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OCR AS Biology…read more

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DNA - Structure Nucleotide
·DNA is a polynucleotide
·Each nucleotide is made up of deoxyribose (a
pentose sugar) phosphate and a nitrogenous base
·Each nucleotide has the same sugar and
phosphate however the base can vary.
·Bases; Adenine, Guanine, Thymine, or Cytosine
·DNA has complementary base pairing, G pairs
with C and A pairs with T
·Three hydrogen bonds form between G and C,
Two hydrogen bonds form between A and T.
·Adenine and Guanine are Purine, Thymine and
Cytosine are Pyrimidines.
·Two anti parallel polynucleotide strands twist to
form the DNA double helix…read more

Slide 3

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DNA - Genes
·DNA molecules are really long and coiled up
very tightly so a lot of genetic information
can fit into a small space in the cell nucleolus
·DNA has a paired structure which makes it
easier to copy itself. This is called self
replication. It's important for cell division
·A gene is a sequence of DNA nucleotides that
codes for a protein (polypeptide)
·Proteins are made from amino acids
·Different proteins have a different number
and order of Amino acids
·The order of nucleotide bases in a gene
determine the order of amino acids in a
particular protein
·Each amino acid is coded for by a sequence of
three bases in a gene
·Different sequences code for different Amino
acids, there are 20 different Amino acids.…read more

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·The sugar in RNA nucleotides is ribose sugar (not deoxyribose)
·The nucleotides form a single polynucleotide strand
·Uracil replaces Thymine as a base. U always pairs with Adenine
Three forms of RNA
1) Messenger RNA (mRNA) is made of a strand complementary to one strand of a DNA
molecule, it is therefore a copy of the other DNA strand of the double helix
2) Ribosomal RNA (rRNA) is found in the Ribosomes, usually shown as a two lobed
3) Transfer RNA (tRNA) carried Amino acids to the Ribosomes where they are bonded
together to form polypeptides.…read more

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Semi-Conservative Replication
DNA replication takes place in inter-phase
and it a process that creates identical sister
1) The double helix is untwisted
2) Hydrogen bonds between the bases are
broken apart, this exposes the bases
3) Free nucleotides are hydrogen bonded onto
the exposed bases according to the
complimentary base pairing rules
4) Covalent bonds are formed between the
phosphate of ones nucleotide and the sugar
of another
5) Each DNA strand now consists of one
conserved strand and one newly built strand.…read more

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Enzymes are globular proteins,
with a specific tertiary structure,
which catalyse metabolic reactions
in living organisms. Enzyme action
may be intracellular or
Enzymes are proteins and thus Lock and Key model
have a specific shape. They are The shape of the active
therefore specific in the reactions site is complementary to
that they catalyse - one enzyme will the shape of the
react with molecules of one substrate. Together they
substrate. form an enzyme-
The reaction occurs in the active substrate complex. They
site. Since the active site for all
fit together hence the
molecules of one enzyme will be
made up of the same arrangement
name lock and key model.
of amino acids, it has a highly We now think the induced
specific shape. fit hypothesis is more
realistic…read more

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