Biology Exam Revision Notes

The Nucleotide

• The nucleotide is the building block of DNA.
• A nucleotide molecule consists of:
  • Phosphate group
  • Deoxyribose sugar
  • Nitrogen-rich base

4 Nucleotides

• There are 4 different nucleotides, determined by the 4 types of bases:
  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)

DNA, Chromosomes & Genes

• Gene:
  • Sections of DNA arranged along the chromosome.
  • Typically, there are 200 – 2000 genes on one chromosome.
• Chromosome:
  • Long pieces of DNA wrapped around proteins.
  • DNA is a long molecule; for example, bacterial DNA can be 1000 times longer than the cell itself.
  • DNA is packaged into chromosomes.

Chromosome Number

• Human cells have 46 chromosomes.
  • Half (23) are inherited from the mother.
  • Half (23) are inherited from the father.
• Gametes (egg and sperm) contain 23 chromosomes.
• Chromosomes come in pairs (23 pairs).
  • One of each pair is from the mother.
  • One of each pair is from the father.

Types of Chromosomes

• Chromosomes come in 23 pairs.
  1. Sex Chromosomes
     • 2 chromosomes (X & Y)
     • Female: XX pair
     • Male: XY pair
  2. Autosomes
     • The other 44 chromosomes.
     • Grouped into 22 pairs.
     • Each pair is homologous (the same):
       • Same length
       • Same genes for particular characteristics at the same location along their length.

Homologous Chromosomes and Alleles

• Alleles:
  • Alternative forms of a gene.
  • Example: the gene for hair color.
    • Alleles include:
      • Allele for brown
      • Allele for black
      • Allele for blonde
      • Allele for red

Gene

• Section of DNA that codes for a polypeptide.
• Codes for a specific sequence of amino acids.
  • Example:
    • A T C code for amino acid X
    • G T A code for amino acid Y
    • T T A code for amino acid Z
    • X-Y-Z Polypeptide (sequence of amino acids)

Cell Division

• Two types of cell division:
  1. Mitosis
     • Makes identical somatic cells (diploid cells).
     • Used for growth and repair (e.g., making more muscle cells).
  2. Meiosis
     • Makes genetically unique sex cells (gametes) - sperm or egg cells (haploid cells).

Comparing Mitosis and Meiosis

• Mitosis
  • Clones body cells (diploid → diploid)
• Meiosis
  • Makes distinct gametes (diploid → haploid)

Mitosis vs. Meiosis Comparison

Punnett Squares

• Example:
  • Parent 1: Homozygous red flowers (alleles = RR)
  • Parent 2: Homozygous white flowers (alleles = rr)

• The genotypes of the offspring are: all Rr = all heterozygous
• The phenotypes of the offspring are: all Red, $4/4$ or $100\%$ red.

Huntington’s Disease Example

• Huntington’s disease is an inherited disorder that affects the nervous system and is caused by a dominant allele.
• Scenario: One parent is heterozygous for Huntington’s, and the other is homozygous normal.
  • Parent 1: Heterozygous for Huntington’s (alleles = Hh)
  • Parent 2: Homozygous normal (alleles = hh)