6.1.1 Cellular Control Flashcards
mutation
a random/spontaneous change to the sequence of bases in DNA
gene mutations, chromasome mutations
gene mutations
chnage to base sequence of DNA in 1 gene
chromosome mutations
change to the structure/ number of chromosomes
where do most mutations occur
somatic body cells - not inherited
associated with mitosis
mutations in meiosis
these mutations can be inherited
chances are low as there is a huge number of sperm cells released at once
mutagens
increase the chance of a mutation occuring
physical, chemical, biological agents
physical mutagens
ionising radiation
e.g. UV, gamma, x-rays
chemical mutagens
deaminating agents
e.g. convert cytosine to uracil
biological agents (mutagens)
e.g. viruses
e.g. agents that change structure of chemical bases
two main classes of gene mutations
point mutations - substitution
insertion or deletion mutations - INDEL
point mutations - substitution
1 base or nucleotide change
INDEL mutations
cause a FRAMESHIFT
1 or more mucleotides are added/deleted - sequence of amino acids may be different from point of mutation onwards
three types of point mutations
silent mutations
missense mutations
nonsense mutations
silent mutations
has no effect on the primary or secondary/tertiary structure of the protein
DNA is degenerate - reduces the effect of point mutation
missense mutations
a change in the base triplet sequence that leads to a change in the amino acid sequence in the protein
sickle cell anaemia
caused by a missense mutation and causes crystallisation of haemoglobin which causes red blood cells to become sickled
decreases SA:Volume
nonsense mutations
a point mutation may alter a base triplet so that it becomes a STOP codon/triplet
results in a truncated/shortened amino acid with no function - protein will be degraded
Duchenne muscular dystrophy
caused by nonsense mutation - protein ** dystrophin** is not produced
muscle cells waste away
insertion and deletion - INDEL mutations
if bases are added or removed NOT in a multiple of 3, the reading frame for RNA polymerase shifts - DNA is non-overlapping
thalassaemia
haemoglobin disorder: due to frameshifts as a result of deletions
Hb cannot sequester Fe3+ ions… must be removed by metal chelation therapy
neutral effects of mutations
if mutation is in non-coding region
if mutation is silent
if mutation causes change to protein structure and a different characteristic but it is not advantageous or disadvantageous
examples of neutral effects of mutations
ability to enjoy coriander
smell honeysuckle
presence of ear lobes
beneficial effect of mutations
example
the ability to digest lactose
allows organism to break lactose down into glucose + galactose to be used as respiratory substrates
harmful effects of mutations
examples
phenylketonuria: caused by substitution mutation in a gene encoding an enzyme involved in phenylalanine conversion
if Phe allowed to build up = brain damage
Duchenne muscular dystrophy: defective gene encoding dystrophin protein
muscles waste away
chromosome mutations
changes in chromosome structure: duplication, deletion, inversion, translocation
can be caused by mutagens + normally occur during meiosis - often lead to developmental abnormalities
duplication in chromosome structure
could lead to over-expression of certain genes
could affect metabolism
deletion in chromosome structure
absence of certain genes
could be fatal
inversion and translocation in chromosome structure
all genes are still present but may inhibit or disable the expression of other genes around them
explain how the degenerate nature of the genetic code reduces the effect of point mutations
there are multiple codons which code for the same amino acid = some mutations do not affect the primary structure of the protein
operons
a cluster of genes controlled by a single promoter region
regulation of gene expression in prokaryotes
bacteria have one circular chromosome
genes controlling related functions are located together to form operons
the lac operon
E.coli normally metabolises glucose as the most efficient respiratory substrate but when glucose is absent and lactose is present = lactose induces production of 2 enzymes
consist of a length of 6000 base pairs
enzymes that are produced in the absence of glucose
lactose permease - lac Y
gene that encodes channel proteins specific to lactose - these get inserted into bacterial plasma memb.
beta-galactosidase - lac Z
breaks glycosidic bond between glucose and b-galactose = glucose + b-galactose can then be used as respiratory substrate
when the lac operon is switched off (at high glucose concentrations)
repressor protein is constantly produced
active repressor binds to operator region = prevents RNA polymerase from binding to the promoter region
when the lac operon is switched on (in the absence of glucose)
lactose (inducer molecule) binds to repressor protein & changes its shape
repressor removed & RNA polymerase binds to promoter region = initiates transcription of lac Z/lac Y genes
genes then translated, folded & modified to become active enzymes
epigenetics
the control of gene expression through the modification of DNA structure & histone structure
regulation of gene expression in eukaryotes
chromatin remodelling
histone modification
chromatin remodelling
heterochromatin = tightly wound around histones
RNA polymerase is unable to bind to promoter region and cause gene expression
2 forms of chromatin
euchromatin: loosely wound around histones (cells in interphase)
heterochromatin: tightly wound around histones (cells in mitosis/meiosis)
histone modification
acetylation: reduces positive charge on histones causing DNA to coil less tightly around histones
methylation: histones become more hydrophobic so they bind more tightly to each other, causing DNA to wrap more tightly around histones
transcription factors
proteins act within nucleus to control which genes in a cell are on/off
they slide along part of a DNA molecule seeking out the correct/specific promoter region - either activate/prevent transcription of the gene (aids/inhibits attachment of RNA polymerase to DNA)
involved in regulating cell cycle checkpoints, protein synthesis
functions of transcription factors in eukaryotic cells
aids/inhibits attachment of RNA polymerase to DNA
regulates cell cycle checkpoints
regulates protein synthesis
regulates cell division
regulates gene expression
introns
non-coding regions of DNA
do not encode proteins
do not encode amino acids
exons
coding regions of DNA
encode proteins & amino acids
post-transcriptional gene regulation
modifies pre-mRNA to make it fit for purpose (mature mRNA)
removal of introns via splicing
exons joined together by a ligase enzyme
cap and tail added to mature mRNA to prevent degradation in cytoplasm (stabilises mRNA)
post-translational gene regulation
involves activation of proteins by cyclic AMP - 2nd chemical messenger
1. binding of signal molecule to specific receptor on plasma memb. activates G-protein
2. adenylyl cyclase is activated
3. ATP converted into cAMP
4. cAMP activates protein kinase = activates proteins - phosphorylates them
formation of cAMP
ATP + adenyl cyclase = pyrophosphate + cAMP
homeotic genes
a large ancient group of genes involved in controlling development of body plan - ensures body parts develop in the correct positions
homeobox sequences
a stretch of 180 DNA base pairs (excluding introns)
homeodomain
act as transcription factors - activate or repress certain genes
mutations in homeobox genes
lead to organisms that are not viable
lead to an organism born with deformities which would quickly eliminate it by natural selection
