Summary notes for AQA A-level Biology gene expression topic. Includes clear information on mutations, stem cells, differentiation, control of transcription, RNAi, epigenetics, cancer and bioinformatics. Summarised from class notes and the official course textbook. From an A* student.
Gene mutations
A gene mutation is a change in the base sequence of DNA
A genome all the genes/genetic material present in a cell or organism
A proteome is all of the potential proteins that a cells genome could produce
Substitution
Where a nucleotide is replaced by another that has a different base
Different consequences:
1. Non sense: formation of a stop codon which leads to the production of the polypeptide ending
prematurely as the stop codon doesn’t code for an amino acid
2. Mis sense: formation of a codon for a different amino acid so the polypeptide differs by one
amino acid. This means it may differ in shape and not function properly
3. Silent mutation: formation of a different codon but produces the same amino acid as before
due to degenerate nature of DNA
Deletion
• Loss of a nucleotide base
• Frame shift as the reading frame has been shifted to the left
• Gene read in the wrong three base groups
• Most triplets and amino acids are changed- different protein produced
• Deleted base at the end of the sequence will have a smaller impact
Addition
• Extra base added
• Similar effect as a deletion as there is also a frame shift, but to the right this time
Duplication
• One or more bases repeated
• Frame shift to the right
Inversion
• Group of bases become separated and rejoin in the same place but in the inverse order
• Affects the amino acid sequence
Translocation
• A sequence of bases is moved from one location in the genome to another
• Can be within the same chromosome or to a different chromosome
• Significant affects on gene expression
• Reciprocal translocation = where group of bases are swapped between chromosomes
• Non reciprocal translocation = where group of bases inserted into one chromosome from
another
, Causes
Spontaneously during DNA replication
• Random but with a predictable frequency which gives a base mutation rate
• This rate depends on the lifespan of organisms
• Useful to evolutionary biologists to compare differences between organisms and find out last
common ancestor
Base mutation rate can be increased by mutagenic agents:
• Ionising radiation such as X rays which disrupt DNA structure
• Chemicals such as nitrogen dioxide which interfere with transcription / disrupt DNA structure
These agents increase the rate of mutations by:
• Acting as a base
• Altering bases
• Changing structure of DNA
Impacts of mutations
• Mutations occur naturally at random and can be inherited if they occur during meiosis and the
formation of gametes.
• Mutations can also occur in somatic cells but these changes aren’t inherited.
• Some mutations cause genetic disorders or increase the risk of cancer
• Produce genetic diversity and new alleles necessary for natural selection and speciation
• Change in amino acid sequence of a protein may change tertiary structure of the protein. This
may change the active site shape and so the substrate cant bind
Stem cells and totipotency
The process by which each cell develops into a specialised structure suited to the role that it will
carry out is known as cell differentiation
Cell differentiation
• Cells of a multicellular organisms are adapted in different ways to perform a different role
• All cells are derived from mitotic divisions of the zygote and they contain the same genes
• Although all cells have the same genes, only certain genes are expressed in one cell at one time
• Some genes are always expressed e.g. enzymes needed for respiration
Development of the zygote
• Develops from one fertilised egg - the nuclei of the sperm and egg fuse
• Zygote divides into a small ball of cells called a morula. These cells are totipotent and form the
embryo and the placenta
• The blastula then forms, containing the inner cell mass which forms the embryo and the outer
cell mass which forms the placenta
Differentiation
• Cells such as fertilised eggs, which can mature into any body cell, are known as totipotent cells
• Some genes in the cell are expressed and transcribed into mRNA
• The mRNA is translated into protein
• The proteins modify the cell structure and cell processes which makes the cell specialised
Stem cells
• Specialisation is irreversible in most animal cells
• A few cells called stem cells retain the ability to differentiate into other cells
• Stem cells are undifferentiated dividing cells in adult animal tissues
• They can divide to form an identical copy of themselves in a process called self-renewal
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