Cambridge (CIE) AS Your notes
Biology
Replication & Division of Nuclei &
Cells
Contents
Chromosome Structure
Mitosis
The Cell Cycle
The Role of Telomeres
The Role of Stem Cells
How Tumours Form
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Chromosome Structure
Your notes
Chromosome Structure
Chromosomes are made of one very long, condensed DNA molecule associated with
proteins (in eukaryotic cells)
The main proteins present are the large positively charged globular proteins called
histones, their role is to organise and condense the DNA tightly so that it fits into the
nucleus
The other proteins are enzymes used in copying and repairing the DNA
The tightly coiled combination of DNA and proteins is called chromatin – this is what
chromatids, and therefore chromosomes, are made of
Chromatin & Histone Proteins Diagram
DNA is coiled around histone proteins to make chromatin
During interphase (S phase) the DNA replicates to create two identical strands of DNA
called chromatids, joined together by a narrow region called the centromere
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
, The two chromatids that make up the double structure of a chromosome are known as
‘sister chromatids’
Your notes
It is important that the sister chromatids are identical (contain the same genes) because
this is key to cell division, as one chromatid goes into one daughter cell and one goes
into the other daughter cell during mitosis, ensuring the daughter cells are genetically
identical
Each chromatid is made up of one very long, condensed DNA molecule, which is made
up of a series of genes
The ends of the chromatids in chromosomes are ‘sealed’ with protective structures
called telomeres
Structure of a Chromosome Diagram
Simplified diagram of the structure of a chromosome
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
Biology
Replication & Division of Nuclei &
Cells
Contents
Chromosome Structure
Mitosis
The Cell Cycle
The Role of Telomeres
The Role of Stem Cells
How Tumours Form
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Chromosome Structure
Your notes
Chromosome Structure
Chromosomes are made of one very long, condensed DNA molecule associated with
proteins (in eukaryotic cells)
The main proteins present are the large positively charged globular proteins called
histones, their role is to organise and condense the DNA tightly so that it fits into the
nucleus
The other proteins are enzymes used in copying and repairing the DNA
The tightly coiled combination of DNA and proteins is called chromatin – this is what
chromatids, and therefore chromosomes, are made of
Chromatin & Histone Proteins Diagram
DNA is coiled around histone proteins to make chromatin
During interphase (S phase) the DNA replicates to create two identical strands of DNA
called chromatids, joined together by a narrow region called the centromere
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
, The two chromatids that make up the double structure of a chromosome are known as
‘sister chromatids’
Your notes
It is important that the sister chromatids are identical (contain the same genes) because
this is key to cell division, as one chromatid goes into one daughter cell and one goes
into the other daughter cell during mitosis, ensuring the daughter cells are genetically
identical
Each chromatid is made up of one very long, condensed DNA molecule, which is made
up of a series of genes
The ends of the chromatids in chromosomes are ‘sealed’ with protective structures
called telomeres
Structure of a Chromosome Diagram
Simplified diagram of the structure of a chromosome
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 3
