Cell Division: Chromosomes | A Level Biology | OCR, AQA, Edexcel

Our A-Level Biology Experts are here to help you ace A-Level Biology!

This week we are revising Cell Division: Chromosomes

A-Level Biology can be tough but fortunately we’ve made this tutorial to help you score the A* you need for questions on everything to do with Cell Division: Chromosomes.

A chromosome consists of lots of DNA molecules.
A DNA molecule has lots of genes.
A gene has lots of base pairs.
In order to fully and properly understand cell division, we first need to spend some time learning the structure of chromosomes.

Chromosomes are large structures of DNA and proteins. DNA in cells is wrapped around proteins called histones. This results in a structure called chromatin. Chromatin is then coiled into structures known as chromosomes.

Humans have 23 different chromosomes which exist in pairs. Humans are diploid organisms, meaning that we have two sets of each chromosome, making a grand total of 46 chromosomes in a single cell nucleus.

Organisms with only one chromosome of each type are called haploid organisms. Organisms with three copies of each chromosome are called triploid, and those with four copies of each chromosome are called tetraploid.

Each copy of a particular chromosome is called a chromatid. Each chromosome is made up of two copies – each one is called a chromatid. A chromatid has two arms which are held together by a specialised structure called a centromere. In most eukaryotic cells, two chromatids, called sister chromatids (which are both nearly identical copies of the same type of chromosome) are joined together at the centromere.

During the S phase of the cell cycle, the cell replicates the DNA. DNA replication results in two copies of each chromosome. Since we normally have 46, after S phase, the cell will have 92 chromosomes, making it tetraploid. The new copies of chromosomes are called homologous chromosomes. The chromosomes will be evenly split during mitosis, with each daughter cell having 46 chromosomes.

Telomeres protect the chromosomes during cell division. Telomeres are disposable sequences of repeated nucleotides at each end of a chromosome. With each successive chromosome replication, the chromosome will become shorter and shorter. The presence of telomeres mean that it will be the telomeres that get shortened during each replication and the genes in the chromosomes remain protected.

#ALevelChemistry #ALevelBiology #Biology #Chemistry #StudyMind