5.1.15 · The Mitotic Cell Cycle
Chromosome Structure — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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5.1.1 Chromosome Structure — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of eukaryotic chromosomes, including chromatids, centromeres, histones and chromatin, and distinguish chromosomes from chromatids.
- Gene expression
- Sister chromatids
- After anaphase begins
- Importance
- Characteristic number
- Homologous pairs
Part 1 of 7DNA packaging in the nucleus
Packaging
- Packaging a human nucleus a few micrometres across holds about two metres of DNA, so eukaryotic DNA is packaged with proteins.
Chromatin
- Chromatin the complex of DNA and protein.
- In interphase it is dispersed, so genes can be transcribed and DNA replicated.
- Before division it condenses and coils into chromosomes, visible with a light microscope after staining.
Histones
- Histones large, positively charged globular proteins that organise and condense DNA so it fits in the nucleus.
Part 2 of 7Levels of DNA packing
1
DNA is negatively charged because of its phosphate groups, so it wraps around positively charged histones.
2
About 147 base pairs of DNA wrap twice around a core of eight histone molecules to form a nucleosome ("beads on a string").
3
A linker histone pulls the nucleosomes together into a fibre about 30 nm across.
4
The fibre forms loops on a protein scaffold, which fold and supercoil into the condensed metaphase chromosome.
Benefits
- Benefits packing shortens the DNA, protects it from damage and lets chromosomes move without tangling or breaking.
Gene expression
- Gene expression genes in tightly packed regions are not readily transcribed, so packing helps regulate expression.
Part 3 of 7Features of a replicated chromosome
1Sister chromatids
- S phase replicates the DNA, so each chromosome has two identical chromatids made by semi-conservative replication.
- Each chromatid contains one very long DNA molecule with its proteins, made up of a series of genes.
2Centromere
- Centromere the constricted region holding sister chromatids together, with specific DNA sequences and a kinetochore where spindle microtubules attach.
- Its position describes chromosome shape (metacentric if near the middle, acrocentric if near the end).
3Telomeres
- Telomeres non-coding repeat sequences at the ends that protect the chromosome.
4Arms
- Arms the short and long arms lie on either side of the centromere.
5After anaphase begins
- After anaphase begins each separated chromatid counts as a chromosome, so the cell temporarily has twice the chromosome number.
Part 4 of 7Chromatid, sister chromatids and chromosome
| Term | Meaning |
|---|---|
| Chromatin | DNA plus histone proteins; uncondensed in interphase |
| Chromosome | A single condensed DNA molecule with proteins, or a replicated pair of sister chromatids |
| Chromatid | One of the two identical copies in a replicated chromosome |
| Homologous pair | Two chromosomes with the same genes, one maternal and one paternal |
- Importance
- one chromatid goes into each daughter cell in mitosis, so the daughters are genetically identical.
Part 5 of 7Chromosome number and homologous pairs
Chromosome number and homologous pairs
Characteristic number
- Characteristic number each species has a fixed number of chromosomes in body cells.
- (humans 46, fruit flies 8, potatoes 48)
Homologous pairs
- Homologous pairs in diploid organisms chromosomes occur in pairs, one from each parent, of the same length and centromere position with the same genes at the same loci.
- (alleles may differ)
Human chromosomes
- 22 pairs of autosomes and one pair of sex chromosomes.
Karyotype
- Karyotype a display of all the chromosomes of a cell in pairs in order of size, used to detect abnormal numbers such as the extra chromosome 21 in Down syndrome.
Counting
- Counting the chromosome number equals the number of centromeres, not chromatids.
Part 6 of 7Prokaryote comparison and seeing chromosomes
1Prokaryotes
- Prokaryotes they have a single circular, supercoiled DNA molecule with no histones and no nucleus, and may have small plasmids.
2Organelles
- Organelles mitochondria and chloroplasts have similar circular DNA, which is evidence for endosymbiosis.
3Visibility
- Visibility chromosomes are visible as discrete structures only when condensed,
- as dark rods in a root tip squash stained with acetic orcein or toluidine blue.
4Metaphase
- Metaphase chromosomes are most condensed, so cells are photographed at this stage for karyotypes.
Part 7 of 7More from the specification
1
Eukaryotic Cell Cycle and Mitosis
- The cell cycle comprises interphase (G1, S,
- and G2 phases) followed by mitosis and cytokinesis, a process where replicated chromosomes segregate into two genetically identical daughter nuclei for growth and tissue repair.
2
Uncontrolled Cell Division and Tumour Formation
- When mutations disrupt cell cycle checkpoints, mutated cells divide uncontrollably through repeated mitosis,
- forming abnormal masses of tissue called tumours.
3
Benign vs. Malignant Tumours and Cancer
- Benign tumours remain localized within a fibrous capsule,
- whereas malignant tumours invade surrounding tissues and undergo metastasis via blood vessels or lymphatics,
- causing cancer.
4
Stem Cells in Adult Tissues
- In adult organisms, populations of multipotent stem cells retain the ability to divide by mitosis and differentiate, replacing damaged cells and maintaining tissue homeostasis.
5
Eukaryotic Cell Cycle and Mitosis
- The cell cycle comprises interphase (G1, S,
- and G2 phases) followed by mitosis and cytokinesis, a process where replicated chromosomes segregate into two genetically identical daughter nuclei for growth and tissue repair.
6
Uncontrolled Cell Division and Tumour Formation
- When mutations disrupt cell cycle checkpoints, mutated cells divide uncontrollably through repeated mitosis,
- forming abnormal masses of tissue called tumours.
7
Benign vs. Malignant Tumours and Cancer
- Benign tumours remain localized within a fibrous capsule,
- whereas malignant tumours invade surrounding tissues and undergo metastasis via blood vessels or lymphatics,
- causing cancer.
8
Stem Cells in Adult Tissues
- In adult organisms, populations of multipotent stem cells retain the ability to divide by mitosis and differentiate, replacing damaged cells and maintaining tissue homeostasis.
9
Multicellular organism
- An organism made of more than one cell.
10
Asexual reproduction
- Reproduction from one parent giving genetically identical offspring.
11
Malignant tumour
- A rapidly growing tumour that invades tissues and may metastasise.
12
Tumour
- A lump of cells formed by uncontrolled division.
13
Cancer
- Disease in which cells divide uncontrollably and spread.
14
Growth
- A permanent increase in size and dry mass.
15
Tissue
- A group of cells of similar structure that perform the same function.
Key terms
| Term | Meaning |
|---|---|
| Chromatin | Complex of DNA and histone proteins in the nucleus. |
| Histone | Basic protein around which DNA is wrapped. |
| Nucleosome | Unit of chromatin: about 147 base pairs of DNA wrapped around eight histones. |
| Chromatid | One of two identical copies of a replicated chromosome. |
| Centromere | Constricted region that joins sister chromatids and binds spindle fibres. |
| Homologous chromosomes | Pair of chromosomes with the same genes at the same loci, one from each parent. |
| Karyotype | Display of the chromosomes of a cell in pairs. |
| Kinetochore | Protein structure at the centromere where spindle microtubules attach. |
Avoid these mistakes
1
Saying the two chromatids of a chromosome are homologous — they are identical copies, while homologous chromosomes are a maternal and paternal pair.
2
Stating that DNA is positively charged and histones negative — DNA is negative and histones are positive.
3
Counting chromatids as chromosomes at metaphase — count centromeres.
4
Saying prokaryotes have histones and linear chromosomes.
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