Reproduction and genetics, Year 11: reproduction, meiosis and DNA

Asexual reproduction

提示Its name begins with a prefix meaning 'without'.

为什么Asexual reproduction needs one parent only. No gametes join, so there is no mixing of genetic information and the offspring are genetically identical to the parent. The only cell division involved is mitosis.

clones

提示The same word is used for an exact copy of anything.

为什么Because only one parent supplies the genetic information, and it is copied by mitosis, every offspring has the same genes as the parent and as each other. Genetically identical individuals are called clones.

mitosis

提示It is the division that makes two identical cells, met in Year 10.

为什么Growing plants from cuttings is asexual reproduction: one parent and no gametes. Asexual reproduction involves mitosis only, which is why the new plants are genetically identical to the parent.

fusion

提示The same word names the nuclear process that powers the Sun.

为什么In animals the gametes are sperm and egg cells; in flowering plants they are pollen and egg cells. When two gametes join, genetic information from two parents is combined in one new cell.

The mixing of genetic information from two parents

提示Think about how many individuals contribute, and what each one passes on.

为什么Each gamete carries genetic information from one parent, and the gametes themselves are all different because they are made by meiosis. When two gametes join, the offspring gets a new combination that neither parent has.

4

提示The cell divides, and then each of the cells made divides again.

为什么In meiosis the genetic information is copied once and the cell then divides twice. One cell becomes two, and those two become four gametes, each with a single set of chromosomes.

19

提示A gamete carries a single set, not pairs.

为什么Body cells hold chromosomes in pairs: here 19 pairs, making 38. Meiosis halves the number, so each gamete has one chromosome from each pair: 38 ÷ 2 = 19.

So that fertilisation restores the full number instead of doubling it

提示Picture what would happen to the count when two join together.

为什么Two gametes join at fertilisation. If each has a single set of chromosomes, the new cell has the normal paired number. Human gametes have 23 chromosomes, so the fertilised egg has 46.

Twice

提示It is one more division than in mitosis.

为什么Copying the genetic information once and then dividing two times is what halves the chromosome number. The result is four gametes, each with a single set of chromosomes. The names of the stages are not needed at GCSE.

different

提示It is the opposite of what is true for the cells made by mitosis.

为什么Meiosis shares out the chromosomes in a new combination each time, so no two gametes carry quite the same genetic information. This is one reason why brothers and sisters are not identical.

The genome

提示Take the word 'gene' and add the ending used in 'chromosome'.

为什么The genome is all of the DNA in a cell: every gene, plus the long stretches between them. The human genome was first read in full by an international project that finished in 2003, and the results are expected to matter a great deal for medicine.

polymer

提示The same word describes poly(ethene) and other long-chain molecules in Chemistry.

为什么A polymer is a long molecule built from many small repeating units. In DNA there are two such strands, wound around each other in the shape called a double helix. DNA is packed into structures called chromosomes.

Amino acids

提示They are the units released when a protein is digested.

为什么A gene is a small section of DNA on a chromosome. It carries the code for the order in which amino acids are joined, and that order decides which protein is made.

The routes along which humans migrated

提示Think about where our ancestors travelled.

为什么Small differences in DNA build up over many generations. Groups of people that separated long ago differ more than groups that separated recently, so the pattern of differences can be used to trace how people spread around the world.

Genes linked to that disease

提示Look for small sections of DNA shared by the first group only.

为什么Now that the whole human genome has been studied, scientists can search it for genes linked to different types of disease. Knowing which genes are involved also helps in understanding and treating inherited disorders.