This document helps inform students how to understand Biology content for the GCSE course, according to the AQA specification. It talks about mitosis, specialisation, differentiation, stem cells and growth and development.
1) To produce new cells
- For growth
- For replacement of old cells
2) To reproduce
- For asexual reproduction
- To produce gametes for sexual reproduction like sperm and egg cells
The cell cycle
Body cells divide in a series of stages known as the cell cycle. Cell division is a process by which a
parent cell divides into two or more daughter cells. There are two main types of nuclear cell division
– Mitosis (for growth and cell replacement) and Meiosis (makes gametes). 90% of the cell of the
cycle is NOT when the cell division occurs. First of all there is STAGE 1, where there is preparation or
nuclear division and later on cell division. Mitosis and meiosis fall under stage 2, the nucleus divides.
Followed by stage 3, is when the cytoplasm is divided (cytokinesis).
Interphase (Stage 1)
Before cell division can occur, there must be growth and DNA replication(doubling the number of
chromosomes) and this happens in the interphase. New organelles are formed e.g., mitochondria
and ribosomes.
Growth 1 -> Cell prepares for replication of DNA. All necessary enzymes are created.
Synthesis -> DNA is replicated.
Growth 2 -> Cell organelles replicate so there are enough for each daughter cells.
Mitosis -> Chromosomes divide evenly between two new daughter cells.
Mitosis (Stage 2)
Mitosis produces two GENETICALLY identical daughter cells. Both the parent and daughter cells
have the same number of chromosomes, which also means they are all diploid, which mean that
, they have 2 complete sets of chromosomes. Red blood cells, sperm and egg cells are not diploid.
Mitosis is used for – growth, repair and in asexual reproduction.
Mitosis is the splitting of a cell to produce two genetically identical cells. Both cells have the diploid
number of chromosomes (The normal number of chromosomes). It occurs when a living thing grows,
repairs and reproduce asexually.
Chromosomes
A chromosome is made up of coiled DNA. A gene is a small section of DNA with a particular code
which will code for a particular protein. DNA is found in the nucleus. It carries genetic code that
determines characteristics of a living organism.
The human karyotype
Humans have 23 pairs of chromosomes. This means that there are 46 chromosomes in total. Each
pair has come from both the egg and sperm (mother and father). We can tell if a karyotype is a male
if the 23rd chromosome is in a XY formation and female if the 23 rd chromosome is XX. People with
down syndrome has a different number of chromosomes in their 21 st chromosome e.g. 3.
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