Module-2-Mitosis-andmeiosis and principle of inheritance that includes different terminologies related to genetics
1. Module 2: MITOSIS and MEIOSIS
(CELL DIVISION)
Heredity - traits are passed on from parents to offspring
Variation - demonstrates differences among individuals
Genetics - is the study of heredity and variation. It aims
to understand how traits can be passed on to the next
generation and how variation arises.
Gregor Mendel – the father of Genetics
- (1822-1884) is widely considered as the father of
experimental genetics for his pioneer work in the
field of genetics. He experimented on pea plants
(Pisum Sativum) and discovered the basic rules
of inheritance of genes.
What is DNA?
These genetic materials, also
known as the deoxyribonucleic
acid or DNA, are passed on from
one generation to the next to
ensure the continuity of life.
In eukaryotic cells (cells with
organelles), the DNA are bound
with proteins and are organized
as beads on strings to form
chromosomes
Phases of the Cell Cycle
The cell cycle is a 4-stage process consisting of Gap 1
(G1), synthesis (S), Gap 2 (G2), and mitosis (M), which
a cell undergoes as it grows and divides. After
completing the cycle it either starts the process again
from G1 or exits through G0. From G0, the cell can
undergo terminal differentiation.
The stages between one mitosis and the next, which
include G1, S, and G2, are known collectively as the
interphase.
G1 phase
Cell increases in size
Cellular contents are duplicated
S phase
DNA replication
Each of the 46 chromosomes (23 pairs) is
replicated by the cell
G2 phase
Cell grows more
Organelles and proteins develop in preparation
for cell division
M phase
Mitosis followed by cytokinesis (cell separation)
Formation of two identical daughter cells
Alternating with the interphase is the cell division phase.
In eukaryotic cells, there are two types of cell division:
mitosis and meiosis.
1. MITOSIS
This type of cell division produces two identical cells
with the same number of chromosomes. Mitosis is
divided into four stages.
STAGE A: Prophase. The nuclear membrane and
nucleoli may still be present. The chromosomes are
thicker and shorter because of repeated coiling. At this
stage, each chromosome is made up of two identical
sister chromatids as a consequence of replication of
DNA during the S phase. The two chromatids
produced from one chromosome are still attached at
one point, called the centromere. The centromere may
divide the chromosome into the shorter arms, also
called the p arms (‘p’ stands for petite in French) and
the longer q arms. If the chromosomes are stained
using Giemsa, alternating dark and light regions will
appear. These are the heterochromatin and
euchromatin, respectively. The heterochromatin are
more coiled and dense than the euchromatin (Figure 3).
Figure 3. Parts of the chromosome.
STAGE B: Metaphase. The nuclear membrane has
disappeared while the highly coiled chromosomes align
at the metaphase plate, an imaginary plane equidistant
between the cell’s two poles. Spindle fibers are also
formed. Each fiber binds to a protein called the
kinetochore at the centromere of each sister chromatid
of the chromosome.
STAGE C: Anaphase. The paired centromeres of each
chromosome separate towards the opposite poles of the
cells as they are pulled by the spindle fibers through
their kinetochores. This liberates the sister chromatids.
Each chromatid is now regarded as a full-fledged
chromosome and is only made up of one sister
chromatid.
STAGE D: Telophase. The chromosomes are now at
the opposite poles of the spindle. They start to uncoil
and become indistinct under the light microscope. A
new nuclear membrane forms around them while the
spindle fibers disappear. There is also cytokinesis or the
division of the cytoplasm to form two separate daughter
cells immediately after mitosis.
2. 2. Meiosis
The number of chromosomes normally remains
the same within the species. It does not double
or triple for every generation.
This suggests that a different kind of cell division
must take place in an individual.
This kind of cell division is called meiosis, from a
Greek word which means “to make smaller.”
Meiosis reduces the chromosome number in
half. It takes place in plants and animals
whenever gametes, or sex cells, are formed
through the process called gametogenesis.
Meiosis is a special type of cell division where
the cell undergoes two rounds of cell division to
produce four daughter cells, each with half the
chromosome number as the original parent cell
and with a unique set of genetic material as a
result of exchange of chromosome segments
during the process of crossing over.
The first round of meiotic division, also known as
meiosis I, consists of four stages:
1. prophase I,
2. metaphase I,
3. anaphase I, and
4. telophase I.
Prophase I of meiosis I, unlike its counterpart in mitosis,
is more elaborate and should be understood well in
order to grasp the mechanisms of heredity.
STAGE A: Prophase I. Meiosis starts with this stage
and includes the following substages:
Leptotene. Each chromosome is made up of two long
threads of sister chromatids as a result of replication
during the S phase of the cell cycle.
Zygotene. The chromosomes begin to pair off. Pairs of
chromosomes are called homologous chromosomes,
and this pairing process is exact.
Pachytene. The chromosomes contract due to
repeated coiling. Crossing over takes place during this
stage where a segment of a sister chromatid of one
chromosome is exchanged with the same segment of
the sister chromatid of the homologous chromosome
through the formation of a cross-linkage of the
segments called a chiasma
Figure 4. Crossing over of homologous chromosomes
during meiosis I.
Diplotene. The chromosomes begin to uncoil.
Diakinesis. The paired chromosomes disperse in the
nucleus.
STAGE B: Metaphase I. The paired chromosomes
arrange themselves along the equatorial plate.
STAGE C: Anaphase I. Spindle fibers form and attach
to the centromeres of the chromosomes. The
homologous chromosomes separate from each other
completely and start their movement toward the poles of
the cells as they are pulled by the spindle fibers. As the
centromere of each chromosome does not divide, the
sister chromatids remain together.
STAGE D: Telophase I. This is the stage when the
chromosomes reach their respective poles. Cytokinesis
follows and two daughter cells are formed. Each cell
now has only half the chromosome number because
only one chromosome from each pair goes to the
daughter cell. This is called the haploid condition, in
contrast to the diploid condition at the beginning of
meiosis I where each chromosome pair is intact.
Telophase I is followed by interphase II.
Note that each chromosome still has two sister
chromatids; it is therefore necessary for the cells to
undergo another round of division.
The second meiotic division, also known as meiosis II,
is mitotic in nature and consists of the following stages:
1. prophase II,
2. metaphase II,
3. anaphase II and
4. telophase II;
- these stages are identical with the mitotic stages
- The results are four cells, two from each
daughter cell from meiosis I, with one half the
diploid chromosome number and with only one
sister chromatid for each chromosome.
Figure 5 shows the different stages of meiosis in the
squash anther.
