Chromosomes and the cell cycle
Chromosomes and the cell cycle
- Your DNA is huge — to fit in the nucleus it is wound up into chromosomes.
- Before a cell divides, all of that DNA must be copied perfectly.
- The whole repeating life of a cell is the cell cycle.
Practice
A chromosome is:
Chromosomes are DNA wound tightly around histone proteins, packing a huge length into the nucleus.
A replicated chromosome
- A chromosome is one long DNA molecule wound tightly around proteins called histones.
- After the DNA is copied (replication), the chromosome is two identical sister chromatids joined at the centromere.
- The tips are capped by telomeres, which protect the ends.
Practice
After replication, a chromosome consists of:
Replication makes an identical copy; the two sister chromatids stay joined at the centromere until anaphase.
The cell cycle
- Interphase — the longest part: grow (G₁), copy DNA (S phase), grow and prepare to divide (G₂).
- Mitosis — the nucleus divides into two identical nuclei.
- Cytokinesis — the rest of the cell splits into two daughter cells.
Practice
During which phase of interphase is the DNA copied?
DNA replication happens in the S (synthesis) phase, between G₁ and G₂.
Practice
Which is the longest part of the cell cycle?
Interphase (G₁, S, G₂) takes up most of the cycle; mitosis and cytokinesis are comparatively short.
Telomeres and the ends
- When DNA is copied, the machinery cannot reach the very end, so a little is lost each time.
- Telomeres are short repeats that carry no genes — they are shortened instead, so no important genes are lost.
Practice
Why does shortening telomeres protect a cell's genes?
The ends cannot be fully copied, so a little is lost; because telomeres hold no genes, the loss is harmless.
You've got it
Key idea
- chromosome = DNA wound on histones; after replication → two sister chromatids joined at the centromere
- cell cycle: interphase (G₁ → S = copy DNA → G₂) → mitosis → cytokinesis
- interphase is the longest part
- telomeres cap the ends, carry no genes, and protect against losing DNA in copying