genetics of living systems Flashcards
1
Q
What is a mutation?
A
A change in the sequence of bases in DNA • Caused by substitution, deletion or insertion of one or more nucleotides within a gene • If only 1 nucleotide is affected, it is called a point mutation • Insertion or deletion leads to frameshift mutation (unless the number of nucleotides changes is multiple of 3)
2
Q
What is a frameshift mutation?
A
The addition or deletion of a nucleotide moves, or shifts, the reading frame of the sequence of bases. This will change every successive codon from the point of mutation
3
Q
What are the effects of
different mutations?
A
No effect - because normally functioning proteins are still synthesised • Damaging - phenotype is affected negatively because proteins are no longer synthesised or synthesised proteins are non functional • Beneficial - protein is synthesised that results in a new and useful characteristic in the phenotype
4
Q
What are mutagens?
A
A chemical, physical or biological
agent which causes mutations
5
Q
What are the main mutagens?
A
6
Q
What is the difference between
gene mutations and
chromosome mutations?
A
Gene mutations occur in single genes or sections of DNA, whereas chromosome mutations affect the whole chromosome, or a number of chromosomes within a cell
7
Q
What causes chromosome
mutations?
A
They can be caused by mutagens
and normally occur during meiosis
8
Q
What types of changes can
occur in chromosome
structure?
A
• Deletion - a section of chromosome breaks off and is lost within the cell • Duplication - sections get duplicated on a chromosome • Translocation - a section of one chromosome breaks off and joins another non-homologous chromosome • Inversion - a section of chromosome breaks off, is reversed, and then joins back onto the chromosome • Non-disjunction - one pair of chromosomes fails to separate, leaving one gamete with an extra chromosome e.g. down syndrome or trisomy
9
Q
How are genes regulated?
A
The ways they are regulated is categorised by the level at which they operate • Transcriptional - genes can be turned on or off • Post-transcriptional - mRNA can be modified which regulates translation and the types of proteins produced • Translational - translation can be stopped or started • Post-translational - Proteins can be modified after synthesis which changes their functions
10
Q
How is DNA stored?
A
It is wound around proteins called histories in order to be packed into the nucleus of a cell The resulting DNA/protein complex is called chromatin
11
Q
What is the difference between
heterochromatin and
euchromatin?
A
• Heterochromatin is tightly wound DNA, causing chromosomes to be visible during cell division • Euchromatin is loosely wound DNA present during interphase
12
Q
Why does protein synthesis not
occur during cell division?
A
• Heterochromatin is present during cell division • Transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes • The genes in euchromatin can be freely transcribed, so protein synthesis can occur during interphase
13
Q
What is the benefit of protein
synthesis not occurring during
cell division?
A
• Ensures the proteins necessary for cell division are synthesised in time • Prevents the complex and energyconsuming process of protein synthesis from occurring when cells are actually dividing
14
Q
Why does DNA coil around
histones?
A
Histones are positively charged and
DNA is negatively charged
15
Q
Why are histones modified?
A
To increase or decrease the degree of packing (or condensation)
16
Q
How are histones modified?
A
• Acetylation and phosphorylation reduce the positive charge on the histones (making them more negative), causing DNA to coil less tightly so certain genes can be transcribed • Methylation makes the histones more hydrophobic so they bind more tightly to each other, causing DNA to coil more tightly and preventing the transcription of genes
17
Q
What is epigenetics?
A
External control of genetic regulation
18
Q
What is an operon?
A
A group of genes that are under the control of the same regulatory mechanism and are expressed at the same time • Very efficient way of saving resources because if a certain gene products are not needed, then all of the genes involved in their production can be switched off
19
Q
Why are operons more
common in prokaryotes than
eukaryotes?
A
Because prokaryotes are smaller
and simpler in the structure of their
genomes
20
Q
What is a lac operon?
A
Operon responsible for the metabolism of lactose • 3 genes: lacZ, lacY and lacA • Structure genes as they code for 3 enzymes, and are transcribed onto a single long molecule of mRNA
21
Q
What is a regulatory gene?
