MEIOSIS
1. Revision: Mitosis
2. MEIOSIS
2.1 TERMINOLOGY:
Autosomes: Chromosomes that are not sex chromosome. There are 22 pairs
of autosomes in a diploid cell of a human.
Centriole: An organelle in the cytoplasm of the cell, which gives rise to
spindle fibres during meiosis and mitosis.
Centromere: Structure that holds two chromatids together to form a
chromosome.
Chiasma: Point where crossing over takes place between chromatids of the
homologous chromosome during prophase 1.
Chromatid: It is a single thread of a double stranded chromosome. Two
chromatids are joined by a centromere to form a replicated
chromosome.
Chromosom A structure made up of two chromatids joined by a centromere
e: that carries the hereditary characteristics within the DNA.
Diploid Complete chromosomal number represented in pairs, which is
number characteristic of an organism.
(2n):
Daughter This refers to each chromatid after it splits from its sister
chromosom chromatid during anaphase II and is moving towards the poles.
e:
Gametes: Haploid cells (n) which contain half the chromosome number of
the diploid generation. Ova and sperm cells are the gametes
necessary in sexual reproduction where the fusion of the two
gametes results in a new individual.
Gene: The unit of heredity transmitted in the chromosome, which
controls the development of the characteristics.
Gonosomes: Sex chromosomes. There is one pair of sex chromosomes in a
diploid cell: the XX chromosomes in females and XY
chromosomes in males.
Haploid Half the number of chromosomes present in gametes after
number (n): meiosis has occurred.
Homologous Maternal and paternal chromosomes having the same shape and
chromosom size which are paired but differs in genetic material.
es
Bivalent: A pair of homologous chromosomes physically held together by at
least one DNA crossover.
Maternal: From the mother / female parent.
Meiosis: A process of cell division whereby the chromosomal number is
halved for the production of haploid gametes (sperm cells and
ova).
Mitosis: A process of cell division where the resulting daughter cells have
the same chromosomal number as the original parent cell.
Non- The homologous chromosomes do not separate due to failure of
disjunction: the centromere to divide during meiosis I & II. The resulting
gametes will have either an extra chromosome/copy or another
gamete will have one less chromosome.
Paternal: From the father / male parent.
Replicated This refers to a chromosome as it appears after DNA replication
chromosom takes place. Hence each replicated chromosome is made up of
e: two chromatids, joined by a centromere.
Somatic Normal diploid body cells.
cells:
Spindle Micro-tubules that form during cell division which radiate out
fibres: from the centrosomes and draw the chromosomes to the poles.
Unreplicated This refers to a chromosome as it appears before DNA replication
chromosom takes place. Because of DNA replication all chromosome material
e: is doubled.
Variation: The morphological and physiological differences that can be seen
between members of the same species.
Zygote: The resulting diploid cell after fertilization has occurred
Meiosis is a special type of cell division that halves the number of chromosomes.
2.2 Importance:
Meiosis is important because:
2.3 Process of meiosis
It is divided into TWO stages (phases):
- Meiosis I
- Meiosis II
Both meiotic divisions have the same FOUR phases (PMAT) as MITOSIS:
- Prophase
-Metaphase
-Anaphase
-Telophase
2.4 Genetic variation
2.5 Differences between meiosis and mitosis
Similarities
2.6 Abnormal meiosis
2.7 Where does it occur in plants and animal cells:
Meiosis is a cell division that usually takes place in sex cells.
- In plants, it occurs in the anther to produce pollen grain and in the ovary to
produce the ovule.
- In humans, it occurs in the testis to produce sperms and in the ovary to produce
an ovum.
