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Biology A level - Genetics of Living Systems Cheat Sheet by

This cheat sheet is based on the OCR A Gateway spec, Chapter 19 of module 6. This will be part of paper 2. Specification reference - 6.1.1

Gene mutations

Substi­tution / point mutation
One base switches place with another. This does not always lead to a change because DNA code is degene­rate. (A change in the third base is the least likely to lead to change.)
Base deleted. Results in frame shift - every codon after deletion is changed, so big change in protein synthesis.
Base inserted. Leads to a frame shift.


Physical mutagens
e.g. ionising radiations
Break one/both DNA strands, mutations can occur in the process.
Chemical mutagens
e.g. deamin­ating agents
Chemically alter based in DNA (e.g. cytosine to uracil).
Biological agents
e.g. alkylating agents
Methyl­/ethyl group added to base --> incorrect pairing.
e.g. base analogs
Incorp­orate in DNA in place of usual base.
e.g. viruses
Viral DNA inserted in genome.
Mutagens are factors that increase the rate of mutations.

Chromosome mutations

Sections of chromo­somes break off.
Section gets duplic­ated.
One section of a chromosome breaks off and joins to non-ho­mol­ogous chromo­some.
Section breaks off, reverses and joins back on.

Chromatin remode­lling

Tightly wound DNA around histones.
Loosely wound DNA. RNA polymerase can only access genes when DNA is euroch­rom­atin.
Chromatin is a DNA-pr­otein complex. Also note the dark green boxes will be related to transc­rip­tional control.

Euroch­romatin v. Hetero­chr­omatin

Histone modifi­cation

Acetyl­ation / Phosph­ory­lation
Lowers the positive charge of histones, so DNA is looser and genes can be transc­ribed.
Makes histones more hydrop­hobic so DNA is wound tighter. Genes cannot be transc­ribed.
DNA is negatively charged and histones are positively charged, which is how they attract. Changing charges will change the attrac­tion.

Operons - Lac Operon

Lac Operon

An operon is a group of genes under control of the same regulatory mechanism --> switch genes in and off depending on enviro­nment.
Lac Operon --> digests lactose if glucose concen­tration is low in E. Coli.

In the presence of glucose, a regulatory genes produces repressor proteins which bind to the operator so RNA polymerase cannot bind and read genes.
In the presence of lactose, lactose binds to the repressor protein, changing its shape so it no longer binds to the operator and the structural genes that code for lactos­e-d­ige­sting enzymes can be transc­ribed.

cAMP acts as a secondary messenger. It speeds up the transc­rip­tional of lactose genes if glucose is low.

Post-t­ran­scr­ipt­ional control

RNA processing
Transc­rip­tional makes pre-mRNA and transforms it in mature mRNA. Once DNA has been transc­ribed, introns are removed from the pre-mRNA.
RNA editing
Change in base sequence to make different proteins (deletion, additi­on...).

Transl­ational control

Degrad­ation of mRNA
Higher resistance of mRNA = longer lasting in the cytoplasm so higher quantity of protein synthesis.
Inhibitory proteins
Prevent binding to ribosomes.
Initiation factors
Aid binding to ribosomes.
Protein kinases
Adds phosphate group to protein, changing tertiary structure and function. Many proteins activated by phosph­ory­lation, important for cell regula­tion.

Post-t­ran­sla­tional control

Addition of non-pr­otein group
Modifying amino acids + bonds
Folding / shortening of proteins
Modifi­cation by cAMP (e.g. Lac operon)

Homeobox gene

Regulatory genes that code for a homeod­omain protein which switches genes on and off.
Code for body plan - basic layout of animals, fungi and plants.
Highly conserved across each species.
180 base pairs long.

Hox genes

Homeobox gene only present in animals.
Codes for correct placement of body parts.
Found in gene clusters (4 in mammals with 39 Hox genes divided in 4 clustered in humans).

Body plans

Body plans
The way the body is arranged.
Segments in embryo from which individual vertebrae develop. Hox genes in the mouth area will code for mouthp­arts, in the thorax for arm placem­ent...
Radial e.g. in jellyfish
Bilateral e.g. humans
None e.g. sponges

Mitosis and apoptosis

Increase number of cells for growth
Programmed cell death to remove unwanted tissue (e.g. webbing between fingers in womb).




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