A
A gene that codes for proteins
involved with DNA regulation
22
Q
What is a repressor protein?
A
Protein that binds to the operator
affecting the rate of transcription
23
Q
How is transcription of the
structural genes prevented in
the absence of lactose?
A
1. A regulatory gene, I, is located near to the operon and codes for a repressor protein (LacI) 2. When this gene is expressed, the repressor protein produced binds to to the operator, preventing RNA polymerase from binding to the promoter region 3. The repressor protein therefore prevents the genes lacZ and lacY from being transcribes 4. The enzymes for lactose metabolism are not made 5. The genes are “off”
24
Q
What happens in the presence
of lactose?
A
1. Molecules of lactose bind to the LacI repressor protein molecules 2. This alters the shape of the LacI repressor protein, preventing it from binding to the operator 3. The RNA polymerase enzyme can then bind to the promoter region, and begin transcribing the structural genes into mRNA that will then be translated into the two enzymes 4. Lactose induces the enzymes needed to break it down
25
What is a transcription factor?
```
Protein or short non-coding RNA
that can combine with a specific site
on a length of DNA and inhibit or
activate transcription of the gene
• Essential for the regulation of gene
expression in eukaryotes
```
26
What are introns and exons?
• Introns are non-coding regions of
DNA which are not expressed
• Exons are the coding, or
expressed region of DNA
27
What happens in RNA
| processing?
```
• The product of transcription is premRNA,
which is modified f to form
mature mRNA
• A cap (modified nucleotide) is
added to the 5’ end, and a tail
(long chain of adenine molecules)
is added to the 3’ end
• These help stabilise mRNA and
delay degradation in the
cytoplasm
• The cap also aids binding of
mRNA to ribosomes
• Splicing also occurs - where RNA
is cut at specific points
• Both processes occur within the
nucleus
```
28
What happens in RNA editing?
```
The nucleotide sequence of some
mRNA molecules can also be
changed through base addition,
deletion or substitution
• These have the same effect as
point mutations
• Increases the range of proteins
that can be produced from a single
mRNA molecule or gene
```
29
Describe post-transcriptional
| gene regulation
```
1. All the DNA of a gene, both
introns and exons, is
transcribed. The resulting mRNA
is called primary mRNA
2. Primary mRNA is then edited,
and the RNA introns (lengths
corresponding to DNA introns)
are removed
3. The remaining mRNA exons
(corresponding to the DNA
introns) are joined together
4. Endonuclease enzyme may be
involved in the editing and
splicing process
5. A length of DNA with its introns
and exons can, according to
how it’s spliced, encode more
than one protein
```
30
What are the mechanisms that
regulate the process of protein
synthesis?
```
• Degradation of mRNA - the more
resistant the molecule, the longer
it will last in the cytoplasm, and so
a greater quantity of protein will be
synthesised
• Binding of inhibitory proteins to
mRNA prevents it binding to
ribosomes and the synthesis of
proteins
• Activation of initiation factors
which aid the binding of mRNA to
ribosomes
```
31
What is post translational gene
| expression regulated by?
The activation of proteins
• Many enzymes are activated being
phosphorylated
32
What is the role of cyclic AMP
| (cAMP)?
```
• The binding of cAMP receptor
protein (CRP) increases the rate of
transcription
• This is only possible when CRP is
bound to cAMP
```
33
How is cAMP used in post
translation gene expression
regulation?
```
1. A signalling molecules e.g.
glucagon, binds to a receptor on
the plasma membrane of the
target cell
2. This activates a transmembrane
protein which then activates a G
protein
3. The activated G protein activates
adenyl class enzymes
4. Activated adenyl cyclase
enzymes catalyse the formation
of many molecules of cAMP
from ATP
5. cAMP activates PKA (protein
kinase A)
6. Activated PKA catalyses the
phosphorylation of various
proteins hydrolysing ATP in the
process. This activated many
enzymes in the cytoplasm e.g.
those that convert glycogen into
glucose
7. PKA may phosphorylate another
protein (CREB, cAMP response
element to binding)
8. This then enters the nucleus and
acts as a transcription factor to
regulate transcription
```
34
What are protein kinases?
