SCIENCE 9-LEedededbevdbedvebvxbSSON 5.pdf

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1. SCIENCE 9
LESSON 5

2. MEIOSIS, GAMETES, AND THE
HUMAN LIFE CYCLE

5. Genesareresponsibleforsuchvariations.
AgeneisasegmentofaDNAmoleculetahtgives
instructionsformakingaprotein.
Theproteincaninfluencethecharacteristicsofaperson.
Fertilizationistheunionofgamete4sfromtheparent
whichhaveundergonegeneticrecombinationinmeiosis;
itmakestheresemblancepromisingyetatotallydifferent
anduniqueindividual.

6. WHAT IS MEIOSIS?
Meiosisisaprocesswhereasinglecell
dividestwicetoproducefourcells
containinghalftheoriginalamountof
geneticinformation.
Thesecellsareoursexcells–spermin
males,eggsinfemales.

7. Duringmeiosisonecelldividestwicetoformfourdaughter
cells.
Thesefourdaughtercellsonlyhavehalfthenumberof
chromosomesoftheparentcell–theyarehaploid.
Meiosisproducesoursexcellsorgametes(eggsinfemalesand
sperminmales).
Meiosiscanbedividedintoninestages.Thesearedivided
betweenthefirsttimethecelldivides(meiosisI)andthe
secondtimeitdivides(meiosisII):

8. MEIOSIS I

9. 1. INTERPHASE
oThe DNA in the cell is copied resulting in two identical
full sets of chromosomes.
oOutside of the nucleus are two centrosomes, each
containing a pair of centrioles, these structures are
critical for the process of cell division.
oDuring interphase, microtubules extend from these
centrosomes.

10. 2. PROPHASE I
oThe copied chromosomes condense into X-shaped structures that
can be easily seen under a microscope.
oEach chromosome is composed of two sister chromatids containing
identical genetic information.
oThe chromosomes pair up so that both copies of chromosome 1 are
together, both copies of chromosome 2 are together, and so on.
oThe pairs of chromosomes may then exchange bits of DNA in a
process called recombination or crossing over.
oAt the end of Prophase I the membrane around the nucleus in the cell
dissolves away, releasing the chromosomes.
oThe meiotic spindle, consisting of microtubules and other proteins,
extends across the cell between the centrioles.

11. 3. METAPHASE I
oThe chromosome pairs line up next to each other
along the centre (equator) of the cell.
oThe centrioles are now at opposites poles of the cell
with the meiotic spindles extending from them.
oThe meiotic spindle fibres attach to one chromosome
of each pair.

12. 4. ANAPHASE I
oThe pair of chromosomes are then pulled apart by the
meiotic spindle, which pulls one chromosome to one
pole of the cell and the other chromosome to the
opposite pole.
oIn meiosis I the sister chromatids stay together. This
is different to what happens in mitosis and meiosis II.

13. 5. TELOPHASE I AND CYTOKINESIS
oThe chromosomes complete their move to the opposite poles of the
cell.
oAt each pole of the cell a full set of chromosomes gather together.
oA membrane forms around each set of chromosomes to create two
new nuclei.
oThe single cell then pinches in the middle to form two separate
daughter cells each containing a full set of chromosomes within a
nucleus. This process is known as cytokinesis.

14. MEIOSIS II

15. 6. PROPHASE II
oNow there are two daughter cells, each with 23 chromosomes (23
pairs of chromatids).
oIn each of the two daughter cells the chromosomes condense again
into visible X-shaped structures that can be easily seen under a
microscope.
oThe membrane around the nucleus in each daughter cell dissolves
away releasing the chromosomes.
oThe centrioles duplicate.
oThe meiotic spindle forms again

16. 7. METAPHASE II
oIn each of the two daughter cells the chromosomes (pair of sister
chromatids) line up end-to-end along the equator of the cell.
oThe centrioles are now at opposites poles in each of the daughter
cells.
oMeiotic spindle fibres at each pole of the cell attach to each of the
sister chromatids.

17. 8. ANAPHASE II
oThe sister chromatids are then pulled to
opposite poles due to the action of the meiotic
spindle.
oThe separated chromatids are now individual
chromosomes.

18. 9. TELOPHASE II AND CYTOKINESIS
oThe chromosomes complete their move to the opposite poles of the
cell.
oAt each pole of the cell a full set of chromosomes gather together.
oA membrane forms around each set of chromosomes to create two
new cell nuclei.
oThis is the last phase of meiosis, however cell division is not
complete without another round of cytokinesis.
oOnce cytokinesis is complete there are four granddaughter cells,
each with half a set of chromosomes (haploid):
oin males, these four cells are all sperm cells
oin females, one of the cells is an egg cell while the other three are polar bodies
(small cells that do not develop into eggs).

20. GAMETES AND FERTILIZATION

21. Sexual reproduction refers to the sexual germ cells (gametes)
produced by the parents, through the combination of bisexual germ
cells (such as sperm and egg cells), become the fertilized egg, and
then the fertilized egg develops into a new individual, called sexuality
reproduction.
Sexual reproduction can also promote the spread of favorable
mutations in the population. If two individuals in a species have
favorable mutations at different sites, in the asexual reproduction
population, the two mutants will compete until one elimination, it is
impossible to retain both favorable mutations at the same time.

22. GAMETES
oGametes refer to mature cells produced by the reproductive system
when the organism is sexually reproducing, referred to as germ cells.
oGametes are divided into male gametes and female gametes. The
female gametes of animals and plants are usually called egg cells,
and the male gametes are called sperm. Sperm is quite small, but it
can move and enter the egg cell in a scorpion shape. The egg cell is
quite large and non-swimming.
oFor example, the egg cell volume of sea urchin is 10,000 times than
that of sperm cells. Although the male and female gametes are
different in volume, the nuclear DNA they provide for the offspring is
equal, that is, each provides a set of genomes.

23. FERTILIZATION
Fertilization is the process by
which eggs and sperm fuse
together into one zygote.

25. Nowadays, couples who cannot bear
children due to certain factors can
Assisted Reproductive Surgery (ART).

26. IN VITRO FERTILIZATION(IVF)
-It is a process by which an egg is fertilized by sperm outside the
body. IVF is a major treatment for infertility. The process involves:
Monitoring and stimulating a woman’s ovulatory process;
Removing ovum or ova (egg or eggs) from the woman’s ovaries;
Letting sperm fertilize the egg or eggs in a fluid medium in a
laboratory.
Culturing the fertilized egg (zygote) for 2-6 days in a growth
medium
Tranferring the fertilized egg to the mother’s uterus with the
intention of establishing a successful pregnancy.

28. INTRACYTOPLASMIC SPERM
INJECTION (ICSI)
-It is an in vitro fertilization procdure in which a single sperm is
injected directly into an egg.

30. INTRAUTERINE INSEMINATION (IUI)
-it involves a laboratory procedure to separete fat moviong sperm
from more sluggish or non-moving sperm cells. The fast moving
sperm cells are then placed into the woman’s womb close to the time
of ovulation when the egg is released from the ovary in the middle of
the monthly cycle.