Topic 5.1. Meiosis 1 and 2

Homologous chromosomes are matched in: similar length, centromere position(attaches sister chromatids together), and gene locations (locus)

Locus - is the position of a gene. Different versions or variations (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.

Meiosis - the process to make cells with half the number of chromosomes for sexual reproduction.

Usually humans and most animals and some plants have diploid (2n) body cells. Meaning that they have two sets of chromosomes (one from each parent).

Meiosis occurs in our germ cells that produce gametes (sperm and egg).

Meiosis results in four cells which are genetically different from parent cell and from each other.

The end products of Meiosis are 4 Haploid (n) cells.

Meiosis is a process that converts diploid nuclei to haploid nuclei.

Diploid cells have 2 sets of chromosomes.

Haploid cells have 1 set of chromosomes.

Meiosis occurs in the sex organs producing gametes - sperm and aegg.

Fertilization is the fusion of a sperm and egg cell.

The zygote has a diploid chromosome number, one set from each parent.

We need meiosis because it is the fundamental basis of sexual reproduction.

Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote.

If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.

Meiosis has two distinct stages.

Meiosis I consists of 5 phases which are Interphase I, Prophase I, Metaphase I, Anaphase I, and Telophase I.

Meiosis II consists of 4 phases which are Prophase II, Metaphase II, Anaphase II, and Telophase II.

Meiosis I

Interphase - cell build up energy, DNA Replication (to make duplicated chromosomes, and cell doesn’t change structurally.

Prophase I

Nucleus - chromosomes coil and become individual chromosomes, nucleolus and nuclear envelope disappear. Homologous chromosomes come together as pairs by synapsis forming a tetrad (each pair with 4 chromatids). Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation. Centrioli move to opposite poles with spindle fibers between them.

During early Prophase I, homologous pair and crossing over occurs.

During late Prophase I, chromosomes condense, spindle forms, and nuclear envelope fragments.

Synapsis - the pairing of homologous chromosomes.

Homologous chromosomes in a tetrad cross over each other, genes are exchanged.

Metaphase I - homologous pairs of chromosomes align along the equator of the cell. The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.

Anaphase I - spindle fibers contract, homologous chromosomes separate and move to opposite poles. Also, sister chromatids remain attached at their centromeres.

Telophase I - nuclear envelope reappear and spindle fibers disappear.

Cytokinesis (when cytoplasm divides) divides cell into two.

Telophase I and Cytokinesis - duplicated chromosomes have reached the poles. A nuclear envelope and nucleolus re-forms around chromosomes. Each nucleus now has the haploid number of chromosomes. Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.

Meiosis II

Prophase II - chromosomes coil and become compact (if uncoiled after telophase I) Nuclear envelope and nucleolus, if re-formed, disappears again. Centrioli move to opposite poles, forming spindle fibers between them.

Metaphase II - Individual duplicated chromosomes align on the equator. One chromosome per spindle fiber attached by means of kinetochore of centromere. Centrioli has reached the poles.

Anaphase II - spindle fibers contract. Duplicated chromosomes split in half (centromere dividing in 2). Sister chromatids separate and move to opposite poles.

Telophase II - daughter chromosomes had reached the poles. Two cells invaginate and form 4 daughter haploid cells (gametes). They uncoil and form chromatin. Nuclear envelope and nucleolus form around chromatin again. Centrioli for centrosome.

Results of Meiosis - gametes (egg and sperm) form, four haploid cells (n) with one copy of each chromosome, one allele for each gene, and different combinations of alleles for different genes along the chromosome.

An extra copy of chromosome 21 causes Down syndrome or also called Trisomy 21.

A. Trisomy 21 - involves the inheritance of three copies of chromosome 21 and is the most common human chromosome abnormality. The symptoms are mental retardation, characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, Alzheimer’s disease, and shortened life span. The incidence increases with the age of the mother.