Enzymes that catalyse the addition
| of phosphate group to proteins
35
What are the modifications to
synthesised proteins that
happen in post-translational
control?
```
• Addition of non-protein groups
e.g. carbohydrate chains, lipids or
phosphates
• Modifying amino acids and the
formation of bonds e.g. disulfide
bridges
• Folding or shortening of proteins
• Modification by cAMP
```
36
What is morphogenesis?
The regulation of the pattern of
| anatomical development
37
What is are homeobox genes?
```
Sequence of 180 base pairs
(excluding introns) found within
genes that are involved in regulating
patterns of anatomical development
in animals, fungi and plants
• It is highly conserved (very similar)
in plants, animals and fungi
```
38
Why are homeobox genes
| regulatory genes?
```
Each homeobox sequence encodes
a 60-amino acid sequence called a
homeodomain sequence, within a
protein
• The homeodomain sequence can
fold into a particular shape and
bind to DNA, regulating the
transcription of adjure genes
• These proteins are transcription
factors and act within the cell
nucleus
```
39
What are Hox genes?
```
Subset of homeobox genes, found
only in animals; involved in formation
of anatomical features in correct
locations of body plan
• Found in gene clusters - mammals
have four clusters on different
chromosomes
• Humans have 39 Hox genes in
total
```
40
How are body plans usually
| represented?
As cross-sections through the
organism showing the fundamental
arrangement of tissue layers
41
What is the difference between
diploblastic and triploblastic
animals?
Diploblastic animals have 2
primary tissue layers
• Triploblastic animals have 3
primary tissue layers
42
What is the common feature of
| animals?
```
They are segmented e.g. the rings of
a worm or the back none of
vertebrates
• These segments have multiplied
over time and are specialised to
perform different functions
```
43
Describe how Hox genes
| control development
```
• Hox genes in the head control the
development of mouthparts
• Hox genes in the thorax control
the development of wings, limbs,
or ribs
• Individual vertebrae and
associated structure have
development from segments in the
embryo called somites
• Somites are directed by Hox
genes to develop in a particular
way depending on their position in
the sequence
```
44
Describe symmetry shown in
the body shape of most
animals
```
• Radial symmetry is seen in
diploblastic animals e.g. jellyfish.
They have no left or right sides,
only a top and a bottom
• Bilateral symmetry which is seen
in most animals means the
organisms have both left and right
sides, and a head and tail
• Asymmetry is seen in sponges
which have no lines of symmetry
```
45
What are the 2 cell processes
that are essential in shaping
organisms?
• Mitosis - which results in cell
division and proliferation
• Apoptosis - programmed cell
death
46
What are the sequence of
| events during apoptosis?
```
1. Enzymes break down the cell
cytoskeleton
2. The cytoplasm becomes dense
with tightly packed organelles
3. The cell surface membrane
changes, and small protrusions
called blebs form
4. Chromatin condenses, the
nuclear envelope breaks and
DNA breaks into fragments
5. The cell breaks into vesicles that
are ingested by phagocytic cells,
so that cell debris does not
damage any other cells or
tissues. The whole process
happens quickly
• Apoptosis is controlled by cell
signalling
• Some signalling molecules may be
released by cells when genes that
are involved in regulating the cell
cycle and apoptosis respond to
internal cell stimuli and external
stimuli e.g. stress
• Signalling molecules include:
cytokines, hormones, growth
factors and nitric oxide
```
47
What are the factors that affect
the expression of regulatory
genes?
```
• Internal and external environment
• Stress, change in temperature,
light intensity
• Drugs e.g. thalidomide which
prevented the normal expression
of a particular Hox gene. This
resulted in the birth of babies with
shortened limbs
```
