BIOLOGY · Stage 9 · Chapter 11: Human Genetics and Development
Biology 11.1 Reproduction: a characteristic of life — Gametes and Gonads — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
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Lesson summary
11.1 Reproduction: a characteristic of life — Gametes and Gonads — Alhussan International Academy — Biology (Stages 7–9) notes: Describe DNA, chromosomes and genes, compare asexual and sexual reproduction, and describe the reproductive systems and gametes.Reproduction = making mo…
- Passing on genes
- Chromosome numbers
- Fertilised egg
- Sperm ducts
- Oviducts (fallopian tubes)
- Uterus (womb)
Section 1 of 6Reproduction as a characteristic of life
- Reproduction = making more of the same kind of organism, and it is one of the seven characteristics of living things.
- It produces new individuals and so keeps a species in existence.
- Passing on genes
- when organisms reproduce they pass on some of their genetic material to their offspring.
Section 2 of 6DNA, chromosomes and genes
- DNA
- it is the genetic material, an enormous molecule that holds the instructions for making a new life and is found in the nucleus.
- Chromosomes
- when a cell is about to divide, the DNA forms long threads called chromosomes.
- Genes = sections of DNA in the chromosomes that carry information.
- Chromosome numbers
- different species have different numbers, and in body cells they form pairs.
- Humans have 46 chromosomes, which form 23 pairs.
- Garlic has 16 (8 pairs), potato 48 (24 pairs) and elephant 56 (28 pairs).
- Each pair carries information about the same characteristics.
Section 3 of 6Asexual and sexual reproduction
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | One | Two |
| Offspring | Identical to the parent, with the same DNA | Similar to but different from both parents |
| Example | Bacteria split in two | Elephants and cocoa seeds |
- Gametes
- sexual reproduction involves special reproductive cells, a male sperm and a female egg (ovum).
- Each gamete contains half the number of chromosomes of the parent cells.
- Human body cells have 46 chromosomes, so human gametes have 23.
- A hen's body cells have 78 chromosomes, so its egg has 39.
- Fertilised egg
- the zygote has the full number, 46 in humans, with half from each parent.
Section 4 of 6The male reproductive system
- Testes
- they lie inside the scrotum, slightly below body temperature, and make sperm and the hormone testosterone.
- Sperm ducts
- they carry sperm to the urethra.
- Glands
- the prostate and seminal vesicles add fluid containing nutrients, and sperm plus this fluid is semen.
- Penis
- it places semen inside the vagina during intercourse, and the urethra passes through it.
Section 5 of 6The female reproductive system
- Ovaries
- they contain eggs and release one egg about once a month (ovulation), and make oestrogen and progesterone.
- Oviducts (fallopian tubes)
- they carry the egg to the uterus using cilia, and fertilisation normally occurs here.
- Uterus (womb)
- it has a thick, muscular wall with a soft lining where the embryo implants and grows.
- Cervix
- a ring of muscle at the lower end of the uterus.
- Vagina
- it receives the penis during intercourse and forms the birth canal.
- Gonads
- gametes are made in the sex organs, the testes in males and the ovaries in females.
Section 6 of 6Adaptations of gametes
| Gamete | Adaptations |
|---|---|
| Sperm | Long tail to swim; many mitochondria for energy; head contains the nucleus and an acrosome with enzymes to digest the egg membrane; very small; made in huge numbers |
| Egg | Large, with a food store of cytoplasm for the early embryo; a jelly coat that changes after fertilisation to block other sperm; made in small numbers; cannot move itself |
- Numbers
- sperm are produced in millions because only a few survive the journey, while eggs are released one at a time.
- Testes position
- they are outside the body because sperm develop best a few degrees below body temperature.
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