CB2 — Cells and Control

DNA structure

DNA (deoxyribonucleic acid) is the molecule that carries genetic information. Key structural features for Edexcel 1BI0:

• Double helix — two strands wound around each other, discovered by Watson and Crick (1953) using X-ray crystallography data from Franklin.
• Each strand is a polynucleotide — a chain of nucleotides.
• Each nucleotide consists of: deoxyribose sugar + phosphate group + one of four nitrogenous bases.
• Four bases: Adenine A, Thymine (T), Guanine (G), Cytosine C.
• Base pairing rules (complementary base pairing): A pairs with T (2 hydrogen bonds); G pairs with C (3 hydrogen bonds). This holds the two strands together.
• The two strands are antiparallel (run in opposite directions: 5'→3' and 3'→5').

A gene is a section of DNA that codes for the production of a specific protein (sequence of amino acids). The genome is the complete set of DNA in an organism.

The cell cycle and mitosis

The cell cycle has three main stages:

1. Interphase (longest phase): cell grows, organelles replicate, DNA is replicated (S phase). Cell approximately doubles in size.
2. Mitosis: nucleus divides (PMAT stages).
3. Cytokinesis: cytoplasm divides; two daughter cells formed.

Mitosis stages (PMAT):

Result of mitosis: two genetically identical daughter cells, each with the same chromosome number as the parent (diploid, 2n). In humans: 46 chromosomes (23 pairs).

Functions of mitosis: growth, repair/replacement of damaged cells, asexual reproduction.

Cancer

Cancer results from uncontrolled cell division due to mutations in genes that regulate the cell cycle. Tumours can be:

• Benign: localised, cells don't invade other tissues.
• Malignant: cells break away and spread via blood/lymph to form secondary tumours (metastasis).

Risk factors: ionising radiation, UV radiation, carcinogens (tobacco smoke), viral infection (HPV), genetic predisposition.

Stem cells (CB2 detail)

Stem cells divide by mitosis. After division, daughter cells can remain as stem cells (self-renewal) or differentiate into specialised cells.

Differentiation: the process by which a cell develops into a more specialised type. It involves genes being selectively switched on/off — not all genes are expressed in every cell. Once specialised, most animal cells cannot revert (unlike plant cells).

Plant meristems: regions of unspecialised cells in shoot tips and root tips. Plant stem cells. Totipotent — can differentiate into any plant cell type. Used in tissue culture (micropropagation) to clone plants.

Therapeutic cloning: creating an embryo genetically identical to the patient → extract embryonic stem cells → use to grow replacement tissues without rejection risk. Ethically controversial.

Bone marrow transplant: donor stem cells (haematopoietic) replace patient's blood cell-producing cells — used for leukaemia treatment.

Growth and percentile charts

Growth is measured as increase in height/mass/head circumference over time. Percentile charts show how a child's growth compares to the population. A child on the 50th percentile is exactly average. Growth follows a characteristic pattern: rapid in infancy, slow in childhood, rapid again in puberty (growth spurt).

Common exam mistakes (CB2, Edexcel Paper 1)

1. Describing mitosis as producing "haploid" cells — it produces diploid (same as parent). Meiosis produces haploid.
2. Confusing stages of mitosis — Anaphase is when chromatids separate (pulled to poles).
3. Saying cancer cells divide "faster" — they divide uncontrollably (no response to normal stop signals).
4. Stating all stem cells are embryonic — adult stem cells exist in bone marrow, liver, skin.
5. Confusing base pairing: A-T and G-C only (not A-G or T-C).