Powerpoint presentation created for Grade 8 Science learnersl

1. Life
Cycle

6. Spermatogenesis is the production of
sperm cell
(spermatozoon/spermatozoa) in males
males by the process of meiosis which
is haploid.
Oogenesis is the production of egg cell
(ovum/ova) in females via meiosis
which is haploid.

7. Human Life Cycle…
Does human life cycle
involves both
mitosis and meiosis?
How can you explain?

8. Human Life Cycle
The human life cycle involves both mitosis
and meiosis.
Mitosis (equational division) occurs
during development and after birth as
part of growth and tissue repair
Meiosis (reductional division) occurs
during sexual reproduction and the
number of chromosomes are reduced
from diploid to haploid

9. Remember!
 Due to meiosis, two individuals can
create off-spring that are genetically
different not only from themselves
but also from their siblings.
 Meiosis is a special type of nuclear
division that results in 4 gamete cells.
• Spermatogenesis in males
• Oogenesis in females

10. Fertilization - Zygote
The fertilization of an egg by a sperm
gives rise to a zygote (a diploid)
The zygote contains 2 sets of
chromosomes – 1 set from father (n)
and 1 set from mother (n)
In humans, 1 set of chromosomes is
equal to 23 pairs thus the diploid
zygote contains 46 chromosomes.

11. Homologous Chromosomes
 The 23 pairs of chromosomes (46 altogether)
are called the diploid (2n) number
Homologous chromosomes (homologues) are
members of a chromosome pair.
 Alternate versions of a gene for a particular
trait are called alleles.
 The larger chromosome is the X and the
smaller is the Y chromosome

12. Concept of
Homologous Chromosomes
 Of the 23 pairs, 22 pairs are called autosomes while
the remaining 1 pair is called sex chromosomes .
 Sex Chromosomes carry genes responsible for the
sex/gender of the individual which is different from
male and females
 Autosomes/Body Chromosomes in each pair are the
same in males and females.
Males contain XY sex chromosomes.
Females contains XX sex chromosomes.

14. Karyogram
Karyogram is the arrangement of the
22 pairs of autosomes and 2 sex
chromosomes from the biggest to
smallest.
Karyotyping is the process of
preparing a karyogram.

15. Sexual Reproduction:
Meiosis
Meiosis serves 2 functions:
1.Reducing the chromosome
number
2.Shuffling the chromosomes
and genes to produce
genetically different gametes

16. Meiosis

17.  Meiosis is the process by which replicated
chromosomes undergo two nuclear divisions to
produce four haploid cells, also called meiocytes
(sperms and eggs).
 Meiosis uses similar mechanisms as those
employed during mitosis to accomplish the
separation and redistribution of chromosomes.
However, several features, namely, the pairing and
genetic recombination between homologous
chromosomes, are unique to meiosis.

18. The steps leading up to meiosis are
similar to those of mitosis – the
centrioles and chromosomes are
replicated. The amount of DNA in the
cell has doubled, and the ploidy of the
cell remains the same as before, at 2n.

19. Meiosis 1

20. Meiosis I proceeds directly to meiosis II
without going through interphase.
Meiosis I is unique in that genetic
diversity is generated through
crossing over and random positioning
of homologous chromosomes
(bivalent chromosomes).

21. Prophase 1
 During prophase I, chromosomal
condensation allows chromosomes
to be viewed under the microscope.
In late prophase I, homologous
chromosomes (also called bivalent
chromosomes, or bivalents) pair
laterally, or side-by-side. At this time
they are said to be in synapsis.
During synapsis/crossovers – cross-
connections that form from breakage
and rejoining between sister
chromatids – can occur between the
paired bivalents, leading to genetic
recombination (exchange of genetic
material) between the strands
involved.

22. The point where a crossover occurs is
called a chiasma (plural chiasmata).
 Note: The bivalents have two chromosomes and
four chromatids, with one chromosome originating
from each parent.

23. Metaphase 1
 In metaphase I, each pair of
bivalents (two chromosomes, four
chromatids total) align on the
metaphase plate. This is different
from metaphase in mitosis, where
all chromosomes align single file
on the metaphase plate.
 The position of each chromosome
in the bivalents is random - either
parental homolog can appear on
each side. This means that there is
a 50-50 chance for the daughter
cells to get either the mother's or
father's homolog for each
chromosome

24. In a diploid cell with 2 pairs of chromosomes,
there are 4 ways to arrange the chromosomes
during metaphase I.

25. Anaphase 1
 In anaphase I,
homologous
chromosomes separate.
Homologous
chromosomes, each
containing two
chromatids, move to
separate poles. Unlike in
mitosis, the centromeres
do not split and sister
chromatids remain paired
in anaphase I.

26. Telophase 1 and Cytokinesis
 In telophase I, the homologs of
each bivalent arrive at opposite
poles of the cell, and a new
nuclear membrane forms around
each set of chromosomes.
 Cytokines is then divides the cell
into two daughter cells. Each of
the two daughter cells is now
haploid (n), with half the number
of chromosomes per nucleus as
in meiosis I.

27. Meiosis 2:

28. Chromosomal replication does not
occur between meiosis I and
meiosis II; meiosis I proceeds
directly to meiosis II without going
through interphase.

29. Prophase II
 Prophase II is the first stage,
after Telophase I, in the second
meiotic division cycle.
 In this stage, the
chromosomes, in both
daughter cells produced in
meiosis I, migrate to the
of the cell.
 Prophase II ends when the
chromosomes are aligned
along the middle of the cell
along a single plane.

30. Metaphase II
 In metaphase II, in each of the two
daughter cells produced by the first
meiotic division (which are known
as secondary germ cells), the spindle
again draws the chromosomes to the
metaphase plate.
 This time, unlike metaphase I, the
two kinetochores of each centromere bind
to spindle fibers from opposite poles (as
in mitotic metaphase).
 This results in separation of the sister
chromatids of each chromosome during
the next phase of meiosis, anaphase II.

31. Anaphase II
 During anaphase II, the sister
chromatids of
each chromosome separate and
move toward opposite poles. As the
chromosomes are dragged along
by the spindle apparatus, their arms
can be seen dragging along behind
so that the chromosomes form V-
shapes.
 The poles themselves move further
apart as cytokinesis begins and the
cell lengthens .

32. Telophase II
 During telophase II,
the chromosomes reach opposite
poles, cytokinesis occurs, the two
cells produced by meiosis I divide
to form four haploid daughter
cells, and nuclear envelopes
 When telophase II is over, the two
cells are entirely separated and
their nuclear membranes are fully
formed.
 Meiosis is then complete.

33. Meiosis Compared to Mitosis
 Meiosis requires 2 nuclear divisions
 mitosis requires only 1 nuclear
division
 Meiosis produces 4 haploid daughter
cells (half the chromosome number
of the parent cell)
 Mitosis produces 2 diploid
daughter cells (same as the
parent cell)
 Meiosis produces 4 daughter cells
that are genetically different to each
other and to the parent cell
 Mitosis produces 2 daughter cells
that are genetically similar to
each other and to the parent cell