3.3 Meiosis Flashcards
meiosis
It involves the reduction division of a diploid cell into four genetically distinct haploid nuclei
One of the two ways in which the nucleus of a eukaryotic cell can divide
The nucleus divides twice; the first division provides two nuclei, each of which divides again to give a total of four nuclei - known as Meiosis I and Meiosis II
The process of meiosis consists of two cellular divisions:
The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)
sister chromatids
The two identical DNA molecules that are held together by a single centromere
stages of meiosis I
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
stages of meiosis II
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
the final outcome of meiosis
the production of four haploid daughter cells
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
the main sources of genetic variation
Crossing over (in prophase I) Random assortment of chromosomes (in metaphase I) Random fusion of gametes from different parents
karyopting
the process by which chromosomes are organised and visualised for inspection (metaphase of mitosis)
Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities
chromosomal abnormalities are conditions that include
Patau’s Syndrome (trisomy 13) Edwards Syndrome (trisomy 18) Down Syndrome (trisomy 21) Klinefelter Syndrome (XXY) Turner’s Syndrome / Fragile X (monosomy X)
How is meiosis related to the sexual life cycles of eukaryotic organisms?
In eukaryotic organisms: fertilization doubles the number of chromosomes each time it occurs; causes a doubling of chromosome number every generation → happens during meiosis
Body cells are diploid and sex gametes (eg. sperm and egg cells) are haploid
In humans: haploid sperm cell and egg cell fertilize to produce a diploid offspring
Why does DNA replicate before meiosis?
Two chromatids that make up each chromosome are genetically identical because DNA replication is accurate and the number of mistakes is very small
What is non-disjunction?
Non-disjunction: when homologous chromosomes fail to separate at anaphase
Both of the chromosomes move to the same pole and neither to the other pole
Results a gamete that either has an extra chromosome or deficient in a chromosome
If the gamete is involved in human fertilization, the result will be an individual with either 45 or 47 chromosomes
How is non-disjunction related to Down Syndrome?
- Non-disjunction happens in a diploid parent cell with two chromosome 21
- A gamete has two chromosome 21 and other has no chromosome 21
- The gamete with two chromosome 21 fuse with a normal haploid gamete
- A zygote with three chromosome 21 (trisomy 21) is produced
What are the two methods of obtaining cells from a fetus for karyogram analysis?
Amniocentesis - Risk of miscarriage: 1%
- A hypodermic needle is inserted through the mother’s abdomen wall and uterus wall
- Amniotic fluid is removed from the amniotic sac around the fetus which contains fetal cells
Chorionic villus sampling (CVS) - Risk of miscarriage: 2%
- A hypodermic needle is inserted through the mother’s abdomen wall and uterus wall
- Cells are removed from fetal tissues in the placenta called chorionic villi, one of the membranes from which the placenta develops
