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DNA, genes and inheritance
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Why do children look a bit like their parents? The answer is in a molecule called DNA.
Chromosomes, genes and DNA
Heredity is the passing of characteristics from parents to offspring.
The nucleus of a cell contains chromosomes. Each chromosome is a long molecule of DNA.
A gene is a short section of DNA that carries the instructions for a characteristic.
Human body cells have 46 chromosomes in 23 pairs: one of each pair from each parent. Sperm and egg cells have 23 chromosomes, so a fertilised egg has 46 again.
The nucleus of a cell contains chromosomes. Each chromosome is a long molecule of DNA.
A gene is a short section of DNA that carries the instructions for a characteristic.
Human body cells have 46 chromosomes in 23 pairs: one of each pair from each parent. Sperm and egg cells have 23 chromosomes, so a fertilised egg has 46 again.
Discovering DNA's structure
In 1953, James Watson and Francis Crick built a model of DNA as a double helix (two twisted strands).
They used evidence from X-ray images of DNA made by Rosalind Franklin and Maurice Wilkins.
Science is often a team effort: new models are built from other scientists' data.
They used evidence from X-ray images of DNA made by Rosalind Franklin and Maurice Wilkins.
Science is often a team effort: new models are built from other scientists' data.
Species
A species is a group of organisms that can breed together to produce fertile offspring.
Different species have different genes, so they have different characteristics. A horse and a donkey can breed, but their offspring (a mule) is infertile, so horses and donkeys are separate species.
Different species have different genes, so they have different characteristics. A horse and a donkey can breed, but their offspring (a mule) is infertile, so horses and donkeys are separate species.
A human sperm cell has 23 chromosomes. How many chromosomes does a fertilised egg have, and why?
- The egg cell also has 23 chromosomes.
- At fertilisation, the two nuclei join.
- 23 + 23
Answer: 46 chromosomes: 23 from each parent.
Chromosomes in the nucleus are made of DNA. A gene is a section of DNA. Human body cells have 46 chromosomes (23 pairs); gametes have 23. Watson and Crick modelled the DNA double helix in 1953 using Franklin's and Wilkins's X-ray data.
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