2. Human Body Cells
*All human body cells have 46 chromosomes (23 pairs)
inside their nuclei (nuclei is plural of nucleus) except
sex cells (23 chromosomes).
Body cells are also called somatic cells. They make up
our body/body parts/organs, e.g. skin, heart, brain, lungs,
intestine, bones, etc and the cells of all these organs have
46 chromosomes or 23 pairs which is a complete set of
chromosomes.
A cell having a complete set of chromosomes i.e. 46
chromosomes is also called a Diploid cell (2n; n=23).
Therefore, a body cell can be called to as somatic cell or
a diploid cell. They all mean the same.
3. Human body/somatic/diploid cell
Inside this type of cell, inside
the nucleus there are 46
chromosomes as shown in
figure1.
They are arranged in pairs.
We can say 46 chromosomes or
23 pairs of chromosomes.
2n means 2xn; n=23;
so 2x23=46
Figure 1. Chromosomes in
a male somatic cell
4. Human Sex Cells
Sex cells are also called reproductive cells or gametocytes or
simply gametes.
They are produced by the reproductive organs in both males
and females.
In males the reproductive organs are the testicles or testes and
they produce the sex cells called as spermatozoa (sperms or
sperm cells).
In males
Singular – spermatozoan/sperm/sperm cell;
Plural – spermatozoa/sperms/sperm cells
In females the reproductive organs are the ovaries and they
produce the sex cells called as ova (eggs/egg cells).
In females
Singular – ovum/egg/egg cell;
Plural – ova/eggs/egg cells
5. All sex cells have an incomplete
set of chromosomes i.e. half the
number of chromosomes which is
23 chromosomes is also called a
Haploid cell (n) (n=23).
As shown in Figure 2., the
chromosomes are not arranged in
pairs.
Therefore, a sex cell can be called
to as gamete or a haploid cell.
They all mean the same. Figure 2. Chromosomes in
sex cell
6.
7. Figure 3. A chromosome contains the DNA which is the genetic material
of our body, and these are all present inside the nucleus of the cell.
8. Figure 4.
A chromosome
consists of two strands
of filaments attached
at a point.
The two strands are
called the sister
chromatids and the
point of attachment is
the centromere
(consisting of two
kintochore) and is
made of protein.
9. From what we have learned in the previous class is that:
Mitosis helps in production of body (somatic) cells that
make up all parts/organs of the body ( skin cells, heart
muscle cells, eyes, bones, nails, lungs, stomach, intestine,
kidneys, red blood cells, white blood cells, testes, ovaries,
uterus and the list goes on). All these cells are diploid (2n)
cells.
Meiosis helps in the production of only sex cells and not
parts/organs of the body. Sex cells are haploid (n) cells
Male sex cells – Sperm cells
Female sex cells – Egg cells
n=23; 2n=46
10. In Meiosis:
A somatic cell (2n) divides to
produce sex cells (n).
The number of chromosomes
originally 46 in the initial cell
has been reduced to half the
number i.e. 23 in the resulting
daughter cells.
That why its is called a
reduction division.
Meiosis occur in both males
and females in their
sexual/reproductive organs,
testes and ovaries
respectively.
11. Meiosis I
Prophase I
In meiosis homologous chromosomes are brought so close together that
they line up side by side in a process called synapsis.
The chromosomes are so close together that they may actually exchange
genetic material in a process called crossing-over. These are the sub-
stages within prophase I:
12. Metaphase I
The homologous chromosomes
line up on the equatorial plate. The
spindle fibers attach to the
centromere region of each
homologous chromosome pair.
Anaphase I
The homologus chromosomes
separate, and are drawn to the
opposite poles by the contraction
of the spindle fibers. The
centromeres in Anaphase I remain
intact.
13. Telophase I
The homologous chromosome pairs
have separated and now a member of
each pair is at the opposite ends of
the spindle.
The number has been reduced from
46 to 23 at each pole. However, each
chromosome still consists of two
sister chromatids attached by a
common centromere.
This “duplication condition” will be
corrected in the second meiotic
division.
Two daughter cells (n) are produced
at the end of Meiosis I
14. Meiosis II
The meiosis II division closely resembles the
occurrences in mitosis.
Prophase II
In each of the two daughter cells produced in the first
meiotic division, spindles reform, the chromosomes
shorten, coil, and thicken. The nuclear membrane
disappears but no duplication of DNA occurs.
15. Metaphase II
In each of the two daughter cells, the chromosomes line up
on the equatorial plate. Spindle fibers bind to both sides of
the centromere. Each chromosome consists of two sister
chromatids and one centromere.
16. Anaphase II
The centromeres of the chromosomes divide. The spindle
fibers contract, pulling the sister chromatids apart and
moving each one to an opposite pole.
17. Telophase II
New nuclear membranes form around the separated chromatids, the
spindle disappears, and the chromosomes uncoil and decondense.
The result is four haploid daughter cells each containing one half the
genetic material of the original parent cell or in our case each cell having
23 chromosomes instead of 46.
