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Strand 1: Organisation of Life

Information Flow in the Cell

DNA and RNA structure, DNA replication, protein synthesis, gene and chromosome mutations.

1. DNA, RNA & the Genetic Code

In brief:DNA is a double-stranded helix containing the genetic instructions for all living organisms. RNA carries this information to make proteins.

Chromosomes, DNA and Genes

Chromosomes are coiled threads of DNA wrapped around proteins (histones). Humans have 46 (23 pairs). A gene is a section of DNA that codes for a protein - only ~1% of DNA codes for proteins. The rest is non-coding DNA which contains regulatory sequences.

Structure of DNA

DNA (deoxyribonucleic acid) is a two-stranded double helix. Each strand is a polynucleotide made of repeating nucleotides. Each nucleotide = phosphate + deoxyribose sugar + nitrogenous base.

Four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

Complementary base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). Mnemonic: All The Genetic Code

The two strands are held together by weak hydrogen bonds and have a sugar-phosphate backbone.

Structure of RNA

RNA (ribonucleic acid) is single-stranded. Differences from DNA:

FeatureDNARNA
StrandsDoubleSingle
SugarDeoxyriboseRibose
BasesA, T, G, CA, U, G, C (Uracil replaces Thymine)

Three types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

Gene Expression (HL)

When a gene is 'expressed', the protein it codes for is produced. Different cells express different genes - a skin cell makes different proteins than a muscle cell.

DNA chemical structure

DNA - double helix with base pairing

Wikimedia Commons (public domain / CC)

DNA nucleotide

Nucleotide - phosphate, sugar, base

Wikimedia Commons (public domain / CC)

DNA replication

Semi-conservative DNA replication at the replication fork

Wikimedia Commons (CC)

Key Points

  • 1DNA is a double-stranded helix of nucleotides (phosphate + deoxyribose + base)
  • 2Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
  • 3A gene is a section of DNA that codes for a protein (~1% of DNA)
  • 4RNA is single-stranded, uses ribose sugar, and uracil replaces thymine
  • 5Three types of RNA: mRNA, tRNA, rRNA
  • 6Gene expression: the activation of a gene to produce a protein

Learning Outcomes

  • Describe the basic structure and function of a DNA and RNA nucleotide
  • Relate genes, proteins and traits in organisms; outline the concept of the genetic code
  • Describe the structure of a chromosome and the role of a gene

2. DNA Replication & Mutations

In brief:Before cell division, DNA must replicate. Errors in replication or during division can cause mutations leading to genetic disorders.

DNA Replication

DNA replication occurs in the nucleus during interphase, before mitosis.

  1. The DNA double helix unwinds
  2. DNA helicase breaks the hydrogen bonds between base pairs, separating the two strands
  3. Free nucleotides enter through nuclear pores and attach to complementary bases on each exposed strand
  4. DNA polymerase joins the nucleotides, forming the sugar-phosphate backbone
  5. Each new double strand = half original + half new DNA
  6. Two identical DNA molecules result - each a double helix

Gene Mutations

A gene mutation is a permanent change in the nucleotide sequence of a gene. Types:

  • Substitution: one base replaced by another
  • Insertion: an extra base added
  • Deletion: a base removed

Example: Sickle-cell anaemia - a single base substitution in the haemoglobin gene changes protein shape.

Chromosome Mutations

Changes in the number or structure of chromosomes during cell division. Example: Down syndrome - an extra copy of chromosome 21 (trisomy 21), giving 47 chromosomes total.

Mutagens

External factors causing mutations: UV radiation, cigarette smoke, viruses (e.g. HPV), and chemical carcinogens.

Key Points

  • 1DNA replication: helicase unwinds, polymerase builds new strands with complementary bases
  • 2Each new DNA molecule is half old + half new
  • 3Gene mutations: substitution, insertion or deletion of bases
  • 4Sickle-cell anaemia: single base substitution in haemoglobin gene
  • 5Chromosome mutations: changes in number/structure (e.g. Down syndrome = trisomy 21)
  • 6Mutagens: UV light, cigarette smoke, viruses cause DNA damage

Learning Outcomes

  • Model how DNA is replicated and the flow of information through mRNA to protein
  • Model the processes of transcription and translation; relate the structure of tRNA to the codon nature of the genetic code
  • Model how point and chromosomal mutations occur, making reference to known examples of both