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

Asexual reproduction

HintIts name begins with a prefix meaning 'without'.

The whyAsexual 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

HintThe same word is used for an exact copy of anything.

The whyBecause 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

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

The whyGrowing 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

HintThe same word names the nuclear process that powers the Sun.

The whyIn 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

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

The whyEach 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

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

The whyIn 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

HintA gamete carries a single set, not pairs.

The whyBody 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

HintPicture what would happen to the count when two join together.

The whyTwo 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

HintIt is one more division than in mitosis.

The whyCopying 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

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

The whyMeiosis 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

HintTake the word 'gene' and add the ending used in 'chromosome'.

The whyThe 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

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

The whyA 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

HintThey are the units released when a protein is digested.

The whyA 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

HintThink about where our ancestors travelled.

The whySmall 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

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

The whyNow 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.