What is DNA, where are genes kept, and how do cells divide to grow and to make gametes?
Describe the structure and role of DNA, genes and chromosomes, and compare mitosis and meiosis as types of cell division
A focused answer to the O-Level Combined Science outcome on DNA and cell division. DNA, genes and chromosomes, the role of DNA in coding for proteins, and a comparison of mitosis for growth and meiosis for gametes.
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What this dot point is asking
SEAB wants you to describe DNA, genes and chromosomes and how they relate, to state that genes code for proteins, and to compare mitosis (for growth and repair) with meiosis (for making gametes). The chromosome-gene-DNA relationship and the mitosis-versus-meiosis comparison are reliable exam marks.
The answer
DNA, genes and chromosomes
The genetic material of a cell is DNA, found in the nucleus:
- DNA is a long molecule that carries coded instructions for the cell.
- A chromosome is a length of DNA coiled up; the nucleus of a human body cell contains 46 chromosomes (23 pairs).
- A gene is a short section of DNA on a chromosome that codes for a particular protein.
So a chromosome is made of DNA and carries many genes, and each gene is a section of that DNA.
What DNA does
Each gene carries the instructions to make one protein. Proteins (including enzymes) control the cell's chemistry and the organism's characteristics. By coding for proteins, genes determine inherited features such as eye colour. DNA can also be copied exactly before a cell divides, so the information is passed on.
Mitosis
Mitosis is cell division that produces two daughter cells that are genetically identical to the parent cell and to each other (each with the full set of chromosomes). It is used for:
- growth of the organism,
- repair of damaged tissue and replacement of worn-out cells,
- asexual reproduction.
Meiosis
Meiosis is cell division that produces four daughter cells that are genetically different from the parent and from each other, each with half the number of chromosomes. It is used to make gametes (sex cells: sperm and egg, or pollen and ovule). Halving the chromosome number means that when two gametes join at fertilisation, the full number is restored.
Comparing the two
- Mitosis: 2 daughter cells, identical, full chromosome number, for growth and repair.
- Meiosis: 4 daughter cells, different, half the chromosome number, for gametes.
Examples in context
Example 1. Healing a cut. When skin is damaged, the surrounding cells divide by mitosis to make identical new skin cells that fill the wound. This shows mitosis at work in repair, producing exact copies so the new tissue matches the old.
Example 2. Why siblings differ. Because meiosis produces genetically different gametes, and fertilisation combines a random sperm and egg, brothers and sisters inherit different mixtures of their parents' genes. This links meiosis directly to the variation we see within families.
Try this
Q1. State where DNA is found in a cell and what a gene codes for. [2 marks]
- Cue. DNA is found in the nucleus (coiled into chromosomes); a gene codes for a particular protein.
Q2. State two differences between mitosis and meiosis. [2 marks]
- Cue. Mitosis makes two identical cells with the full chromosome number; meiosis makes four different cells with half the chromosome number.
Q3. Explain why gametes must have half the number of chromosomes. [2 marks]
- Cue. So that when two gametes join at fertilisation, the full chromosome number is restored (and does not double each generation).
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksExplain the relationship between a chromosome, a gene and DNA, and state what a gene codes for.Show worked answer →
DNA is the molecule that carries the genetic information. It is found in the nucleus of the cell, coiled up into structures called chromosomes.
A chromosome is a long, coiled molecule of DNA. A gene is a short section of DNA on a chromosome.
So the order, from largest to smallest, is: chromosome (made of DNA) contains many genes, and each gene is a section of the DNA.
A gene codes for a particular protein (it carries the instructions for making one protein, which controls a characteristic).
Markers reward DNA being in the nucleus and coiled into chromosomes, a gene being a section of DNA on a chromosome, and a gene coding for a (specific) protein.
Original4 marksCompare mitosis and meiosis by stating, for each type of cell division, the number of daughter cells produced, whether they are genetically identical to the parent cell, and one place or purpose for which it is used in the body.Show worked answer →
Mitosis: produces two daughter cells. They are genetically identical to the parent cell (and to each other). It is used for growth, repair of tissues and replacing worn-out cells (and asexual reproduction).
Meiosis: produces four daughter cells. They are genetically different from the parent cell and have half the number of chromosomes. It is used to make gametes (sex cells: sperm and egg / pollen and ovule).
Markers reward two identical daughter cells for mitosis used in growth/repair, and four genetically different daughter cells with half the chromosomes for meiosis used to make gametes.
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