Cellular Control Summary

OCR F215 - detailed summary for cellular control A2 biology in pdf. lotsa colourrz

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  • Created on: 07-05-16 11:26
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Cellular Control
DNA Genetic Code
Transcription and Translation
Transcription: creation of single-stranded mRNA copy of DNA coding strand.
Translation: the assembly of polypeptides at ribosomes.
Genes code for polypeptides such as:
· structural proteins (collagen and keratin)
· haemoglobin
· immunoglobulins
· cell surface receptors
· channel proteins
· enzymes.
The genetic code - Sequence of bases on a gene provide a code which has a number of
· Triplet code - three bases code for 1 amino acid
· Degenerate code - amino acids have more than one code (except methionine), some indicate
· Widespread but not universal - base sequence TCT codes for serine in any organism, however
there are some variations.
First stage in protein synthesis. mRNA molecule made from template strand of DNA. There are free
DNA nucleotides in the nucleoplasm and free RNA nucleotides in the nucleus. Nucleotides are
activated - they have two extra phosphoryl groups attached. Four different activated RNA
nucleotides: ATP, GTP, CTP and UTP.
· Gene unwinds and unzips, length of DNA dips into nucleolus, hydrogen bonds between bases
· Activated RNA nucleotides
bind w/ hydrogen bonds to
their exposed
complementary bases on
the template strand,
catalysed by RNA
· Two extra phosphoryl
groups released to release
energy for bonding
adjacent nucleotides.
· mRNA strand is
complementary to template
strand so is therefore a
copy of the coding strand
of DNA.
· mRNA released from the Figure 1 - Transcription of a gene, the DNA unwinds and rewinds,
DNA and passes out of the forming a template strand and a coding strand. mRNA is formed
nucleus through a pore in from the template strand and is a copy of the coding strand.
the nuclear envelope to a

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I. Molecule of RNA binds to ribosome. Two codons (a codon is three bases,
one codon codes for an amino acid) attach to small subunit on the
ribosome, exposed to the large subunit. First exposed codon
is always AUG. Using ATP energy and an enzyme, a tRNA
w/ methionine and anticodon UAC forms hydrogen bonds
with this codon.
II. Second tRNA binds to the second exposed codon w/
complimentary anticodon.
III.…read more

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Mutation: change in the amount of, or
arrangement of, the genetic material
in a cell.
· May be a mutation to the DNA, for
example base deletion, addition or
substitution or by inversion/repeat of
a triplet.
· Chromosome mutation involves
change to the structure of a
· Mutations associated with mitosis
are somatic mutations, and are not
passed on to offspring.
· Mutations associated with meiosis
and gamete formation can be
Figure 3 - The lac operon: Part a).…read more

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Biological Processes
Meiosis and its significance
The chromosome number in gametes needs to be haploid. Meiosis achieves this through two
separate divisions, which each have four stages.
There is an interphase during which DNA replication takes place.
Meiosis I:
Prophase I
· Chromatin condenses, supercoil, shorten and thicken. Come together in homologous pairs
forming bivalents (same genes at same loci, one paternal one maternal). Non-sister chromatids
wrap around each other and attach at chiasmata
where crossing over may occur.
· Nucleolus disappears, nuclear envelope
disintegrates.…read more

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Allele: a version of a gene, an allele of the gene has a difference in DNA base sequence that is
expressed as a slightly different polypeptide.
Locus: the position of a gene on a chromosome.
Crossing over: lengths of DNA are swapped from one chromatid to another.…read more

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Homeobox genes are arranged in
Hox clusters.
· Nematodes (roundworms) have
1 Hox cluster.
· Drosophila has 2 Hox clusters.
· Vertebrates have 4, of 9-11
genes on separate
Increase in Hox clusters probably
arose by duplication of a single
complex present in segmented
worms, and has allowed more
complex arthropods to evolve from
Homeobox gene expression:
· Expressed in specific patterns in
certain stages in both
vertebrates and invertebrates.…read more

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Figure 6 - The process of apoptosis. Hydrolytic enzymes are released from lysosomes inside the cell,
which then results in chromatin condensing and the cell shrinking. Organelles become tightly packed. The
membrane forms blebs and DNA breaks into fragments. The cell breaks into vesicles which are then taken
up by phagocytosis.
Genetics and Inheritance
Genotype: the genetic makeup of an organism. It describes the organism in terms of the alleles
it contains. An organism with two identical alleles for a gene is described as homozygous.…read more

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Using genetic diagrams
Problems involving sex linkage -
Haemophilia A - Several factors needed for blood to clot following wound, one is a protein, factor
VIII, coded for by a gene on the X chromosome. Recessive allele expresses a protein that does
not function. Males that suffer from haemophilia re hemizygous, and haemophilia A shows a
recessive inheritance pattern.
Figure 7 -
Inheritance of
haemophilia from
a carrier mother
and a normal
father, resulting in
50% normal
zygotes, 25%
carrier, and 25%
sufferer.…read more

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Problems involving codominance -
Sickle-cell anaemia - -strands of Hb differ
by one animo acid at position 6, when
abnormal Hb is deoxygenated it is not
soluble and becomes crystalline and
aggregates to more linear structures,
deforms red blood cells so can no longer
squeeze through capillaries. After many
cycles some cells irreversibly sickled, some
destroyed. Blood flow is impeded. Organs
do not receive enough oxygen, leading to a
painful crisis.…read more

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Population genetics
The Hardy-Weinberg Principle
Mathematical model to calculate the allele frequencies in populations for traits with dominant and
recessive alleles.
· Population is very large.
· Mating is random.
· No selective advantage for any genotype.
· There is no mutation, migration, or genetic drift.…read more


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