f215 Complete summary

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Control, Genome and Environment
Cellular Control
(a) state that genes code for polypeptides, including enzymes
(b) explain the meaning of the term genetic code
The sequence of the bases on a gene is a code with instructions for the construction of proteins. It
has a number of characteristics:
It is a triplet code three bases code of an amino acid
It is a degenerate code All amino acids bar one have more than one code
Some codes don't code for amino acids but are `stop' codons they indicate the end of the
polypeptide chain
It is widespread but not universal Codons generally always code for the same amino acid
in every organism, but this is not always the case.
(c) describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid
sequence in a polypeptide
DNA is copied into mRNA by a process called transcription
Only one strand of the DNA is copied by complementary basepairing (hydrogen bonds)
The code is a triplet code which is read in threes (a codon is 3 bases)
The base sequence determines the amino acid sequene
The mRNA is released from the DNA and passes out of the nucleus through a pore in the
nuclear envelope to a ribosome
(d) 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
1. A molecule of mRNA binds to a ribosome. Two codons are attached to the small subunit of the
ribosome and exposed to the large subunit. The first exposed mRNA codon is always AUG.
Using ATP energy and an enzyme, a tRNA molecule with the amino acid methionine and the
anticodon UAC forms hydrogen bonds with this codon
2. A seconds tRNA molecule, bearing a different amino acid, binds to the second exposed codon
with its complementary anticodon
3. A peptide bonds forms between the two adjacent amino acids. This is catalysed by an enzyme
4. The ribosome now moves along the mRNA reading the next codon. A third tRNA brings another
amino acid and a peptide bonds forms between it and the dipeptide. The first tRNA leaves and
is able to collect and bring another of its amino acids.
5. The polypeptide chain grows until a stop codon is reached, for which there are no
corresponding tRNAs and the polypeptide chain is complete
(e) state that mutations cause changes to the sequence of nucleotides in DNA molecules
A 13 base pair deletion has serious consequences as it would cause a frameshift. As the
genetic code is triplet (read in groups of 3 bases), it alters all amino acids that are coded for
after the mutation
A 21 base pair deletion causes 7 amino acids to be lost
A substitution changes one or possibly no amino acids
(f) explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions
Beneficial
The mutation changes the sequence of amino acids and therefore the phenotype, but this
gives the organism an advantageous characteristic
E.g. Paler skin in more temperate climates absorbs more vitamin D
Neutral
It is a mutation in a noncoding region of the DNA
It is a silent mutation although the base triplet has changed, it still codes for the same
amino acid and so the protein is unchanged.
Harmful
The mutation changes the sequence of amino acids and therefore the phenotype, and the
resulting characteristic is harmful
E.g. paler skin in a hotter climate burns more easily
(g) state that cyclic AMP activates proteins by altering their threedimensional structure

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This involves both regulatory and structural genes.
Regulatory genes make a repressor protein which is a transcription factor that switches a
structural gene on or off
Structural genes make enzymes, polypeptides or proteins
So the regulatory gene controls and affects the expression of structural gene
E. coli grown in a culture medium with no lactose can be placed in a growth medium with lactose.
At first they cannot metabolise the lactose because they only have tiny amounts of the enzymes
needed to catalyse the reaction.…read more

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Meiosis and Variation
(a) 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 I
Prophase I
1. The chromatin condenses and supercoils
2. The chromosomes come together in their homologous pairs to form a bivalent.
Each member of the pair has the same genes at the same loci.…read more

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Observable characteristics of an organism
genotype
The combination of alleles possessed by organism
dominant
Characteristic in which the allele responsible is expressed in the phenotype, even in those
with heterozygous genotypes
codominant
A characteristic where both alleles contribute to the phenotype
recessive
Characteristic in which the allele responsible is only expressed in the phenotype is there is
no dominant allele present.…read more

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If both the A and the B alleles are present in the genotype, the phenotype will be AB they
are codominant. The O allele is recessive, so it will not be expressed in the phenotype
unless the alleles A or B are not present
(f) describe the interactions between loci (epistasis). (Production of genetic diagrams is not required)
Epistasis is the interaction of different gene loci so that one gene locus makes or suppresses the
expression of another gene locus.…read more

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Continuous variation
Different alleles at the same gene locus have small effects
Different gene loci have the same, often additive effect on the trait
A large number of gene loci may have a combined effect on the trait
(k) explain that both genotype and environment contribute to phenotypic variation. (No calculations of
heritability will be expected)
While an organism may have the genetic potential to achieve a certain characteristic, e.g. length of
corn cob, the environment also has an influence.…read more

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If two subpopulations are separated from each other, they will evolve differently as they have
different selection pressures, so different alleles will be eliminated or increased within each sub
population. Eventually the sub populations will not be able to interbreed and so will be different
species.
The sub populations may be split by various isolating mechanisms
Geographical barriers e.g. rivers or mountains
Seasonal barriers e.g. climate change throughout the year
Reproductive mechanisms e.g.…read more

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Triticum aestivum)
Dairy cow
Characteristics selected for
large yield (volume) of milk
long lactation period
high milk quality
large udders or the correct udder shape (for milking machine)
resistance to disease (eg mastitis) or effective immune system
calm temperament
Process of selection
Each cow's milk yield is measured and recorded
The progeny of bulls is tested to find out which bulls have produced daughters with high
milk yields
Only a few goodquality bulls need to be kept are the semen from one bull can be…read more

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Biotechnology and Gene Technologies
Cloning in Plants and Animals
(a) outline the differences between reproductive and nonreproductive cloning
Reproductive cloning is the production of offspring which are genetically identical to either the
mother (nuclear transfer), or the other offspring (splitting embryos)
Nonreproductive cloning is the use of stem cells in order to generate replacement cells, tissues or
organs which may be used to treat particular diseases or conditions of humans
(b) describe the production of natural clones in plants using the example of vegetative propagation…read more

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The beetles only live on elm trees
and attempts at control contributed to spread. As more trees became diseased then more tree surgery
was necessary and as more trees became infected then more saws were contaminated.…read more

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