Transcript Communication - Miss Hanson's Biology Resources

F215 control genomes and environment
Module 1 Cellular Control and Meiosis
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state that genes code for
polypeptides, including enzymes;
explain the meaning of the term
genetic code;
describe, with the aid of diagrams, the
way in which a nucleotide sequence
codes for the amino acid sequence in
a polypeptide;
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describe, with the aid of diagrams,
how the sequence of nucleotides
within a gene is used to construct a
polypeptide, including the roles of
messenger RNA, transfer RNA and
ribosomes;
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state that mutations cause changes to
the sequence of nucleotides in DNA
molecules;
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Definitions
 Mutation
▪ A change in gene or chromosome structure
 Mutant
▪ An individual showing or carrying a mutation
 Mutagen
▪ A chemical or physical agents causing a
mutation
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There are two types of mutation
 Gene mutation
▪ Affects a single gene
 A chromosome mutation
▪ Affects a single chromosome or set of
chromosomes
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Occur at random
Are spontaneous
Are rare
Can be increased by chemicals or
radiation
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This results from a change in base
sequence of the DNA of a gene
 This means a different protein is coded for
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Look at the sequence of bases below:
CCT AGT ATT CGC TGA GGC TAA TG
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Now look at the three sequences
below
Describe the change you see in each
sequence
1. CCT AGA ATT CGC TGA GGC TAA TG
1. CCT AGT TTC GCT GAG GCT AAT G
2. CCT AGT AGT TCG CTG AGG CTA ATG
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Use one of the three terms below to
describe each sequence
 Substitution
 Deletion
 insertion
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Substitutions will only alter one codons
 This results in only one amino acid in the protein
being changed
 This is known as a point mutation
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Insertions and deletions cause a shift in the
whole sequence of bases and all the
codons after that point are altered.
 This is a frame shift.
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explain how mutations can have
beneficial, neutral or harmful effects
on the way a protein functions
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Give suggestions for each of the three
effects of mutations below:
 Beneficial
 Neutral
 Harmful
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These are mutation which offer a
selective advantage to an individual.
 Well-adapted organisms can out-
compete those in the population without
the advantageous characteristic
 This is the driving force behind natural
selection
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If the mutation occurs in the noncoding part of DNA
Silent mutation
 Base triplet is changed but has no effect
on the amino acid coded for.
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70% of cystic fibrosis sufferers, the
mutation is a deletion of a triplet of
base pairs
Protooncogenes can be changed into
oncogenes by a point mutation,
which promote uncontrolled cell
division
Huntington disease is caused by a
stutter – this is repeating sections of
CAG sequences,
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Haemoglobin
 Globular protein
▪ Two α polypeptide chains
▪ Two β polypeptide chains
 A mutation in the gene coding for the β
chain causes sickle cell anaemia
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When the four polypeptide chains curl up
they form a specific 3-D shape
 Some amino acids have hydrophobic side chains
e.g. valine
 Some amino acids have hydrophillic side chains
e.g. glutamate
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If the O2 level in blood falls, valines form
bonds with themselves that stick
haemoglobin molecules together,
producing long chains of stuck-together
haemoglobin molecules, the RBC is pulled
out of its usual biconcave shape.
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If a mutation changes a
characteristic, there can be an
advantage or a disadvantage to
having this new character.
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the environment plays a role in
determining the likelihood of this
characteristic being maintained
through natural selection.
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Background:
 Melanin is a skin pigment that protects
cells from the harmful effects of UV
radiation.
 Vitamin D is synthesised when skin is
exposed to sunlight.
Dark skin protects from
harmful UV rays.
 Sunlight is intense
enough to synthesis
vit.D.
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Light skin does not
shield against harmful
UV- causes skin
cancer.
Vitamin D can be
synthesised
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Dark skin not needed
to protect against UV.
Melanin prevents
sunlight synthesising
vitamin D.
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Sunlight less intenselowered intensity of UV
(no skin cancer)
Vitamin D can be
synthesised
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The Inuit people still retain
some skin pigments, but do
not live in an environment
with high levels of
UV/sunlight.
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Is melanin a
disadvantage?
 Diet is high in Vitamin D, so no
need to reduce melanin
levels in order to synthesise
vitamin D.
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state that cyclic AMP activates
proteins by altering their threedimensional structure;
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explain genetic control of protein
production in a prokaryote using the
lac operon
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E. coli is capable of synthesising a
variety of different enzymes,
depending on their environment.
E. coli only produce enzymes needed
to metabolise lactose when lactose is
present in the substrate
 ß-galactosidase:
▪ catalyses hydrolysis of lactose.
 Lactose permease:
▪ transports lactose into the cell.
The operon consists of several genes
Regulatory gene
for lac operon
Control sites
Structural genes
P: Promoter region. RNA polymerase binds here to start
transcription of Z & Y.
