16.1 Nuclear division
Chromosomes are made of DNA and carry genetic information in the form of genes. They are found in the nucleus.
- A haploid nucleus contains a single set of chromosomes (as in gametes).
- A diploid nucleus contains two sets of chromosomes (as in body cells).
- In a diploid cell, there is a pair of each type of chromosome. In a human diploid cell there are 23 pairs, which makes 46 chromosomes in total.
Mitosis
Roles of mitosis: growth, repair of damaged tissues, replacement of dying cells and asexual reproduction.
Stem cells are unspecialised cells that divide by mitosis to produce daughter cells, which can become specialised for specific functions (for example nerve cells, muscle cells).
Meiosis
| Mitosis | Meiosis | |
|---|---|---|
| cells made | two | four |
| chromosome number | stays the same (diploid) | halved (haploid) |
| genetically | identical | different |
| used for | growth, repair, asexual reproduction | making gametes |
Cancer
Cancers form as a result of uncontrolled cell division, which makes a mass of cells called a tumour.
16.2 Asexual and sexual reproduction
Examples of asexual reproduction: bacteria dividing, yeast budding, and potato tubers and strawberry runners (in plants).
| Asexual | Sexual | |
|---|---|---|
| parents | one | two |
| offspring | genetically identical (clones) | genetically different (variation) |
| advantages | only one parent needed; fast; many offspring; no need to find a mate; useful in a stable environment | variation, so some offspring may survive a change in the environment or a new disease; can adapt (evolve) |
| disadvantages | no variation, so all can be killed by one disease or a change | needs two parents and time and energy to find a mate; slower; fewer offspring |
16.3 Sexual reproduction in plants
The flower
| Part | Function |
|---|---|
| sepals | protect the flower when it is in bud |
| petals | in insect-pollinated flowers, large and brightly coloured to attract insects |
| stamen | the male part: anther (makes pollen, containing the male gametes) and filament (holds the anther up) |
| carpel | the female part: stigma (receives pollen), style (holds the stigma up) and ovary (contains the ovules, which contain the egg cells) |
Insect-pollinated and wind-pollinated flowers
| Insect-pollinated | Wind-pollinated | |
|---|---|---|
| petals | large, brightly coloured, scented | small or absent, dull |
| nectar | present | absent |
| anthers | inside the flower | hang outside the flower, so wind can blow the pollen away |
| stigma | inside, sticky | feathery and hangs outside, to catch pollen |
| pollen | sticky or spiky, relatively little | smooth and light, produced in large amounts |
Pollination
| Self-pollination | Cross-pollination | |
|---|---|---|
| variation | little: offspring are similar | more variation |
| response to environmental change | poorer: all may be affected | better: some offspring may survive |
| reliance on pollinators | less (can happen without insects or wind) | more |
Fertilisation
After pollination, a pollen tube grows from the pollen grain down the style, into the ovary and into the ovule. The nucleus of the male gamete travels down the tube and fuses with the nucleus of the egg cell: this is fertilisation. It produces a zygote.
After fertilisation, the ovules develop into seeds and the ovary develops into a fruit.
Seeds
A seed contains an embryo (radicle: the future root; plumule: the future shoot), cotyledons (the food store, or seed leaves) and a protective testa (seed coat).
Dispersal
Seed and fruit dispersal by wind and by animals spreads the offspring to colonise new areas and reduces competition with the parent plant (and between seedlings) for light, water and minerals.
| Wind-dispersed | Animal-dispersed | |
|---|---|---|
| features | light, with a wing or a parachute of hairs (for example sycamore, dandelion) | hooks that catch on fur, or brightly coloured, fleshy, edible fruits with seeds that pass through the gut (for example berries) |
Germination
- Water is absorbed, and activates the enzymes in the seed.
- Oxygen is needed for aerobic respiration, which releases the energy for growth.
- A suitable temperature is needed for the enzymes to work.
Process. The enzymes (such as amylase) break down the stored food (starch) in the cotyledons or endosperm to sugar, which is used in respiration and to build new cells. The radicle grows first, down into the soil, then the plumule grows upwards.
Investigating the conditions. Set up tubes of seeds with: water, air and warmth (germinates); no water (does not); no oxygen (water boiled and covered with oil: does not); and low temperature (a refrigerator: does not).
16.4 Sexual reproduction in humans
The male reproductive system
| Part | Function |
|---|---|
| testes | make sperm and the hormone testosterone |
| scrotum | holds the testes outside the body, where it is slightly cooler (better for sperm) |
| sperm ducts | carry sperm from the testes to the urethra |
| prostate gland | makes fluid that nourishes the sperm (semen) |
| urethra | carries sperm (or urine) through the penis |
| penis | places sperm inside the vagina during sexual intercourse |
The female reproductive system
| Part | Function |
|---|---|
| ovaries | make eggs and the hormones oestrogen and progesterone |
| oviducts (fallopian tubes) | carry the egg to the uterus; fertilisation takes place here; lined with cilia |
| uterus (womb) | where the embryo implants and the fetus develops |
| cervix | a ring of muscle at the opening of the uterus |
| vagina | receives the penis and sperm; the birth canal |
The gametes
| Sperm | Egg cell | |
|---|---|---|
| size | small | large (the largest human cell) |
| numbers | millions | one released a month |
| motility | swims by its tail (flagellum) | does not move itself: carried along the oviduct by cilia |
| structure | flagellum (tail), many mitochondria to supply energy, enzymes in the acrosome to digest the egg's jelly coat | energy stores (food) in the cytoplasm; a jelly coat that changes at fertilisation so that no more sperm can enter |
Puberty and secondary sexual characteristics
At puberty, testosterone (from the testes) causes the development of male secondary sexual characteristics (for example facial and body hair, a deeper voice, broader shoulders), and oestrogen (from the ovaries) causes female characteristics (breast development, widening of the hips, body hair, the start of the menstrual cycle).
The menstrual cycle
The cycle is about 28 days. Each month an egg is developed and released (ovulation) from an ovary, and the lining of the uterus thickens to prepare for a pregnancy. If there is no pregnancy, the lining breaks down and is lost in menstruation (a period).
| Hormone | Made by | Role |
|---|---|---|
| FSH (follicle-stimulating hormone) | pituitary gland | makes a follicle (an egg) develop in the ovary; stimulates the ovary to make oestrogen |
| oestrogen | ovary | makes the uterus lining thicken; stimulates the release of LH |
| LH (luteinising hormone) | pituitary gland | triggers ovulation (release of the egg), at about day 14 |
| progesterone | ovary (the empty follicle) | maintains the lining of the uterus; inhibits FSH and LH |
Early development
After fertilisation, the zygote divides to form a ball of cells (an embryo), which moves down the oviduct and becomes implanted in the lining of the uterus.
- The amniotic sac surrounds the embryo and contains amniotic fluid, which protects the fetus from knocks and supports it, and allows it to move.
- The placenta grows into the wall of the uterus and is joined to the fetus by the umbilical cord.
Functions of the placenta and umbilical cord. The mother's blood and the fetus's blood come close together but do not mix. Dissolved nutrients (glucose, amino acids) and oxygen diffuse from the mother's blood to the fetus's blood, and excretory products (urea) and carbon dioxide diffuse from the fetus to the mother's blood. The umbilical cord carries blood between the fetus and the placenta.
Some viruses (for example HIV and rubella) can pass across the placenta and affect the fetus. Harmful substances such as nicotine and alcohol can too.
A flagellum (tail) for swimming, and enzymes in the acrosome to digest the jelly coat of the egg (also many mitochondria to release energy).
Practise this chapter
Every past-paper question on these topics, with the mark scheme.