3.3 Meiosis Flashcards
Types of cell division: Asexual vs Sexual reproduction
Asexual: Mitosis
Sexual: Meiosis
Number of daughter cells produced, genetic variation, haploid or diploid: Asexual vs Sexual reproduction
Asexual: 2, genetically identical, diploid
Sexual: 4, genetically varied, haploid
Function: Asexual vs Sexual reproduction
Asexual: Growth, repair, cloning
Sexual: Produce gametes
What is synapsis?
The process during prophase I where homologous chromosomes come together
How are homologous chromosomes connected?
By a protein-RNA complex called the synaptonemal complex
Why is genetic variation an advantage?
Evolution, natural selection, adaptation
What are the 3 methods by which meiosis creates genetic variation?
- Crossing over - prophase I
- Independent assortment - metaphase
- Random fusing of gametes (fertilisation)
What is crossing over? Where does it occur? What is the result?
Crossing over can occur multiple times on the same homologous pair.
It occurs in prophase I when homologous chromosomes undergo a process synapsis, whereby they pair up to form a bivalent (or tetrad). The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Then crossing over of genetic material between non-sister chromatids can occur at these chiasmata
- As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
- Chromatids of each chromosomes are no longer genetically identical
What does the exchange of alleles form?
Recombinant chromatids
Chromatids of each chromosomes are no longer genetically identical
How are recombinant chromatids different?
Recombinant chromatids still contain the same genes but a different combination of alleles
Where does independent assortment occur? How does it produce genetic variation?
During metaphase I, the orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes (metaphase II)
- This affects which chromosome of the pair (maternal or paternal) goes into which daughter so when they are pulled apart during anaphase I
- Will occur for each homologous pair
What is the number of possible gamete combinations dependent on?
As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs
- Gamete combinations = 2n (where n represents the haploid number)
What needs to happen for the formation of a diploid zygote that can grow via mitosis?
Fertilisation of two haploid gametes (egg + sperm)
What happens if chromosome number was not halved in gametes?
The total chromosome number would double each generation (polyploidy)
How does random fusing of gametes produce genetic variation?
Haploid gametes undergo random fusion with gametes derived from another organism (of the same species) at fertilisation.
The resulting zygote will therefore contain genetic material from two unrelated individuals, introducing further genetic variation.
What are two ways genetic variation can be introduced by mutations?
Gene mutations e.g. point mutations
Chromosome mutations e.g. non-disjunction - failure of homologous chromosomes or sister chromatids to separate properly during anaphase
What are examples of non-disjunction:
Down syndrome - triplets in chromosome 21
Klinefelter syndrome - triplet in heterosome (male also with female attributes, wider hips etc)
Turner syndrome - 1 heterosome chromosome (female but ovaries not developed properly, developmental issues)
What does the age of the parent have to do with chances of non-disjunction?
Studies show that the chances of non-disjunction increase as the age of the parents increase
- There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events
- This may be due to developing oocytes being arrested in prophase I until ovulation as part of the process of oogenesis
What two procedures are used for obtaining cells containing the fetal chromosomes needed for producing a karyotype.
- Amniocentesis - passing a needle through the mother’s abdomen wall, suing ultrasound to guide the needle. The needle is used to withdraw a sample of amniotic fluid containing fetal cells from the amniotic sac. Risk of miscarriage from amniocentesis is 1%. Usually carried out around week 16 of preganancy
- Chronic Villi sampling - the sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops. This can be done earlier in the pregnancy than amniocentesis (around week 11). Risk of miscarriage is 2%.
Why would parents produce a karyotype?
So they can choose not to carry on pregnancy, because they can choose not to carry on the pregnancy because some can’t afford the medical bills to support a child with a syndrome
What is a recombinant
Chromosomes that consist of genetic material from both homologues are called recombinant chromosomes
What is a chiasmata
A region where non-sister chromatids remain physically connected at points of exchange during crossing over
What can happen while in synapsis?
Non-sister chromatids may break and recombine with their homologous partners (crossing over)
