MEIOSIS

In sexual reproduction, two diploid individuals each contribute half their DNA to produce offspring with a new and unique combination of alleles. Two haploid sex cells (n) – one from each parent – are fused together to form a zygote (2n).

Meiosis is a form of nuclear division that produces four haploid nuclei (n) from one diploid nucleus (2n). Before meiosis, the DNA of the cell is replicated during the S phase of interphase.

Meiosis involves two cycles of division – Meiosis I and Meiosis II, and each cycle has the following four phases: Prophase, Metaphase, Anaphase, and Telophase (PMAT). At the end of both Meiosis I and II, cytokinesis also occurs.

Meiosis I

Reductive Division – Cells begin with two copies of each chromosome and end with only one.

PROPHASE I:

Chromosomes become visible due to supercoiling
Replicated chromosomes form closely linked homologous pairs, which have two chromosomes and four chromatids
Non-sister chromatids may cross over at points called chiasmata and exchange DNA. Crossing over creates new combinations of alleles that were not present in either original chromosome to increase genetic variation
Centrioles, if present, migrate to opposite poles and spindle fibers being to form
Nucleolus and nuclear membrane begin to disintegrate

METAPHASE I:

Homologous pairs move together along the metaphase plate (lies halfway between both poles)
Maternal and paternal homologues show random orientation towards the poles
Spindle fibers attach to centromeres of each chromosome and gently pull to align them along the equatorial metaphase plate

ANAPHASE I:

Spindle microtubules shorten, pulling homologous chromosomes apart towards opposite polls
Unlike mitosis, sister chromatids remain connected at the centromere and move to the same pole

TELOPHASE I:

First meiotic division effectively ends when chromosomes arrive at poles
Chromatids partially uncoil and a nuclear membrane reforms around each nucleus
Cytokinesis occurs and two daughter cells with haploid nuclei are formed

Meiosis II

Separation of chromatids in haploid cells

PROPHASE II:

Chromosomes condense again
Centrioles, if present migrate to opposite poles and spindle fibers form
Nucleolus and nuclear membrane begin to disintegrate

METAPHASE II:

Spindle fibers attach to the centromere of chromosomes and connect each centromere to both poles
Chromosomes are aligned across metaphase plate

ANAPHASE II:

Centromeres divide and chromatids are moved to opposite poles by spindle fibers
Once sister chromatids are separated, they are called chromosomes

TELOPHASE II:

Chromosomes reach opposite poles and uncoil
Nuclear envelope formed and cytokinesis occurs
Meiosis complete, resulting in 4 haploid daughter cells
Each daughter cell is genetically distinct

Genetic Variation

Sexual reproduction promotes genetic variation by:

Crossing over between homologue in prophase I
Random orientation of homologous chromosomes in metaphase I
Fusion of gametes from two individuals

Fetal Karyotyping: Methods and risks

A karyogram is an image of a cell’s homologous chromosome pairs ordered by decreasing size. Karyotyping is used to check for number and type of chromosomes. The two ways to take a karyotype of the fetus are:

Amniocentesis: Performed between 14 and 20 weeks of pregnancy. A syringe needle is used to withdraw a small amount of amniotic fluid, as fetal cells are floating in the fluid. An ultrasound is used to guide the syringe to go through the abdomen and uterine wall without piercing the fetus. The risk of miscarriage is between 0.1 and 1%.
Chorionic Villus Sampling (CVS): Early on in the pregnancy, when there is not enough amniotic fluid to perform amniocentesis safely, CVS is used between 10-13 weeks. Ultrasound imaging is used to guide the suctioning tool (catheter or syringe) through the vagina or abdomen, to reach the fetal cells in the chorion (a membrane that surrounds the fetus and develops into part of the placenta). Risk of miscarriage is between 0.5-2.0%.