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HUMAN BIOCHEMISTRY
NUCLEIC ACIDS
THE GENETIC CODE
STRUCTURE OF NUCLEOTIDES
AND NUCLEIC ACIDS
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Nucleic acids are very long thread-like macromolecules with altering
sugar and phosphate molecules forming the ’backbone’. There are two
types of nucleic acid found in living cells: deoxyribonucleic acid (DNA),
and ribonucleic acid (RNA). Nucleic acids are polymers made up of
nucleotides.
Nucleotides join together to form nucleic acids. A nucleodite consists of
three substances combined together. These are:
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A nitrogenous base the four bases are cytosine (C), guanine (G),
adenine (A), thymine (T).
A pentose sugar deoxyribose occurs in DNA and ribose in RNA.
A phosphoric acid.
Nucleic acids are joined by covalent bonds between the phosphate of
one nucleotide and the sugar of the next, resulting in a backbone with
a repeating pattern of sugar–phosphate–sugar–phosphate.
Nitrogenous bases are attached to the sugar of the backbone.
A NUCLEODITE, A NUCLEIC ACID
AND THE FOUR NITROGENCONTAINING BASES
THE DNA AND RNA MOLECULES
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The DNA molecule consists of two polynucleotide strands paired
together and held by hydrogen bonds. The two strands take the shape
of a double helix. The pairing of bases is between adenine (A) and
thymine (T), and between cytosine (C) and guanine (G). Adenine and
thymine form 2 hydrogen bonds when they combine and cytosine and
guanine form 3 hydrogen bonds.
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The RNA molecule is a single strand of polynucleotide in which the
sugar is the ribose the bases found in RNA are cytosine (C), guanine
(G), adenine (A) and uracil (which replaces thymine of DNA). There are
three types of RNA, known as messenger RNA (mRNA), transfer RNA
(tRNA) and ribosomal RNA.
DIFFERENCES BETWEEN THE
STRUCTURES OF DNA AND RNA
FEATURE
DNA
RNA
NUMBER OF
STRANDS IN THE
MOLECULE
Two strands forming
a double helix
One strand only
TYPE OF SUGAR IN
EACH NUCLEOTIDE
TYPES OF BASES
CONTAINED
Deoxyribose
(Deoxyribose lacks
an oxygen atom
on C2)
A, C, G and T
Ribose
A, C, G and Uracil
replaces thymine
DNA AND RNA
REPRESANTATION
DNA REPLICATION
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The cell produces many free nucleotides for DNA replication.
Helicase uncoils the DNA double helix and splits it into two template
strands.
RNA primase adds a short length of RNA attached by base pairing to
the template strand of DNA. This acts as a primer, allowing DNA
polymerase to bind and begin replication.
DNA polymerase III adds nucleotides in a direction 5’-3’ next to the
RNA primer.
Short lengths of DNA are formed between RNA primers, called Okazaki
fragments.
DNA polymerase I removes the RNA primer and replaces it with DNA.
A nick is left where two nucleodites are still unconnected. This nick is
sealed up by DNA ligase.
DNA REPLICATION
REPRESENTATION
DNA TRANSCRIPTION
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The one strand that forms the template and is transcribed is called
antisense strand and the other one is called sense strand.
DNA is unwound by the front of RNA polymerase.
Free nucleoside triphosphates are used by RNA polymerase to extent
the growing mRNA molecule.
Two phosphates are removed as they are linked on, converting them
into RNA molecules.
The 5’ end of the nucleotide is added to the 3’ end of the growing
chain-transcription thus moves in a 5’3’ direction.
DNA is rewound into a double helix by the rear of RNA polymerase
REPRESENTATION OF DNA
TRANSCRIPTION
DNA TRANSLATION
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Messenger RNA carries the information needed for making
polypeptides out from the nucleus, to the cytoplasm of eukaryotic cells.
The information is in a coded form which is decoded during translation.
The base sequence of mRNA is translated into the amino acid sequence
of a polypeptide.
Three bases (codon) code for one amino acid and this is why the
genetic code is called a triplet code.
It is possible for two or three codons to code for the same amino acid
and this is why the genetic code is called degenerated.
DNA TRANSLATION
REPRESENTATION
TRANSLATING THE GENETIC CODE
DNA PROFILING
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Humans and other organisms have short sequences of bases that are
repeated many times called satellite DNA. This satellite DNA varies
greatly between different individuals in the number of repeats.
If it is coped using a methods which is called PCR and then cut up into
small fragments using restriction enzymes, the length of the fragments
vary greatly between individuals.
Gel electrophoresis can be used to separate fragmented pieces of DNA
according to their charge and size.
The pattern of bands on the gel is very unlikely to be the same for two
individuals.
This technique has many applications, including forensic investigations
and investigating paternity.
REPRESENTATION OF DNA
PROFILING