3.3 Meiosis Flashcards
What is meiosis?
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs
What does meiosis involve?
It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
What does the first meiotic division involve?
The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
What does the second meiotic division involve?
The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)
What is mitosis preceded by and what is its purpose?
Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies
What are the two identical DNA molecules termed as and what are they held together by
The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere
When are sister chromatids separated?
The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I
What does meiosis consist of?
Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)
What phase occurs between meiosis I and meiosis II?
A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage
What is the first meiotic division termed as?
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
What occurs in P-1? (prophase in meiosis I)
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
What occurs in M-1?
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
What occurs in A-1?
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
What occurs in T-1?
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
What is meiosis II termed as?
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
What is P-II?
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
What is M-II?
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
What is A-II?
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
What is T-II?
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
What is the final outcome of meiosis?
The final outcome of meiosis is the production of four haploid daughter cells
What is the difference between the daughter cells produced by meiosis?
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
What process fo homologous chromosomes undergo in prophase I (P-I)?
In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad)
What points are homologous chromosomes held together at?
The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Where can crossing-over occur?
Crossing over of genetic material between non-sister chromatids can occur at these chiasmata
What does crossing over lead to?
As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
What happens once chiasmata are formed?
Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis
How does crossing over affect daughter cells?
If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)
What do homologous chromosomes do in metaphase I?
During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
What is random in metaphase I?
This orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes
How will random assortment affect gametes?
The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I
What will affect the number of possible gamete combinations?
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)
Why do organisms need to make gametes?
Most sexually reproducing organisms are diploid, meaning they have two copies of every chromosome (maternal / paternal copy)
In order to reproduce, these organisms need to make gametes that are haploid (one copy of each chromosome)
What will the fusion of two haploid gametes result in?
Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis
What can happen if chromosome number was not halved in gametes?
If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)
What is the advantage of meiotic division?
The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring
What are the 3 main sources of genetic variation?
Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents
What is crossing over?
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
Where does the exchange of genetic material occur?
The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
What is a consequence of crossing over, how do chromosomes differ?
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
What are recombinants?
Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
What will offspring with recombinant chromosomes posses?
Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
What is random orientation?
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random
What does the orientation of each bivalent occurring independently mean?
The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
What does the orientation of each bivalent occurring independently mean?
The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
What is the total number of combinations that can occur in gametes?
The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
How many chromosomes do humans have and therefore how many gametes combos can be formed?
Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (2^23) by random orientation
How many chromosomes do humans have and therefore how many gametes combos can be formed, considering crossing over?
If crossing over also occurs, the number of different gamete combinations becomes immeasurable
What happens to the zygote?
This zygote can then divide by mitosis and differentiate to form a developing embryo
What will generate different zygotes?
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes
When are identical twins formed?
Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses
What is non-disjunction?
Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
What can be two causes of non-disjunction?
Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)
What will happen if a zygote is formed from a gamete that has experienced a non-disjunction event?
If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body
What are examples of conditions that arise from non-disjunction effects?
Patau’s Syndrome (trisomy 13)
Edwards Syndrome (trisomy 18)
Down Syndrome (trisomy 21)
Klinefelter Syndrome (XXY)
Turner’s Syndrome (monosomy X)
What chromosomal abnormality do individuals with down syndrome present?
Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
How does a zygote therefore present trisomy 21 (what events lead to it0?
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21
What may the chance of non-disjunction increase alongside?
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 is karyotyping?
Karyotyping is the process by which chromosomes are organised and visualised for inspection
What can karyotyping be used for?
Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities
How is karyotyping done in fetuses?
Cells are harvested from the foetus before being chemically induced to undertake cell division (so chromosomes are visible)
What will determine whether chromosomes appear wth sister chromatids in karyotyping?
The stage during which mitosis is arrested will determine whether chromosomes appear with sister chromatids
What is done to chromosomes so they’re visible on a karyogram?
Finally, chromosomes are stained and photographed, before being organised according to structure
The visual profile generated is called a karyogram
What does chorionic villi sampling (CVS) involve?
Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix
It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)
What does amniocentesis involve?
Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle
Does amniocentesis or CVS have a higher risk of miscarriage?
It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)
AMNIOCENTESIS
It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)
