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Structure of DNA
Nucleotide Structure
DNA is made up with 3 components:
Deoxyribose (sugar)
Phosphate group
Function of DNA
outside forces
Organic bases ­ Cytosine (double ring) + Guanine
(single ring), Thymine (single) + Adenine (double)
Combine by condensation reaction
and protein synthesis
DNA Structure
By Watson and Crick
2 strands of nucleotide join by hydrogen bonds
Stable can pass without change
It carried out it's function by being:
Pairing of bases
The bases contain nitrogen and there are two
types double (adenine and guanine) and single
Large molecule carries a lot of genetic info.
(thymine and cytosine)
DNA passes info from cell to cell, generation to
info. is somehow protected from chemicals and
Adenine and Thymine (2 hydrogen bonds)
Has 2 strands which separates in DNA replication
The bases are between the strands so the genetic
Cytosine and Guanine (3 hydrogen bonds)
So the pairs are complementary
The Double Helix
The ladder is being twisted
Deoxyribose and Phosphate wind around one
another, they are the backbone of DNA…read more

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The Triplet Code
What is a gene
Genes, sections of DNA that contains coded info
about making polypeptides (you)
It's in the form a genetic sequence of bases
A gene is a sequence of polypeptide and a
polypeptide is a sequence of amino acid
The triplet code
There is a minimum of 3 bases that code for amino
acid the reason is:
20 amino acid regularly occur in proteins
Each amino acid has to have it's own code
There are only 4 bases
If each base codes for one amino acid only 4 amino
If a pair (4^2=16) not enough
If 3 (4^3=64) it's more than enough
Since there are 64 possible codes it means some
amino acids has more than one code…read more

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DNA and Chromosomes
Chromosome Structure
Only visible when cell is dividing
2 chromatids 1 centromere
DNA is held together by proteins it's coiled
(DNA combine with protein, DNA-protein complex
genetic variation
is coiled, it forms loops which pack together to
form chromosomes)
They join with partner
Number of chromosomes usually the same within
the species but vary from species to species e.g.
Recombination by crossing over
humans = 46 potato = 48 dogs = 76
Usually equivalent is exchanged
Homologous Chromosomes
Tension is created portions break off
Chromatids twist around one another
They are even because one set of chromosomes is
maternal the other paternal they are known as
homologous pairs the total number is diploid
One pair may contain info on the eyes doesn't mean
New genetic combination created this increases
it's the same
What is an allele?
Two forms of genes each is called allele 1 allele
from 1 parent
allele is different it will code different polypeptide
Different sequence of amino acid, different peptide…read more

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Meiosis and Genetic Variation
Why is Meiosis Necessary
If 2 gametes fuse together with full set of
chromosome that would mean the diploid number
will double (92) this will happen in every
generation so the diploid number has to half this is
due to meiosis
In meiosis the two sets of chromosomes separates
arrangement (see p. 142)
(haploid number)
The Process of Meiosis
1. Meiosis 1: homologous pair up and chromatids
Independent Segregation of Homologous
1. One pair of chromosome is eye colour
wrap around each other, portions may exchange
2. End of meiosis 1 HC segregated into 2 cells
which daughter cell is random (independent
by cross over, the homologous separates one
During Meiosis 1 which chromosome goes to
going to two daughter cells
allele for blood group A/B. 2 arrangements P/Q
3. Segregated into chromatids 4 gametes for each
blue/brown allele the other chromosome carries
2. Meiosis 2: chromatids separate making four cells
each of the cell has 23 chromatids
Meiosis brings genetic variation by 2 ways:
Independent segregation of HC
Recombination of HC by cross over
Locus ­ position of gene on DNA…read more

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Genetic Diversity
Similarities/differences can be defined in variation of
DNA this leads to genetic diversity
of Body
All members of the same species have same genes
e Surface
they differ in their alleles
The greater the number of allele the greater the
Respiring Low
Selective Breeding
Individuals with desired characteristics and pairing
them together, if they don't show desired
Oxygen CO2
tration ration
characteristics they are killed e.g. wheat that has high
gluten, short stem and resistant to disease
Concen Concent of
The Founder Effect
Few individuals from a population colonise a new
region, the alleles wont represent the population
as a whole, this is often seen when new volcanoes
Affinity Result
arise they can develop to new species, less able to
Genetic Bottlenecks
Population drop in numbers, survivors have fewer
alleles e.g. elephant seal in Mexico have less
genetic diversity than southern elephant seals…read more

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