Reproduction

Asexual Reproduction

Asexual reproduction – a process resulting in genetically identical offspring from one parent.

Sexual Reproduction

Sexual reproduction - a process involving the fusion of the nuclei of two gametes to form a zygote and offspring that are genetically different from each other.

• Fertilisation – fusion of nuclei of gametes

Nuclei of gametes are haploid (contain only 1 set of chromosomes)

Nuclei of zygotes are diploid (contains 2 sets of chromosomes)

Sexual reproduction in plants

• Stigma - sticky or feathery end where the pollen lands/deposited
• Style - supports the stigma
• Ovary - produces female sex cells contained in ovules
• Sepals - a leaf-like structure that encloses and protect the bud of a flower
• Pistil - female part of flower (stigma, style, ovary, ovule)
• Ovule - contains female sex cells (ovum)
• Anther - site where the male gametes (pollen) are produced
• Filament - supports the anther
• Stamen - male part of flower (anther, filament)
• Petals - a plain or colourful leaf-like structure of some flowers
• Nectary - site where nectar is found (usually present in insect pollinated flowers)
• Pollen grains - male sex cells

Pollination – The transfer of pollen grains from an anther to a stigma.

Diagram

Self-pollination – The transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant.

Cross-pollination – The transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.

Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule

1. A pollen tube grows
2. The nucleus of pollen grain slips down the tube as it grows down the style towards the ovary.
3. The ovary contains one or more ovules which each contain a female nucleus that a male pollen nucleus can fuse with.
4. The pollen tube enters the ovule through a small hole - the micropyle.

After fertilisation,

1. The stamen, petals and sepals wither and fall off.
2. Each ovule now contains a zygote, which is formed at fertilisation.
3. The zygote divides by mitosis to form an embryo.
4. The ovule is now a seed, while the ovary forms the fruit.

Germination is the development of a new plant from the embryo in a seed. Conditions to germinate:

• Water - activate enzymes which digest stored food. allows the seed to swell up.
• Oxygen - needed for aerobic respiration to release energy needed for germination
• Warmth - needed for the enzymes to work effectively at their optimum temperature

Sexual reproduction of humans

Fertilisation – the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell)

In early development, the zygote forms an embryo which is a ball of cells that implants into the lining of the uterus.

• Umbilical cord : allows exchange of dissolved nutrients, gases and excretory products between the blood of the mother and the blood of the fetus.
• Placenta : provides oxygen & nutrients to fetus, and removes excretory products.
• Amniotic sac : protect fetus from mechanical damage, Amniotic fluid: protect fetus from mechanical damage, maintains temperature and fetal drinking

Some pathogens and toxins can pass across the placenta and affect the fetus.

• Oesteogen → stimulates the development of sex organs and sexual characteristics in girl’s body.
• Progesterone → prepares the uterus so that it is ready to receive an embryo in the case of a pregnancy.

Fetal development:

• Fertilisation takes place in the oviduct
• The fertilised egg is now referred to as Zygote.
• Zygote will continue dividing and becomes embryo (a ball of cells that implants (implantation) into uterus lining to grow & develop)
• Major development of organs takes place in first 12 weeks, where the embryo gets nutrients from the mother by diffusion through the uterus lining.
• After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus.
• Fetus develops and grows by gaining dissolved nutrients (glucose, amino acids, fats, water and oxygen).
• The fetus’s blood connects to the placenta by the umbilical cord
• The mother’s blood absorbs the waste (CO2, urea are removed so it won’t build up to dangerous levels) from the fetus’s blood in the placenta.

Sexual hormones in humans

• Testosterone – development and regulation of secondary sexual characteristics during puberty in males.
• Oestrogen – development and regulation of secondary sexual characteristics during puberty in females.

Menstrual cycle:

• Day 1 – 7 : First day of a woman’s period. Lining of the uterus comes away and exits through the vagina as blood.
• Day 7 - 13 : Lining of the uterus begins to build up again. An egg starts to mature in one of the ovaries.
• Day 14 : An egg is released by an ovary into the oviduct.
• Day 14 - 17 : The egg can last up to 3 days after release from ovary. The egg travels down the oviduct and into the uterus hoping to be fertilised.
• Day 18 - 28 : If not fertilised, uterus lining begins to break down.

The cycle repeats

FSH & LH is produced in the pituitary gland.

Oestrogen & progesterone is produced in the ovaries, but placenta during pregnancy.

• FSH : Stimulates growth & maturation of egg, causes the release of oestrogen.
• Oestrogen : Thickening of uterus lining, stops release of FSH, stimulates release of LH.
• LH : Ovulation.
• Progesterone : Maintains the lining of the uterus, Inhibits LH and FSH release.

Sexually transmitted infections

Sexually transmitted infection (STI) - an infection transmitted through sexual contact.

Human Immunodeficiency Virus (HIV) - a pathogen that causes an STI. It attacks white blood cells. There is no cure, but it is treatable with medicine. It can lead to AIDs.

Transmission of HIV: sexual contact, pregnancy, childbirth, breastfeeding, injection, blood transfusion, etc.

How to prevent?

• Use condoms
• Have only 1 sex partner
• Only sterile drug injection equipment and water, and never share your equipment with others.
• Education programmes to make people aware of HIV, etc
• Zidovudine (ZDV), is an antiviral medication that delays the development of problems that result from AIDS/HIV.