Reproduction, meiosis and DNA
Sexual reproduction mixes genetic information from two parents, which needs special cells with half the usual number of chromosomes. Meiosis makes those cells. All of it rests on DNA, the molecule that carries the instructions.
Part 1 of 3: Learn it
In short
- Meiosis makes four gametes, each with half the chromosomes and all genetically different.
- Fertilisation joins two gametes, restoring the full number of chromosomes.
- A gene is a short section of DNA that codes for one protein.
Where this is in your specification
Spec points: AQA 4.6.1.1 to 4.6.1.5 (Combined Trilogy 4.6.1.1, 4.6.1.2 and 4.6.1.4), Edexcel Topic 3, OCR B5.1
| Board | Topic: Inheritance, variation and evolution |
|---|---|
| AQA 8461 | 4.6 (8461); 4.6 (8464) |
| Edexcel 1BI0 | Topics 3 and 4 |
| OCR J247 | B5 and B6 |
| Eduqas C400 | 7.1 to 7.4 |
| Cambridge IGCSE 0610 | 16.1 to 16.4, 17.1 to 17.4, 18.1 to 18.3, 21.3 |
| National 5 C807 75 | Cell biology: 3 DNA and the production of proteins, 5 Genetic engineering; Multicellular organisms: 3 Reproduction, 4 Variation and inheritance; Life on Earth: 6 Evolution of species |
Sexual and asexual reproduction
| Sexual | Asexual | |
|---|---|---|
| Parents | two | one |
| Cells involved | gametes (sperm and egg; pollen and egg cells) | ordinary body cells |
| Cell division | meiosis to make gametes | mitosis |
| Offspring | varied | genetically identical clones |
Meiosis and fertilisation
- Meiosis happens in the reproductive organs (testes and ovaries in humans).
- The genetic information is copied, then the cell divides twice.
- That makes four gametes, each with one set of chromosomes (23 in humans) and each genetically different.
- At fertilisation, a sperm and an egg join, giving a cell with the full number (46 in humans). It then divides by mitosis to grow into an embryo.
DNA, genes and the genome
DNA is a polymer made of two strands twisted into a double helix. It is coiled into chromosomes in the nucleus. A gene is a small section of DNA that codes for a particular sequence of amino acids, which makes one protein. The genome is the entire genetic material of an organism.
Knowing the human genome helps doctors find genes linked to diseases, understand inherited disorders and trace how humans migrated in the past.
DNA structure (separate Biology)
Each strand is a chain of nucleotides, each with a sugar, a phosphate and one of four bases: A, T, C and G. The bases pair across the strands: A always with T, C always with G. So in any DNA molecule the amount of A equals the amount of T, and C equals G.
How many chromosomes are in a human sperm cell?
Show the answer
23.
Part 2 of 3: See it worked
Worked examples
Example 1
A cat's body cells have 38 chromosomes. How many are in each of its egg cells?
- Gametes are made by meiosis, which halves the number
- 38 ÷ 2 = 19
Answer: 19 chromosomes.
Example 2
In a DNA sample, 30% of the bases are A. What percentage are G? (separate Biology)
- A pairs with T, so T is also 30%
- A + T = 60%, leaving 40% for C and G
- C pairs with G, so each is 20%
Answer: 20%.
Common mistakes
- Saying meiosis makes two cells. It makes four.
- Saying gametes are identical. Each one is genetically different.
- Mixing up a gene, an allele and a chromosome.
- Pairing A with G or C with T.
What is a genome?
Show the answer
The entire genetic material of an organism.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
How many chromosomes are in a human sperm cell?
23.
What is a genome?
The entire genetic material of an organism.
Give one advantage of sexual reproduction.
It produces variation, so some offspring may survive if the environment changes.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses