Download Human sexual reproduction notes

Human sexual reproduction notes
Sexual and asexual reproduction
There are two types of reproduction: sexual reproduction and asexual reproduction.
∙
Sexual reproduction:
Specialised sex cells called gametes are produced.
In humans, the sperm moves to the ovum and fertilises it.
In plants, the pollen nucleus fertilizes the egg cell (after pollination)
The single cell formed by fertilisation is called a zygote. This develops into
an embryo - and then, in humans, a foetus and in plants, a seedling.
Advantage of sexual reproduction: genetic diversity, so some offspring are
better adapted than others to changing conditions
∙
Asexual reproduction:
There are no gametes and no fertilisation.
A structure (e.g. a bud or a runner) formed by mitosis splits off or grows
away from the main body.
This structure grows into a new organism
All offspring produced asexually are genetically identical.
Advantage: large numbers of genetically identical offspring can be
produced which will be as well adapted to conditions as their parents.
∙
Sperm, pollen, ova and egg cells are gametes produced by meiosis, and thus
have only half the number of chromosomes, making them haploid cells.
∙
Sperm are produced in the testes and ova are produced in the ovaries.
Mitosis and meiosis
Feature
Mitosis
Meiosis
Number of cell divisions
1
2
Number of cells formed
2
4
Number of chromosomes in
Diploid
Haploid
Type of cells formed
Body cells
Sex cells
Genetic variation in cells
None
Variation
cells formed
formed
Human sexual reproduction notes
Fertilization
∙
Occurs when the male gamete nucleus fuses with the female gamete nucleus
∙
Some animals reproduce through external fertilisation in which the males and
females release their gametes into the water
∙
Most animals reproduce through internal fertilisation, in which the male ejaculates
his sperm into the bodies of the females.
∙
As each gamete has half the number of chromosomes, the zygote will have the full
number.
Human sexual reproduction:
1. Sperm are produced in the testes by meiosis.
2. During sexual intercourse they pass down the sperm duct and are mixed
with fluid from the seminal vesicles to produce semen.
3. The semen is ejaculated into the woman’s vagina and travels towards the
fallopian tubes.
4. One ovum is released every month into a Fallopian tube, at which point it
may be fertilised by sperm.
5. The zygote will develop into an embryo and implant itself in the lining of the
uterus.
6. The embryo then develops a placenta which allows the embryo to receive
materials such as food and oxygen from its mother.
7. The amnion develops around the embryo, and it secreted amniotic fluid,
which acts as a shock absorber for the embryo.
8. When the embryo becomes recognisably human we call it a foetus,
Childbirth:
1. Cervix dilates.
2. Uterus contracts to burst the amnion.
3. Uterus contracts to deliver the baby.
4. After birth, the uterus continues to contract to deliver the placenta.
Human sexual reproduction notes
Hormones and secondary sexual characteristics
∙
Presence of sexual organs is called primary sexual characteristic.
∙
During puberty, secondary sexual characteristics become apparent.
∙
Testosterone causes male secondary sexual characteristics and oestrogen causes
female secondary sexual characteristics.
∙
Testosterone is produced in the testes and stimulates sperm production.
∙
Oestrogen is released by the ovaries (under the influence of FSH).
In boys
In girls
Sperm production starts
The menstrual cycle begins.
Growth and development of male sexual
Growth and development of female sexual
organs.
organs.
Growth of armpit hair, pubic hair, chest
Growth of armpit hair and pubic hair.
hair, and facial hair.
Increase in body mass; growth of muscles,
Increase in body mass; development of
e.g. chest.
“rounded” shape to hips.
Voice breaks
Voice deepens
Sexual “drive” develops
Sexual “drive” develops
Breasts develop
Human sexual reproduction notes
Hormones and the menstrual cycle:
stage in cycle
pituitary
ovary
uterus
Start
releases FSH
FSH stimulates follicle to
grow
menstruation takes
place
growing follicle releases
oestrogen
lining is repaired after
menstruation
After menstruation
Middle of cycle
releases LH
LH causes ovulation
LH causes follicle to turn
into corpus luteum
Second half of cycle
corpus luteum releases
progesterone
lining thickens and is
ready to support the
implantation of an
embryo
End of cycle
If implantation does
not take place
the corpus luteum shrinks menstruation begins
and oestrogen and
progesterone are no longer
released produced
If implantation of an
embryo takes place
Corpus luteum continues
lining of the uterus
to release progesterone
supports the
until the placenta develops implanted embryo