Genes, Chromosomes and DNA

In I/GCSE Biology, please remember that:

• In normal human cells there are only 23 pairs of chromosomes.
• They consist of long, coiled molecules of DNA (Deoxyribonucleic acid)
• A gene is a small section of DNA which codes for a particular inherited characteristic e.g. blue eyes
• Each gene codes for a particular combination of amino acids which make a specific protein
• The DNA molecule consists of two strands which are coiled to form a double helix
• Each person has unique DNA (apart from identical twins who have the same DNA)
• A way of identifying each individual is using DNA fingerprinting.
• Look at the diagram of gene below !!

Mitosis

• In body cells the chromosomes are normally found in pairs
• Body cells divide by mitosis to produce additional cells during growth or to replace other cells.
• Mitosis also occurs during asexual reproduction.
• Before each cell divides, a copy of each chromosome is made so the new cell has exactly the same genetic information.
• This means that the cells of asexually produced offspring have the same genes as the parents.

Meiosis

• It's important in I/GCSE Biology,
• Body cells have two sets of genetic information; sex cells (gametes) have only one set.
• Cells in reproductive organs (testes and ovaries in humans) divide to form gametes
• This is done by the process of meiosis
• When a cell divides to form gametes:

1. Copies of the chromosomes are made
2. Then the cell divides twice to form four gametes, each with a single set of chromosomes

• Here's the diagram showing the difference of mitosis and meiosis in I/GCSE Biology!

When we come to fertilisation in I/GCSE Biology,

Remember that:

• When gametes join at fertilisation, one chromosome comes from each parent and a single body cell with new pairs of chromosomes is formed.
• This then divides repeatedly by mitosis to form a new individual.
• This causes variation between people and animals.
• It's important in I/GCSE Biology that sexual reproduction gives rise to variation because each pair of alleles comes from each parent.

References:

1. "B2: Cell Division and Inheritance". Document of I/GCSE, https://getrevising.co.uk/resources/b2_cell_division_and_inheritance

2. "Mitosis and Meiosis Diagram", https://o.quizlet.com/po-byCS8xfaKpxuoLKwWgA_b.png

3. "AC: What is a Chromosome? What is a Gene?", http://4.bp.blogspot.com/-097gP2W3GTM/UyPv0PafztI/AAAAAAAACvA/nE_buj36SxQ/s1600/gene+chromosome.png

And we're done with this topic! Well Done!

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Drafted by Alyssa (Biology)