Summary
OCR A A-level Biology Module 6 Notes Summary COMPLETE
- Course
- Institution
Concise notes with diagrams and key words highlighted, sticking to the OCR A Biology specification
[Show more]
Seller
Follow
abdurrahmaanhasan
Content preview
A Level BiologyGENE MUTATIONS OCR A Module 6
Gene mutations
• A change in the base sequence of DNA which results in the formation of a new allele
• Occur spontaneously during DNA replication
• Substitution mutations → one base is substituted for another
• Deletion mutations → a base (or more than one) is deleted
• Insertion mutations → a base (or more than one) is inserted
Effects of mutations on proteins
• Some gene mutations affect protein structure
→ the base sequence of DNA is altered
→ the mRNA codon is altered so a different tRNA anticodon is complementary during translation
→ the tRNA brings a different amino acid
→ this changes the amino acid sequence (the primary structure)
→ the position of bonds (e.g. ionic bonds, disulfide bridges) between R groups alters
→ the tertiary structure is changed
→ mutations involving a change in the number of nucleotides cause a frameshift (all base triplets
downstream of the mutation are altered) and change many amino acids in the encoded polypeptide
→ could be beneficial if the altered protein increases chances of survival and reproductive success
(e.g. the organism is better camouflaged due to a different coloured pigment)
→ could be harmful if the altered protein decreases chances of survival and reproductive success
(e.g. an enzyme involved in digestion has reduced function)
• Some gene mutations do not affect protein structure
→ mutation could be in an intron or other non-coding base sequence
→ the genetic code is degenerate so a substitution mutation might not affect the amino acid even
though the base triplet has changed
→ a change in an amino acid might not always affect the tertiary structure if the properties of
the amino acid are similar
→ these mutations are neutral
• Mutations can happen in promoter regions
→ might prevent a transcription factor binding to the DNA in the promotor region
→ could prevent transcription of the gene being stimulated or inhibited by the transcription factor
original DNA A T CCC A
C T T A mutation in a gene coding for
base
sequence a transcription factor may
I 1.
triplets
I
mean that the transcription
substitution A T C CCC C T T factor has an altered tertiary
mutation structure and is not able to bind
I I I 11 I
to DNA.
deletion A T
/
cC A CT T
mutation
I I I I I
frameshift
acce
insertion A T T
mutation
I I frameshift
Page 1 of 1 © Dr Zoë Huggett
, A Level Biology
GENE EXPRESSION OCR A Module 6
Transcription factors
• Proteins which regulate transcription in eukaryotic cells
• Move from the cytoplasm to the nucleus to bind to DNA
• Promotor regions → DNA base sequences before genes where transcription factors bind
→ have binding sites for specific transcription factors depending on the gene
• Different types of transcription factor
→ activator proteins stimulate transcription of target genes by helping RNA polymerase to bind to
the promoter region
→ repressor proteins inhibit transcription of target genes by preventing RNA polymerase from
binding to the promoter region
transcription factor
⑧↳ /DNA
I promoter I I gene I
The lac operon
• An operon is a cluster of genes under the control of a promoter
• An example of regulation of gene expression at the transcriptional level in prokaryotic cells (E. coli)
• The lac operon allows bacteria to metabolise lactose for use in respiration if it is present
• Components of the lac operon
→ lacl = a regulatory gene coding for the lac repressor protein
Regulatory genes code for
→ promoter (P) = region of DNA where RNA polymerase binds proteins that control
→ operator (O) = region of DNA where the lac repressor binds expression of other
→ lacZ = a structural gene coding for the enzyme lactase genes. Structural genes
→ lacY = a structural gene coding for the enzyme lactose permease code for proteins with
→ lacA = a structural gene coding for the enzyme transacetylase other functions.
RNA
• If lactose is absent polymerase
dry
Deene
-
→ the lac repressor binds to the operator
→ blocks RNA polymerase from binding to the promoter
→ the three structural genes are not transcribed
(lactose metabolism is not required) .
