OCR Biology A level Module 6 Summary notes

6.1.1 Cellular Control
Genes code for polypeptides, including enzymes;
Gene:
 A length of DNA (sequence of nucleotide bases) that codes for one or more polypeptides
 Each gene occupies a specific locus on the chromosome
o Each chromosome consists of one molecule of DNA
o Each gene is just one part of a DNA molecule

Codes for (e.g):
 Enzymes
 Structural proteins - collagen/keratin
 Haemoglobin
 Immunoglobulins (antibodies)

The genetic code;
 TRIPLET CODE:
- Sequence of three nucleotides codes for an amino acid
 DEGENERATE CODE:
- All amino acids (except methionine) have more than one code
 Some codes don't correspond to an amino acid but indicate "STOP" - end of a polypeptide chain
 Widespread but NOT universal
- Some base sequences code for the same amino acids in all organisms but it is not always the same
(they may code for different things/more than one thing)


Mutations cause changes to the sequence of nucleotides in DNA molecules (unpredictable)


Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions;
Occur:
 During DNA replication Can be SILENT MUTATION:
 Meiosis  Triplet still codes for same amino acid
 No effect on polypeptide produced

Can be NEUTRAL MUTATION:
Substitution (point mutation):  Resultant differentiation gives organism no
 One base pair replaces another advantage/disadvantage
o Mutation causes change to structure of protein
o Another version of same gene - different ALLELE
o Different characteristic
 OR mutation in non-coding region of DNA
 OR silent mutation
Harmful = e.g pale skin in hot weather
Beneficial = e.g pale skin - absorb more vitamin D

Insertion: Causes a FRAME SHIFT:
 Nucleotide base inserted into genetic code  Inserted/deleted base pair changes entire coding of
DNA molecule
Deletion:  Triplet codons change
 One base removed from genetic code  Alters all amino acids after mutation

State that cyclic AMP activates proteins by altering their three-dimensional structure;
 Cyclic AMP is a nucleotide derivative
 Activates proteins by changing their 3D shape
 Shape is a better fit to their complementary shape

Explain genetic control of protein production in a prokaryote using the lac operon;

,Bacteria produce two enzymes in the presense of lactose – a potential respiratory substrate:
- B-galactosidase (break down lactose to glucose + galactose)
- Lactose permease (transport lactose into bacterial cell)
Bacteria need to produce this enzyme; only produced when lactose present. Lactose is the inducer for these
enzymes.

Lac operon



Operon:
- Length of DNA
- Made up of structural genes; code for proteins
o and control sites – operator/promotor region; regulate operon

Operator region –
 Can switch structural genes on and off

Promotor region –
 Enzyme RNA polymerase bind to here in order to being transcribing mRNA for synthesis of enzymes from Z/Y

Regulator gene (I) –
 Not part of operon
 It is expressed (transcribed/translated into protein)
 Makes repressor protein/transcription factor
o Can bind to operator region
o Product switches structural genes on and off
 Controls expression of structural gene

When lactose is absent from growth medium:
 Regulator gene expressed and repressor protein produced
 No lactose present to bind to repressor protein
o Repressor protein binds to operator region of
operon (which also binds to part of promotor
region)
 RNA polymerase cannot bind to promotor region
 Structural genes not transcribed into mRNA
 Enzymes not synthesized

Why useful?
 Useful resources/energy not wasted by making unnecessary enzymes


When lactose is present in growth medium:
 Regulator gene expressed as normal, repressor protein still
produced
 Lactose (inducer molecule) binds to repressor protein
 Repressor protein changes shape
o Binding site no longer complimentary to operon
 Repressor protein does not bind to operator region/breaks
away
 RNA polymerase therefore binds to promotor region
 Transcribes Z and Y into enzymes
 Lactose can be brought into cell, broken down and respired

,e.g Suggest how the protein encoded by allele T regulates transcription
 Ref promoter/operator/"on" switch
 Allele T is regulator
 Protein binds to DNA
 Binds to repressor, preventing it binding to DNA
 Allows RNA polymerase to bind

The genes that control development of body plans are similar in plants, animals and fungi, with reference to
homeobox sequences;
Homeobox genes:
 Homeotic/regulatory gene
 Contains 180bp/homeobox sequence
 Codes for homeodomain (on protein)
 Gene product binds to DNA
 Intiates transcription/switch genes on/off
 Control the development of the body plan of an organism including polarity (head/tail ends) and positioning
of the organs
Perform different functions: Mutation of these genes can change one body
 Maternal genes; affect polarity (anterior/posterior) part to another
 Segmentation genes; specify polarity of segments
 Homeotic selector genes; specify the identity of each segment; direct development of individual body
segments

They regulate the expression of other genes
 Switch genes on and off
Homeobox genes sequence for specific polypeptides (transcription factors/regulatory protein/repressor protein)
that in turn bind to genes and prevent the expression of them
 Others act as TRANSCRIPTION FACTORS;
o Bind to gene
o Initiate transcription in protein synthesis, thus specialising the cell

Explain how the products are coded for by the genes interact to give different pigments
 Gene products are enzymes
o Multi enzyme pathway
 3 steps (specific to q)
 Enzymes change tryptophan to red pigment
 Product of one reaction is substrate of another
 Dominant allele gives functional enzyme
 Recessive gives non-functional/different enzme

(j) outline how apoptosis (programmed cell death) can act as a mechanism to
change body plans.
It is a speedy process
 Important for cell development where mitotic divisions are regular
o Balance of too much apoptosis (cell loss/degeneration) and too little
(tumours)

 Enzymes break down cell cytoskeleton
 Cytoplasm becomes densely packed
o (Organelles bundle together)
 Cell surface membrane alters in shape - blebs form
 Chromatin in nucleus condenses
 Nuclear envelope breaks down
 DNA fragments
 Cell packages its contents into vesicles
 Vesicles taken up by phagocytosis

, Cancer:
If cells have a mutation to genes involved in regulating mitosis
 Do not undergo apoptosis
 Keep on dividing by mitosis
 Tumour

Why?
 Reuse cell components
 No harmful enzymes released
 Weeds out ineffective/harmful cells
 Separation of limbs/digits during development




6.1.2 Patterns of Inheritance
Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the
associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are
expected, but not the subdivisions of prophase);

Meiosis
 Nuclear division where original number of chromosomes in the organism is halved
 Cells produced are haploid
 Means when two cells combined in sexual reproduction chromosome number is restored
 Produces non-identical cells

Meiosis I
Prophase
 Chromosomes supercoil and become visible
 Chromosomes consist of two SISTER chromatids (genetically identical)
 Chromosomes come together in their homologous pairs (each member has same
genes at the same loci) (one maternal, one paternal)
 Form bivalent

 Non-sister chromatids wrap around each other at points called chiasmata; "crossing
over"
o (they may break and swap sections of DNA - leading to alleles being switched
around)

 Nucleolus disappears
 Spindle forms
 Nuclear envelope breaks down
 Centrioles at opposite ends of cell


Metaphase
 Bivalents line up at equator
 Spindle fibres attached to bivalents at centromere