Information Flow in the Cell

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:

Feature	DNA	RNA
Strands	Double	Single
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, 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 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.

The flow of information in a cell is DNA → mRNA → protein. DNA stays in the nucleus, so a working copy, messenger RNA, carries the message out to the ribosome.

Transcription (in the nucleus)

1. The DNA double helix unzips at the gene when hydrogen bonds between base pairs are broken.
2. RNA polymerase moves along the template (antisense) strand.
3. Free RNA nucleotides pair with the exposed bases: C with G, and A with U, because RNA has uracil instead of thymine.
4. The new mRNA strand is released, the DNA rezips, and the mRNA leaves through a nuclear pore.

Translation (at the ribosome)

1. The mRNA attaches to a ribosome, which reads it in codons - groups of three bases.
2. Each tRNA molecule carries one specific amino acid and has an anticodon that pairs with the matching codon.
3. The ribosome holds two codons at a time, and a peptide bond forms between the amino acids.
4. The ribosome moves on one codon, the empty tRNA leaves to collect another amino acid, and the chain grows.
5. At a stop codon the polypeptide is released and folds into its functional shape.

The genetic code

There are 64 codons for 20 amino acids, so the code is degenerate - several codons can code for the same amino acid. It is also universal, meaning the same codons mean the same amino acid in bacteria, plants and humans. That universality is exactly why genetic engineering works: a human insulin gene can be read correctly by a bacterium.

Common mistake

Codons are on mRNA, anticodons are on tRNA. If a question gives you the DNA template strand, transcribe it first, then read the codons.