3.3 Meiosis Flashcards
What kind of human cells are made in meiosis?
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs.
Meosis starts with one diploid nucleus being divided to produce
Four genetically distinct haploid nuclei
What happens to the number of chromosomes in the 1st cellular division in meiosis
The first meiotic division separates pairs of replicated homologous chromosomes (X X) to halve the chromosome number (diploid → haploid)
(X) (X)
46 chromosomes (92 chromatids) → 23 chromosomes (46 chromatids)
What happens to the number of chromosomes in the 2nd cellular division in meiosis
The second meiotic division separates sister chromatids from the homologous pair of replicated chromosomes that were separated during meiosis I
(X) (X)
Results in four haploid daughter cells:
(I) (I) (I) (I)
23 chromosomes (46 chromatids) → 23 chromosomes (23 chromatids)
Stages of meiosis 1 and meiosis 2
Meiosis 1:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
Meiosis 2:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
What type of cell do we begin with before meiosis?
A diploid cell (I I)
What happens during the S phase in interphase?
DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
The chromosomes replicate and now each have sister chromatids.
46 chromosomes → 46 chromosomes (92) chromatids
Prophase I
Meiosis I
- Chromosomes condense
- Nuclear membrane dissolves
- Spindle fibres form
- Homologous chromosomes pair up and form a bivalent (or tetrad) through a process called synapsis. The homologous chromosomes are held together at points called chiasmata
- Crossing over occurs (between non-sister chromatids at these chiasmata)
What is crossing over and what are its consequences?
- When the chromosomes undergo synapsis and pair up to form a bivalent, they are held together at points called chiasmata
- At these points, crossing over occurs, which refers to the exchange of non genetic material (between non-sister chromatids)
- This means that the sister chromatids will no longer be identical
- Results in recombinant chromosomes
- Promotes genetic variation
Metaphase I
- Homologous pairs of chromosomes align along the equator of the cell
- The spindle fibres connect to the homologous pairs at their centromeres
- There is random ortientation/independent assortment, meaning that each chromosome may face either pole of the cell. This means different combinations of maternal and paternal chromosomes can be inherited when the chromosomes separate in anaphase.
Random orientation/independent assortment
- Occurs during methaphase I
- Random ortientation/independent assortment means that each chromosome may face either pole of the cell.
- This means different combinations of maternal and paternal chromosomes can be inherited when the chromosomes separate in anaphase.
- Leads to genetic variation
Anaphase I
- Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
Note: the sister chromatids remain attached at their centromeres.
Telophase I
- Nuclear envelopes reassemble.
- Spindle fibres disappear.
- Cytokinesis divides cell into two.
- Results in two haploid daughter cells
Prophase II
- Nuclear envelope breaks down
- Spindle forms.
- Centrosomes move to opposite poles
Metaphase II
- Chromosomes align along equator of cell.
- Spindle fibres attarch to the chromosomes at their centromere
Anaphase II
- Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
Telophase II
- Nuclear membrane reforms
- Chromosomes decondense
- Spindle fibres dissapear
- Cells divide (cytokinesis) to form four haploid daughter cells
What does meiosis allow on sexually reproducing organisms?
Meiosis allows sexually reproducing organisms, which have diploid cells, to halve their chromosome numbers (to produce gametes which are haploid cells) and allow a sexual life cycle to occur.
The fusion of two haploid gametes results in…
the formation of a diploid zygote.
- The production of for distinct gamete cells allows genetic variation when the fusion of sperm and eggs occurs to create a diploid zygote cell.
- This zygote can then divide by mitosis and differentiate to form a developing embryo.
Non-disjunction
When chromosomes fail to separate.
- This can result in gametes with one extra or one missing chromosome
What causes Down syndrome
Down syndromw is caused by non-disjunction and happens when a zygote receives an extra copy of chromosome number 1.
Relationship between age of parents and chances of non-disjunction
Studies have shown that as the age of parents increases, the chances of non-disjunction increases.
Methods used to obtain cells for karyotype analysis – e.g. chorionic villi sampling and amniocentesis + the associated risks
Application
Karyotyping is the process by which chromosomes are organised and visualised for inspection. Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities.
Fetal cells can be obtained so karotyping can take place. There are two ways of obtaining these cells:
1. Amniocentisis: a needle is inserted through the abdomen and into the amniotic sac, amniotic fluid is withdrawn which contains the fetal cells.
- This procedure can be done later in pregnancy with a low risk of miscarriage
2. Chorionic Villi Sampling: a tube is inserted through the cervix and is used to remove part of the placenta called called the chorionic villi which contains the fetal cells.
- This procedure can be done earlier in pregnancy but runs a greater risk of miscarriage
