Download Molecular Genetics

Molecular
Genetics
Lesson Objectives
• What is DNA?
• What is gene?
• Protein production
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DNA: Deoxyribonucleic acid
• Molecule that carries genetic information.
• Found in the nucleus of cells.
• Genetic information is important for all cellular
activity and functions.
• Almost all cells in your body contains DNA.
• Name some examples of cells that do
not contain DNA.
• About 2 metres of DNA is found in each cell
nucleus.
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How is DNA organized inside
your cells?
• Each DNA consists of two parallel strands.
• The two strands are twisted around each
other to form a double helix.
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• A molecule of DNA is
wrapped around proteins to
form a chromatin thread.
• Chromatin threads are tightly
coiled into chromosomes
inside the cell nucleus.
DNA wrapped around
proteins to form chromatin threads
nuclear pore
Chromatin threads coiled
tightly to formed chromosomes
nucleus
nuclear envelope
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What is a gene?
• A gene is a small segment of DNA that
contains information (instruction) to make
a single protein.
• Each protein is responsible for determining
a particular characteristic of an
organism.
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Basic units of DNA
• The basic unit of DNA is called a
nucleotide.
Each nucleotide is made up of:
• a sugar called deoxyribose
• a phosphate group
• a nitrogen-containing base
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4 types of nitrogen-containing
bases
•
•
•
•
Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)
4 types of nitrogen-containing bases
deoxyribose
Phosphate
group
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Nucleotides are the building
blocks of DNA
• Deoxyribose, nitrogen-containing bases
and phosphate group are joined together to
form four different nucleotide molecules.
Adenine nucleotide
thymine nucleotide
guanine nucleotide
Cytosine
nucleotide
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• Nucleotides can be joined together to form long chains
called polynucleotides.
• Since there are four different nucleotides,
they can be arranged at many different
combinations.
• Hence the sequence of polynucleotides varies.
• A gene is made up of a sequence of nucleotides.
• Different genes will have different sequence of
nucleotides
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Putting nucleotides together
• DNA is made up of two parallel strands joined
together.
• The two chains run in opposite direction.
• They are said to be anti-parallel.
• The two strands are bonded by the
bases according to the rule of base
pairing.
Sugar and
Phosphate groups
base
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Rule of base pairing
•
•
•
•
Adenine (A) always pairs with Thymine (T)
Cytosine (C) always pairs with Guanine (G)
(A) and T are complementary bases
(C) and (G) are complementary bases
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Model of a DNA molecule
• An uncoiled DNA molecule can be represented by a
model which resembles a ladder.
• Note that the two strands are anti-parallel
• The two stands will twist and coil to
make a double helix.
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Genes
• A DNA molecule may contain many genes along its
length.
• A gene is a small segment of DNA which controls the
formation of a single protein.
• Each gene stores information or message that
determines how a protein should be made in the cell.
• This protein will determine a particular characteristic of
an organism.
• If the message to make a particular protein is altered
(mutation), your body structure or function may be
affected.
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gene
Part of DNA molecule unzip to show a gene
A gene is a segment of DNA which
contains message to a protein
A protein coded by the gene
The message stored by the gene to make a protein is called the genetic code
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Structure of a gene
• A gene is a sequence of nucleotides to make
proteins.
• Proteins are made up of amino acids.
• Hence the sequence of nucleotides will code for
different amino acids.
• In a DNA sequence, three bases will
code for one type of amino acid.
• This is known as the triplet code or
codon.
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• To see the sequence of a gene in a DNA, the DNA must
first be unziped to form a DNA template.
• Let us say the triplet codes for the following amino acids
are as follows:
Triplet
code
TAC
TAT
CAT
GAG
ACA
Amino
acid
coded for
M
A
K
E
S
DNA template
Polypeptide made up
of five amino acids.
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• Note that a single gene carries message to
synthesize only one polypeptide.
• If a protein is made up of many
polypeptides, more that one gene will be
involved in the synthesis of this protein.
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How are proteins made?
• Proteins are made in a two-step process.
• Transcription and Translation
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1.
2.
DNA must be unzipped to exposed the template DNA strand.
The message of the gene on the template DNA must be copied
into a RNA molecule called messenger RNA (mRNA)
This copying process is known as transcription.
Transcription occurs in the nucleus.
Three bases in the mRNA makes up a
codon.
DNA template
Transcription
(transcription follows the base pairing rule)
mRNA contains uracil (U) instead of thymine (T)
(U) Pairs with (A)
mRNA
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3.
After transcription, mRNA detached itself from DNA
template and carries the message out of nucleus into
cytoplasm.
The DNA molecule revert back into its double helix
structure.
A ribosome helps to convert (translate) the message
in the mRNA into a protein molecule.
This is called translation.
transcription
translation
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Transcription and Translation
1
part of a gene
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Transcription and Translation
1
part of a gene
First, the gene unzips.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
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Transcription and Translation
template
1
One of the strands in the gene is
used as the template to make
mRNA. This is transcription. The
mRNA molecule copies the genetic
code in the DNA template,
following the rule of base pairing.
Note that mRNA does not contain
T (thymine). A (adenine) in DNA
pairs with U (uracil) in mRNA.
Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.
mRNA molecule is
made
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Transcription and Translation
mRNA molecule is
made
2
The mRNA leaves the nucleus
and attaches to a ribosome in
the cytoplasm.
nuclear
envelope
nuclear pore
mRNA
ribosome
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What is RNA?
• Ribonucleic acid.
DNA
RNA
Sugar unit is deoxyribose
Sugar unit is ribose
Bases are A, C, T and G
Bases are A, U, C, G
Ratio of A:T and C:G is
1:1
No fixed ratio
Large insoluble molecule
Small soluble molecule
Permanent molecule in
nucleus
Temporary molecule
(made only when
needed)
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Control of genes
• Cells can control their genes.
• Each cell in the body contains a complete
set of genes.
• However, many of these genes are
switched off.
• Genes that are switched off do not
produce any proteins.
• We say that such genes (switched off) are
not expressed.
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• Different cells express different genes.
• For example the genes for insulin production can
be found in both liver cells and pancreas cells.
• Since only pancreas produce insulin and not
liver, gene for insulin production in liver is not
expressed (switched off). It is only expressed in
the pancreas (switched on).
• Pancreas cells can also control when they want
to switch on or off the gene for insulin
production.
Pancreas switched the genes on when they
need to produce insulin and switched them off
when they do not need to produce insulin.
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