processed to form the active hormone
=> pre-pro GnRH.
What are advantages of sexual reproduction?
- prevents the accumulation of genetic mutaions
-increase in gentic diversity
-maintenance ocuurs because of the advantage of genetic variability - variation in offspring -> survival of fittest , better adapt to changing environment and evolve.
how did X and Y develop?
- thought to have differentiated from a pair of identical chromosomes … 300 million years ago.
- mammal developed a variation which made it male.. gradually this chromosome become the Y and the other the X
- with evolution, genes advantageous to either sex become focussed on X or Y and those maleness close to SRY gene
-with evolution, genes advantageous to either sex become focused on X or Y and those for ‘maleness’ close to SRY gene.
What to gametes originate from?
- gametes are highly specialised cells formed from germ line cells: primordial germ cells that migrate into gonad and then differentiate to either male or female gametes.
- the process producing oocytes - oogenesis (incorporated producing sperm- spermatogenesis)
- undergo cycles of mitosis to increase numbers.
- then undergo meiosis
-then combine at fertilisation.
alternative splicing and glycolysylation of FSH and LH
- Alternative splicing to make different proteins , some protein forms may cause FSH and LH not binding well
- different glycosylated FSH and LH have different efficacy (as woman approaches menapause they get more glycolylation)
What happens before mitosis - interphase
-interphase
- chromatin - uncondensed form of DNA
-DNA duplicates , not only DNA but centreoles and organelles start to form also duplicate(growth)
-preparation for cell division
what are 4 stages of mitosis
1.prophase- duplicated chromosome condenses, centrioles migrate to opposite ends and spindles start to form
2. metaphase - chromosomes line up in the centre
3.anaphase - attachment between sister chromatids breaks and get pulled apart to opposite poles
4. telophase - new nuclei form, each with two complete set of chromosomes , cytokinesis divides cell into two daughter cells.
What happens in prophase 1 in meiosis?
- chromosome condense
- synapsis occurs : homologous chromosomes pair up and become perfectly aligned with one another
-crossing over takes place
What happens in metaphase 1?
- homologous pairs of chromosomes line up at the midline of cell
-spindle fibre from each end attach to the homologue
What happens in anaphase 1?
- homologous pairs of chromosomes separate and move to opposite ends of the cell
- each homologue still consists of two chromatid
What happens in telophase 1?
- one member of each homologue pair is at each pole
-cytokinesis occurs and forms two haploid daughter cells. - each chromosome still consists of two chromatid
What is synapsis/recombination -in prophase 1?
- 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
-each of the affected chromatids has a mixture of maternal and paternal genetic information
What are homologous pairs chromosomes?
- in metaphase 1 chromosomes line up in the equator
-chr1 from mum and chr 1 from dad line up next to each other, chr2 from mum and chr2 from dad etc (homologous pairs) - so during anaphase the chromatids dont split at the centromere like they did in mitosis they seperate as a whole , eg chr1 from mum goes to one side and chr1 from dad goes to the opposite pole so each homologue consists of two chromatid still.
What is independent assortment - in metaphase 1?
-maternal and paternal members of homologous pairs align randomly at the equatorial plate, creating a random assortment of maternal and paternal chromosomes in the daughter cells.
-the orientation of the members of the pair is random with respect to which member is closer to which pole
What happens in meiosis II?
- prophase II - chromosomes condense again , occurs in both daughter cells
- metaphase II - chromosomes line up at the midline of the cell
- anaphase II - centromeres of sister chromatids separate, chromatids of each pair are now called chromosomes and they move to opposite poles
- telophase II - one complete set of chromosomes is located at each pole , cytokines occurs in both daughter cells, forming 4 haploid daughter cells.
compare and contrast mitosis and meiosis
mitosis :
- involves one cell division not two
- produces two diploids not 4 haploids
-occurs in somatic cells , meoisis occurs only in ovaries and testes during formation of gametes
- results in growth and repair, results in gamete production
- no exchange of genetic material , parts of chromosomes are exchanged in crossing over
- daughter cells are genetically identical , not genetically identical .
how is genetic variability achieved?
- independent assortment
- crossing over (recombination)
why does crossing over not happen in sex chromosomes?
- X and y chromosomes align but crossing over doesn’t happen bc they are hemizygous (only has one segment of chromosome rather than two) so recombination may be harmful
-X-inactivation occurs in which one of the copies of the X-chromosomes is inactivated to prevent females from having twice as many gene products
What is the only region of the sex chromosome where crossing over happens?
-pseudoautosomal regions (PAR)
Chromosomal abnormalities: aneuploidies
- gain or loss of chromosomes from the normal 46 is called aneuploidy
- each chromosome contains hundreds of genes so addition/loss of chromosome dirupts the phenotypes
- cytogenetics (karyotyping) can be used to identify aneuploidies
-majority are lethal