• If lactose concentration is high
→ lactose binds to the lac repressor and it RNA
polymerase lac
changes shape lactos ⑧
repressor
binds to X ⑤
→ the lac repressor cannot bind to the promoter
operator &
lacZ lacY lacA
→ RNA polymerase can bind to the promoter
→ the three structural genes are transcribed
lactase transcribed
lactose permease transcribed -
transacetylase transcribed
-
↓
hydrolysed to
lactose
-
permeability of the cell
glucose + galactose another step in lactose
surface membrane to
lactose increases
metabolism is
catalysed
Page 1 of 2 © Dr Zoë Huggett
, A Level Biology
GENE EXPRESSION OCR A Module 6
Post-transcriptional mRNA editing in eukaryotes
• Happens in the nucleus Prokaryotic DNA does
• Transcription produces a strand of primary mRNA not have introns.
→ complementary to the DNA base sequence of the gene
→ contains coding base sequences (exons) and non-coding base sequences (introns)
• Primary mRNA is capped and tailed to make it more stable and protect it from being broken down
→ a 5’ cap is added at the start of primary mRNA
→ a 3’ poly-A tail (lots of A nucleotides) is added to the end of primary mRNA
• Splicing → introns are removed from primary mRNA to form mature mRNA containing only exons
→ introns are non-coding DNA so they are not translated
→ exons code for polypeptides and are translated
→ sometimes certain exons are also removed in certain tissues, meaning different versions of
the polypeptide can be produced in different tissues (this is called alternative splicing)
primary - mRNA mRNA
exon / 2 3 exonl exon2 exon3
splicing,
exon exon exont exon4
·
.... 3333 ⑧
· ...3333
intron intron intron
exonl exon3 exon4
I
33333
alternative
splicing in
differenttissues exon / exon2 exon4
-
⑧
· -
Post-translational regulation
regulatory
-↓
• Some proteins are translated in an inactive form subunits
• They can be activated by cyclic AMP (cAMP)
• Example: protein kinase A is activated by cAMP: cat
alytiC
->
subunits
-> &&
1) Protein kinase A has four subunits → the enzyme is inactive when
the regulatory subunits are bound to the catalytic subunits. activation
CAMP
by
2) Two cAMP molecules bind to each of the regulatory subunits CAMP
I
which changes their tertiary structure. ↳
. ⑤
3)The catalytic subunits are released and activated.
& &
Page 2 of 2 © Dr Zoë Huggett
, A Level Biology
BODY PLAN OCR A Module 6
Homeobox genes
• Similar base sequences across the plant, animal, and fungi kingdoms
→ highly conserved (very similar) base sequences within the each of the plant, animal, and fungi
kingdoms
• Code for protein transcription factors called homeodomains
• Mutations are often lethal to the organism
• Hox genes → a subset of homeobox genes specific to animals
→ have a homeobox sequence coding for a homeodomain protein transcription factor
→ control development of body segments from front to back to ensure they are in the
right order
→ genes arranged in the same order on the chromosome as the order of the body segments
they control
→ highly conserved (very similar) DNA base sequences in all animals
· D
determine the
eightHox genes
position of the differentsections Fruit flies are often used to
investigate these genes.
*
⑧-
Gene
·↑iI
They have a simple body plan
*
!
-
I
-
A
↑M
x s ⑲
N-
I
i and reproduce rapidly.
*↑
&
do
⑲ -
-
-
⑦
-
⑦
↑
e
i
Mitosis and apoptosis
• Important processes in controlling development of body plan
• Mitosis → produces genetically identical cells See the Module 2 notes
→ enables the organism to produce more cells and grow for more on mitosis
and the cell cycle.
→ cells differentiate to specialise into different cell types
• Apoptosis → genetically programmed cell death
→ the cell shrinks and enzymes digest cell contents into fragments
→ fragments are engulfed by phagocytes to protect surrounding cells
→ destroys cells which are no longer needed e.g. important for destroying cells between the
fingers during human development so that the fingers are not webbed
→ destroys cells with irreparably damaged DNA detected during cell cycle checkpoints
• Genes regulating the cell cycle and apoptosis respond to internal and external stimuli
→ an internal stimulus could be damage to DNA in the cell or cellular stress
→ an external stimulus could be the presence of a hormone outside the cell or presence of a
pathogen
Page 1 of 1 © Dr Zoë Huggett
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller abdurrahmaanhasan. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $10.63. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
80461 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now