Chromosomes: MDCAT Biology notes
Chromosomes MDCAT notes: chromatin, histones, nucleosomes, centromere, telomeres, karyotype, chromosome numbers and non-disjunction syndromes.
Discovery and nature
Chromosomes were first observed by Walther Flemming. The word means "coloured body": the nucleus takes up basic dyes because it contains chromatin. Between divisions, the chromosomes stay loosely coiled as chromatin threads. At division they condense into visible chromosomes. A chromosome is one extremely long DNA molecule folded tightly with proteins. Its main function is the storage and transmission of genetic information.
Chemical composition and packing
- Chromosomes are made of DNA and proteins, making them nucleoproteins, with a little RNA.
- Histones are positively charged proteins, rich in the basic amino acids arginine and lysine. They bind the negatively charged DNA, and they are what arrange DNA into chromosomes.
- DNA wraps around a core of eight histones to form a nucleosome. Nucleosomes look like "beads on a string" and are the scaffold for higher-order coiling of chromatin.
- Nucleoproteins pass hereditary information to the next generation.
Structure of a chromosome
- After S phase, a duplicated chromosome has two sister chromatids. Each chromatid is one double-helical DNA molecule, so the chromosome holds two identical DNA molecules.
- Centromere (primary constriction): the narrowed region that holds the sister chromatids together. The kinetochore on it is where spindle fibres attach.
- Telomeres: the terminal ends. They protect chromosomes from damage and from sticking together.
- Some chromosomes have a secondary constriction, the nucleolar organizer, and a knob-like satellite.
Chromosome number
| Organism | Diploid number (2n) |
|---|---|
| Human | 46 (22 pairs of autosomes + 1 pair of sex chromosomes) |
| Drosophila (fruit fly) | 8 |
| Pisum sativum (pea) | 14 (7 homologous pairs) |
- Somatic cells and the zygote are diploid (2n). Gametes are haploid (n), with half the somatic number.
- All body cells are genetically identical except the reproductive cells (gametes), because meiosis reshuffles genes.
- Human females are XX and males are XY. So females differ in having two X chromosomes.
- A karyotype is the particular set of chromosomes an individual has, arranged by size and shape.
Chromosomes in the cell cycle
- DNA content doubles during interphase (S phase).
- In G2, tubulin for spindle microtubules is made, energy is stored for chromosome movement, and mitosis-specific proteins are produced.
- Sister chromatids separate and move to opposite poles in anaphase.
Non-disjunction syndromes
Non-disjunction means chromosomes fail to separate during meiosis. This gives gametes with an extra or missing chromosome.
| Syndrome | Karyotype | Key features |
|---|---|---|
| Down's (autosomal) | Trisomy 21 (47); arises from an egg with 24 chromosomes | Learning disability, characteristic facial features |
| Turner's | 44 + XO (45) | Female, short, webbed neck, low hairline, infertile |
| Klinefelter's | 44 + XXY (47) | Male, small testes, enlarged breasts, infertile |
| Jacob's | 44 + XYY (47) | Tall male |
Common MDCAT traps
- Spindle fibres attach at the centromere (kinetochore), not the telomere.
- Telomeres are the ends; the centromere is the constriction.
- Down's is the autosomal example. Turner's, Klinefelter's and Jacob's are sex-chromosome syndromes.
- Humans have 22 pairs of autosomes, not 23. The 23rd pair is the sex chromosomes.
- Histones are positive because of arginine and lysine, not alanine.
Quick revision
- Chromosome = DNA + histone proteins.
- Nucleosomes are DNA wrapped around histones.
- Turner's syndrome is XO.
- The human zygote is 2n = 46.
- Karyotype is an individual's chromosome set.