Chromosomes, Genes and ProteinsCambridge IGCSE Biology: Revision notes
Section 1
What Are Chromosomes, Genes and Alleles?
Chromosomes are made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene.
In humans, biological sex is inherited via the X and Y chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
Section 2
How Does a Gene's Base Sequence Determine a Protein?
The sequence of bases in a gene determines the sequence of amino acids used to build a specific protein. Since different genes have different base sequences, they code for proteins with different amino acid sequences — and different amino acid sequences fold into different shapes. A protein's shape determines its function, for example the shape of an enzyme's active site or an antibody's binding site.
Link the chain of logic: base sequence → amino acid sequence → protein shape → protein function. Exam answers often need this whole chain, not just one link.
Section 3
How Is a Protein Made from a Gene?
- The gene coding for the protein remains in the nucleus
- Messenger RNA (mRNA) is made in the nucleus as a copy of the gene
- mRNA molecules move from the nucleus into the cytoplasm
- The mRNA passes through a ribosome
- The ribosome assembles amino acids into a protein molecule
- The specific sequence of amino acids is determined by the sequence of bases in the mRNA
Section 4
Why Don't All Body Cells Look and Behave the Same?
DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers, and receptors for neurotransmitters. Although most body cells contain the same complete set of genes, only some of those genes are expressed (switched on and used) in any particular cell — a cell only makes the specific proteins it needs for its function. This is why cells with identical DNA, such as a muscle cell and a nerve cell, can look and behave completely differently.
A red blood cell and a white blood cell both contain the same genes, but the red blood cell expresses genes for making haemoglobin while the white blood cell expresses genes for making antibodies.
Must Know
- Chromosomes are made of DNA and carry genes
- A gene is a length of DNA coding for a protein; an allele is an alternative version of a gene
- Sex is determined by X and Y chromosomes (XX = female, XY = male)
- The base sequence of a gene determines the amino acid sequence of its protein, which determines the protein's shape and function
- Protein synthesis: gene stays in the nucleus → mRNA copy made → mRNA moves to cytoplasm → passes through a ribosome → amino acids assembled into protein
- Most body cells contain the same genes, but only some genes are expressed in each cell type
That's the notes covered.
Carry on to the next subtopic.