cellular control
The genetic code
The sequence of base triplets in DNA or mRNA that code for the sequence of amino acids in a polypeptide
triplet code
Each amino acid is coded for by a sequence of 3 DNA bases
degenerate code
Most amino acids are coded for by more than one codon
Nonoverlapping
each base is only read once
universal
The same codons code for the same amino acid in all living organisms
mutations
Changes in the sequence of DNA nucleotides
substitution
One nucleotides replaced with another
three types of substitution mutation
Silent mutation- Still codes for the same amino acid, No effect on protein
Missense mutation- Codes for a different amino acid, May alter protein
nonsense mutation- Codon changes to a stop codon
insertion
One or more nucleotide are added to the DNA sequence
Causes a frameshift mutation
changes every code on downstream, Alters amino acid sequence
deletion
One or more nucleotides are removed
Causes a frameshift mutation
Altera amino acid sequence, Results in a non-functional protein
possible effects of gene mutations
Antibiotic resistance
No effect on phenotype
Disrupt protein structure, Can lead to genetic disorders or increased risk of disease
causes of mutations
mutagen- Anything that causes a mutation
X-rays
High energy radiation
chemicals in cigarette smoke
UV light
how mutations affect protein structure
Primary- Change one or more amino acids (missense) Or introduce a stop codon (nonsense)
Secondary- Disrupt hydrogen bonding patterns, Prevent normal folding
Tertiary- Hydrogen bonds formed differently, Ionic bonds are lost or gained, disulphide bridges may be prevented, Hydrophobic interactions may be disrupted, changes in Proteins 3D shape alters or destroys its function
quaternary- Prevent correct assembly of subunits, Reduce protein stability
overall effects of mutations on protein structure
No effect- If mutation is silent or occurs in a non-critical region
Loss of function- Protein misfolds or active site is destroyed
Gain of function- Mutation may produce a new or altered function
Gene expression
The process by which the information stored in a gene is used to produce a function functional protein
steps in gene expression
1. Transcription- Gene is copied into mRNA
2. RNA processing- Primary mRNA is modified, Splicing forms mature mRNA
3. Translation- tRNA brings amino acids, Polypeptide chain is formed
4. Protein folding and modification- Correct 3D shape
importance of gene expression
Determines which protein A cell makes
Ensures specialisation of different cell types
transcriptional level- Prokaryotes
Lac operon controls expression of genes involved in lactose metabolism
structure:
regulator gene- codes for repressor protein
Operator region- Binding site for repressor protein
Promoter region- Binding site for RNA polymerase
Structural genes- Code for enzymes E.g. beta-galactosidase, lctose permease
operon
A cluster of genes under the control of a promoter
transcriptional level- No lactose
Repressir protein binds to operator
Prevent prevents RNA polymerase from binding to the promoter because its in the way
Therefore, structural genes are not transcribed, no transcription
transcriptional level- Lactose present
Lactose bones to repressor protein
Repressor protein changes shape And can no longer bind to the operator
Therefore, RNA polymerase can bind to promote
Social transcription of structural genes occur
transcriptional stage
Post transcriptional stage
Post translational stage
(gene expression)
determines whether transcription occurs or not
Determines if the protein is made or not
Determines whether protein is active and functional
eukaryotes - role of transcription factors
Proteins that bind to DNA at promoter regions
They can activate (enchancers) or suppress (repressors) transcription
Important in cell differentiation and development
Bind to promoter region allowing or preventing RNA polymerase from binding
oestrogen- transcription factor
1. oestrogen moves into nucleus of cell and binds to an inactive transcription factor
2. transcription factor is now activated and binds to promoter region
3. this allows rna polymerase to bind, causing transcription
post-transcriptional level
occurs in eukaryotes
editing primary mrna- splicing removes introns (non-coding regions of dna), exons (coding regions of dna) are joined together to form mature mrna
(different exons may be spliced together in different combinations- alternative splicing)
post-translational level
changes to proteins after translation affects whether they are active or not
protein activation by cyclic AMP
cAMP- binds to inactive proteins making them active by changing their 3d shape
body plan
basic structural layout of an organisms body
refers to:
symmetry
head-tail orientation
segmentation
limb formation
organisation of tissues
how is bosy plan controlled
homeobox, homeodomain, hox
homeobox genes
group of genes that contain a short dna sequence called homeobox
180 base pairs, highly conserved across plants, animals, fungi (very similar) (mutations are lethal)
codes for a homeodomain protein (part of a transcription factor)
homeodomin proteins
bind to dna and controls transcription of other genes
hox genes
a subset of homeobox genes in animals
controls the development of body plans
determines limb formation
found in clusters
e.g. limb formation in vertebrates, antenna instead of legs
studying homeobox genes- fruit flies
have a simple body plan
develop quickly
genes are highly conserved
experimental advantages-
easy to breed in large numbers
short life cycle
clear embryonic segmentation patterns
relatively cheap
mitosis
growth and repair of tissues
produces geneticallt identical cells
allows cell proliferation
apoptosis
programmed cell death
removed unwanted or damanged cells
sculpts body structures e.g. removal of webbing between digits
mitosis and apoptosis together
ensure proper development of the body form
regulated by genes that respons to both internal and external cues
how does apoptosis occur
blebbing- plasma membrane forms small bulges
cell breaks apart into small apoptotic bodies
apoptotic bodies are engulfed by phagocytes