Chromosomes, DNA & Mitosis

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CognitoDNA, genes and chromosomes

CognitoMitosis and the cell cycle

Key facts & methods

DNA and chromosomes

DNA is a polymer made of nucleotides. It is a double helix.
Chromosomes: long molecules of DNA. Human body cells have 23 pairs (46 total).
Gene: a section of DNA that codes for a particular protein (determines a characteristic).
Allele: different versions of the same gene (e.g. blue eye vs brown eye alleles).
Genome: the entire genetic information of an organism.
Most body cells are diploid (2 copies of each chromosome). Gametes are haploid (1 copy).

The cell cycle and mitosis

Cell cycle: growth (G1) → DNA replication (S phase) → growth (G2) → mitosis (M) → cell division.
Mitosis: cell division producing two genetically identical daughter cells.
Used for: growth, repair, asexual reproduction.
Before mitosis: DNA is copied so each cell gets a complete set of chromosomes.
Result: 2 daughter cells, same chromosome number as parent (diploid → diploid).
Uncontrolled mitosis = cancer (cells divide without regulation).

Meiosis (contrast with mitosis)

Meiosis: cell division producing 4 genetically different daughter cells.
Occurs in reproductive organs to produce gametes (sperm, eggs, pollen).
Chromosome number is halved: diploid (46) → haploid (23).
Crossing over during meiosis creates genetic variation.
Fertilisation restores diploid number: 23 + 23 = 46.

DNA replication and protein synthesis (Higher)

DNA is replicated before cell division — each strand acts as a template.
Complementary base pairing: A-T, C-G.
Each daughter cell receives one original strand and one new strand (semi-conservative replication).
Genes code for proteins. Each sequence of 3 bases (codon) codes for one amino acid.
Mutations: random changes in DNA base sequence. Can alter protein structure.

Exam questions — 5 questions · 12 marks · AQA 8461 style

1 markMitosis productAQA 8461 style

How many cells are produced by mitosis and what is their genetic relationship to the parent cell?

A Four genetically different cells

B Two genetically identical cells

C Two genetically different cells

D Four genetically identical cells

1 markChromosome numberAQA 8461 style

A human body cell has 46 chromosomes. How many chromosomes does a human egg cell contain?

A 46

B 92

C 23

D 22

3 marksCell cycleAQA 8461 P2 style

Describe the stages of the cell cycle that occur before mitosis takes place. (3 marks)

Hint: Think about what a cell needs to do to prepare for division: grow, duplicate organelles, copy its DNA.

+30 XP

4 marksMeiosis vs mitosisAQA 8461 P2 style

Compare mitosis and meiosis in terms of: (a) the number of daughter cells produced, (b) whether they are genetically identical, (c) where each process occurs in the human body. (4 marks)

Hint: Three paired comparisons. Each comparison earns a mark. Be specific about location for meiosis.

+40 XP

1 markRole of mitosisAQA 8461 style

Which of the following is NOT a role of mitosis in humans?

A Growth of the organism

B Repair of damaged tissue

C Production of gametes

D Replacement of worn-out cells

Chromosomes, DNA & Mitosis

DNA and chromosomes

The cell cycle and mitosis

Meiosis (contrast with mitosis)

DNA replication and protein synthesis (Higher)

Module complete! 🎉