Cell Division & Protein Synthesis

The cell cycle describes the stages a cell goes through from one division to the next.

The Cell Cycle - Three Main Phases

1. Interphase (longest phase, ~90% of cycle):
   • G1 phase: cell grows, organelles replicate, proteins synthesised
   • S phase: DNA is replicated (each chromosome becomes two sister chromatids joined at the centromere)
   • G2 phase: cell continues to grow, prepares for division
2. Nuclear division: mitosis OR meiosis
3. Cytokinesis: cytoplasm divides, producing separate daughter cells

Mitosis - PMAT

Produces 2 genetically identical diploid cells. Purpose: growth, repair, asexual reproduction.

Phase	What Happens
Prophase	Chromatin condenses into visible chromosomes. Nuclear membrane breaks down. Spindle fibres form.
Metaphase	Chromosomes line up at the equator (middle) of the cell, attached to spindle fibres at centromeres.
Anaphase	Centromeres split. Sister chromatids are pulled to opposite poles by shortening spindle fibres.
Telophase	Nuclear membranes reform around each set of chromosomes. Chromosomes decondense back to chromatin.

Meiosis

Produces 4 genetically different haploid cells (gametes). Involves two divisions. Creates genetic variation through independent assortment of chromosomes.

Haploid and Diploid

Diploid (2n): full set of chromosomes in pairs (e.g. 46 in humans). Found in body cells.
Haploid (n): half the chromosome number (e.g. 23 in humans). Found in gametes (sperm and egg).
At fertilisation, two haploid gametes fuse → diploid zygote.

DNA Replication

Occurs during S phase of interphase, facilitated by enzymes:

1. Helicase unwinds and separates the double-stranded DNA
2. Each single strand acts as a template for a new complementary strand
3. DNA polymerase adds free nucleotides to each template strand following base pairing rules (A-T, C-G)
4. Two identical DNA molecules are produced - each containing one original and one new strand (semi-conservative replication)

The Central Dogma: DNA → mRNA → Protein

Genetic information stored in DNA is used to make proteins via two steps:

1. Transcription (in the nucleus)

• DNA double helix unwinds at the gene to be expressed
• RNA polymerase reads one strand of DNA (template strand) and builds a complementary mRNA strand
• Base pairing: A→U, T→A, C→G, G→C (note: RNA uses uracil (U) instead of thymine)
• mRNA exits the nucleus through nuclear pores

2. Translation (at ribosomes in the cytoplasm)

• mRNA attaches to a ribosome
• The ribosome reads the mRNA in groups of 3 bases called codons
• tRNA molecules carry specific amino acids to the ribosome. Each tRNA has an anticodon that is complementary to the mRNA codon
• Amino acids are joined by peptide bonds in the correct sequence
• The polypeptide chain folds into a functional protein

The Genetic Code (HL)

The genetic code is the set of rules by which codons (3-base sequences) specify amino acids. It is universal (same in all organisms), degenerate (several codons can code for the same amino acid), and non-overlapping.

Roles of RNA Types

• mRNA: carries the genetic message from DNA to the ribosome
• tRNA: carries amino acids to the ribosome; has an anticodon that matches the mRNA codon
• rRNA: structural component of ribosomes

Mutations

Point mutations (gene mutations): changes in a single nucleotide during DNA replication:

• Substitution: one base replaced by another (may change one amino acid)
• Insertion: an extra base added → frameshift (all downstream codons changed)
• Deletion: a base removed → frameshift

Chromosomal mutations: larger-scale changes affecting whole genes or chromosomes:

• Deletion: part of chromosome lost
• Duplication: section copied
• Inversion: section reversed
• Translocation: section moved to another chromosome

Cancer

Cancer results from uncontrolled cell division caused by mutations in genes that regulate the cell cycle. A mass of abnormal cells forms a tumour.

• Benign tumours: localised, non-invasive, do not spread
• Malignant tumours: invasive, can spread to other parts of the body (metastasis)

Factors contributing to cancer: UV radiation, cigarette smoke, certain viruses (HPV), chemical carcinogens, genetic predisposition. Strategies: early detection, surgery, chemotherapy, radiotherapy, vaccination (HPV), lifestyle changes.