O: Operator region. Switches Z & Y on and off.
Z: Codes for ß-galactosidase.
Y: Codes for lactose permease.
What Happens Without Lactose?
mRNA
ribosome
repressor protein
Regulator gene is expressed and produces REPRESSOR PROTEIN.
One binding site on Repressor protein binds to operator region, covering
promoter region where RNA polymerase would attach.
RNA polymerase cannot bind to promoter region and neither gene Z or Y is
expressed.
What Happens With Lactose?
lactose
ß-galactosidase
Lactose permease
Lactose binds to other binding site on repressor protein, changing the
shape.
Repressor protein cannot bind to operator region
RNA polymerase binds to promoter region and genes Z & Y are
expressed.
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explain that the genes that control
development of body plans are similar
in plants, animals and fungi, with
reference to homeobox sequences
(HSW1);
Homeobox genes determine how an
organism’s body develops as it grows from
a zygote into a complete organism.
 They determine the organism’s body plan
 These sequences are highly conserved,
which implies that their activity is
fundamental to the development of an
organism
 Homeobox genes have been discovered in
animals, plants and fungi
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These are the sequences of 60 amino acids in the
proteins coded for by the homeobox genes Antp in
a fruit fly and HoxB7 in a mouse.
All animals have homologous homeobox genes –
they are recognisably similar
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Drosophila melanogaster a.k.a. fruit fly
Body is divided into
 Head
 Thorax
 abdomen
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Development is mediated by homeobox
genes
 Maternal effect genes determine the embryo’s
polarity e.g. anterior (head) & posterior (tail /
abdomen)
 Segmentation genes determine polarity of each
segment
 Homeotic selector genes identify and direct the
development of each segment
▪ Two groups exist, that control development of (i) head +
thorax segments and (ii) thorax + abdomen segments.
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The Thorax of the fruit fly is split into 3 segments
 T1 – a pair of legs
 T2 – a pair of legs and a pair of wings
 T3 – a pair of legs and a pair of halteres
A homeobox gene
called Ubx stops the
formation of wings
in T3.
 A mutation in both
copies of Ubx, wings
grow in T3 instead
of halteres.
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Ubx
Mutant
Ubx
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If the homeobox gene
Antp is usually turned on
in the thorax, where it
causes legs to
develop.
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In mutant flies where
Antp is switched on in
the head, legs grow
instead of antennae
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Homeobox genes code for the
production of transcription factors
These proteins can bind to a particular
region of DNA and cause it to be
transcribed
 A single homeobox gene can switch on a
whole collection of other genes,
regulating gene expression
• Hox clusters are aggregations of
homeobox genes and are found in all
animals.
• Examples
• Nematodes have one Hox cluster
• Fruit flies have 2 Hox clusters
• Vertebrates have 4 Hox clusters
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Effect of thalidomide in embryo
development
 Homeobox genes HoxA11 and HoxD11
switch on genes that cause forelimb
development.
 The drug thalidomide affected the
behaviour of these homeobox genes at a
critical stage in embryonic development.
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Retinoic acid and birth defects
 Retinoic acid
▪ is a derivative of vitamin A
▪ activates homeobox genes in vertebrates
▪ Is a morphogen (substance governing pattern
of tissue development).
 If a pregnant woman takes too much
Vitamin A, it can interfere with the
expression of these genes causing birth
defects in the central nervous system and
axial skeleton
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http://www.dnaftb.org/dnaftb/37/con
cept/index.html
 If you have time, sit and watch this
animation, as well as investigating other
aspects of this web site.
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outline how apoptosis (programmed
cell death) can act as a mechanism
to change body plans
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Apoptosis is programmed cell death in
development
 Series of biochemical events leading to an
orderly and tidy cell death
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Hayflick Constant
 Cells undergo about 50 mitotic divisions before
apoptosis
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Necrosis
 Untidy and damaging cell death occurring after
trauma
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Enzymes breakdown cell cytoplasm
Cytoplasm becomes dense
Organelles are tightly packed
Cell surface membrane changes and
blebs form
Chromatin condenses, nuclear
envelope breaks
Cell breaks into vesicles
phagocytosis
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Apoptosis is controlled by cell
signalling
 Cytokines from the immune system
 Hormones and growth factors
 Nitric oxide
▪ Makes inner mitochondrial membrane more
permeable to hydrogen ions
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The rate of cells dying should balance
the rate of cells produced by mitosis
 Not enough apoptosis leads to the
formation of tumours
 Too much leads to cell loss and
degeneration
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Cell signalling plays a role in
maintaining the correct balance
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The formation of
the digits (fingers
and toes) occurs
due to apoptosis
during the
development of
the embryo.
As tadpoles grow
they develop legs,
change their body
shape and lose
their tails
 The tail is lost by
apoptosis
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