3.3 Meiosis Flashcards
What is meiosis?
- The process by which sex cells (gametes) are made in the reproductive organs
- the reduction division of a diploid germ line cell into 4 genetically distinct haploid nuclei
What are the two processes of cellular division in meiosis?(2)
- the first meiotic division separated pairs of homologous chromosomes number (diploid -> haploid)
- the second meiotic division separates sister chromatids (created by replication of DNA during interphase)
What comes before meiosis?
Interphase
What occurs in interphase?(3)
- DNA is replicated to produce 2 genetically identical copies
- 2 identical DNA molecules are identifies as sister chromatids and are held together by a single centromere
- sister chromatids are separated during meiosis II following the separation of homologous chromosomes in meiosis I
What is the growth stage that may occur between meiosis I and II?
Interkinesis
What is the process of meiosis I? (5)
it is a reduction division (diploid -> haploid) where homologous chromosomes are separated
- PI: chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
- MI: spindle fibres from opposing centromeres connect to bivalents (@centromeres) and align them along the middle of the cell
- AI: spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
- TI: chromises decondense, nuclear membrane may reform, cell divides (cytokinesis) to form 2 haploid daughter cells
What is the process of meiosis II? (5)
The second division separates sister chromatids (these sister chromatids may not be identical due to crossing in PI)
- PII: chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
- MII: spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
- AII: spindle fibres contract and separate the sister chromatids, chromatids (called chromosomes) move to opposite poles
- TII: chromosomes decondense, nuclear membranes reforms, cells divide (cytokinesis) to from four haploid daughter cells
What is the final outcome of meiosis?
The production of 4 haploid genetically distinct daughter cells (if crossing occurs in PI)
What process do homologous chromosomes undergo?
- Synopsis - whereby they pair up to from a bivalent (or tetrad)
- homologous chromosomes are held at chismata
Where does crossing over of genetic material occur between non-sister chromatids?
Chismata
What is the result of genetic exchange in crossing over?
- resulting in the exchange of genetically material , new gene combinations are formed on chromatids (recombination)
What occurs once chismata are formed? (2)
- homologous chromosomes condense as bivalents and then separates in meiosis
- If crossing occurs then all 4 haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)
What are the arrangements of homologous chromosomes metaphase I line up at the equator?
- maternal copy left /paternal copy right
- paternal copy left / maternal copy right
What is random assortment? (2)
- the orientation of pairs of homologous chromosomes as the subsequent assortment of chromosomes into gametes
- the final gametes will differ depending on whether they got the maternal or paternal copy of a chromosomes in anaphase I
how to find the gamete combinations?
Gametes combination = 2^n
What will the fertilisation of 2 haploid gametes result in?
Result in the formation of a diploid zygote that can grow via mitosis
What will occur is the chromosome number is not halved?
The total chromosomes numbers would double each generation (polyploidy)
What are the 3 main sources of genetic variation from sexual reproduction?
- crossing over (PI)
- random assortment of chromosomes (MI)
- random fusion of gametes from different parent
What is crossing over? (2)
- involves the exchange of segments of DNA between homologous chromosomes during PI
- the exchange of genetic material occurs between non-sister chromatids called chiasmata
What is consequence of recombination? (3)
- all four chromatids that comprise the bivalent will be genetically different
- chromatids that consist of a combination of DNA derived from both homologous chromosomes (recombinant)
- offspring with recombinant chromosomes will have unique combinations that are not present in either parent
What is random orientation? (2)
- When homologous chromosomes line up in MI their orientation towards the opposing poles is random
- the orientation of each bivalent occurs independently meaning different combinations of mat/pat chromosomes can be inherited when bivalents separate in AI
What is random fertilisation?
- the fusion of haploid gametes results in the formation of a diploid zygote
- the zygote can be divided by mitosis and differentiate to form a developing embryo
Why will random fertilisation always generate different zygotes?
As meiosis results in genetically distinct gametes
what is non-disjunction?
the chromosome failing to separate correctly resulting in games with 1 extra or missing chromosome (aneuploidy)
how does the failure of chromosomes to separate occur? (2)
- failure of homologues to separate in AI (resulting in 4 affected daughter cells)
- failure of sister chromatids to separate in AII (resulting in only 2 daughter cells being affected)
what are 5 examples of diseases which result due to a zygote from a gamete from a non-disjunction event?
- patau’s syndrome (trisomy 13)
- edward’s syndrome (trisomy 18)
- down syndrome (trisomy 21)
- klinefelter syndrome (XXY)
- turner’s syndrome (monosomy X)
what does it mean if someone has down syndrome?
- they have 3 copies of chromosome 21 (from non-disjunction)
- 1 of the parental gametes had 2 copies of chromosome 21 due to non-disjunction
- other parental gamete is random was normal and had a single copy of chromosome 21
- when the 2 gametes fused during fertilisation the resulting zygote had three copies of chromosome 21
what did studies show about the chance of non-disjunction and age? (3)
- chances of non-disjunction increase as the age of parent increase
- especially maternal age (30)
- due to developing oocytes being arrested in PI until ovulation as part of the process of oogenesis
what else do studies suggest about non-disjunction?
there is higher incidence of chromosomal errors in offspring as a result of non-disjunction in MI
what is karyotying?
the process by which chromosomes are organised and visualised for inspection
what is the process of karyotyping? (4)
- cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible)
- the stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids
- chromosomes are stained and photographer before being organised according to structure
- the visual profile generated = karyogram
what is chronic villi sampling? (2)
- removing a sample of the placental tissue (chorionic villus) via a tube inserted through the cervix
- can be done at -11 weeks of pregnancy (slight risk of inducing miscarriage)
what is aminocentesis?
- the exrtaction of a small amount of amniotic fluid (fetal cells) with a needle
- can be done at -16 weeks (lower risk of miscarridge)
what are 2 ways that cells can be harvested?
- amniocentesis
- chronic villi sampling
