Download DNA (Deoxyribonucleic acid)

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Biology AS
DNA (Deoxyribonucleic acid)
A DNA molecule is a chain of nucleotides. A nucleotide is a 5-carbon sugar molecule
joined to a phosphate group (–PO3) and an organic base.
PO3
Adenine
phosphate
pentose sugar
(deoxyribose)
organic base
Above is an example of a nucleotide.
In DNA, the sugar is deoxyribose and the organic base is either:
A
T
-
adenine
thymine
C
G
-
cytosine
guanine
The nucleotides are joined by their phosphate groups to form a long chain: often thousands
of nucleotides long. The phosphate and sugar molecules are the same all the way down, but
the organic bases may be any of the four listed.
The sequence of bases down the DNA molecule forms a code, which instructs the cell to
make particular proteins. Proteins are made from linked amino acids. Different amino acids
produce different types of proteins.
A DNA molecule consists of two chains of nucleotides, which run anti-parallel to each
other: the two chains are joined by hydrogen bonds between the bases. This forms a double
helix (same shape as a twisted ladder).
DNA double helix:
Phosphate group
Bases
Pentose sugar
A
T
Hydrogen bonds
G
C
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The ladder is twisted (double helix). Note the base pairings:
Adenosine only pairs with Thymine
Cytosine only pairs with Guanine.
From this, it is clear that if you know the pairings on one side, you can work out the pairings
on the other side (because of the specific complementary base pairings).
Fill in the organic bases for the following (only the first letter of each organic base should
be used):
A
C
T
A
The Genetic Code
C
-
DNA carries instructions (i.e. a code) for the assembly of amino acids (which
form proteins)
-
If one base was equal to one amino acid, the maximum codes would be 4.
If two bases was equal one amino acid, the maximum would be 16 codes.
If three bases equal one amino acid, the maximum codes would be 64.
There are at least 20 amino acids present in the human body.
the triplet code of bases provides sufficient amino acid codes for instructions to
produce or stop the assemble of proteins: three bases equal one amino acid.
-
some amino acids have more than one code
Mutation (as will be discussed later) can be explained by the fact that if a base is inserted
into the wrong place, the wrong amino acid is added. This results in a different protein being
produced.
The code is carried by a sequence of one of the DNA strands. This is called the “SENSE”
strand. The other strand is called the “NON-SENSE” strand. The DNA is universal: it is
interpreted by every organism in the same way.
OK……so that was all GCSE stuff…let’s get on to the more advanced stuff….fun fun fun!!
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Biology AS
The difference between DNA and RNA
Ribonucleic acid (RNA)
Bases include :
Adenine,
Uracil,
Cytosine,
Guanine
Ribose as pentose sugar
Single stranded
Generally smaller
Deoxyribonucleic acid (DNA)
Bases include:
Adenine,
Thymine,
Cytosine,
Guanine
Deoxyribose as pentose sugar
Double stranded
Generally larger
Complementary base pairing
As we have previously mentioned, the bases pair together using hydrogen bonds. However,
the bonding between the pairs is different according to the class of base. There are two
different classes:
Purines
-
Adenine and Guanine
-
these have two rings of carbon and nitrogen atoms
Pyrimidines -
Cytosine and Thymine/Uracil
these have a single ring of carbon and nitrogen atoms
PURINES ALWAYS BOND TO A PYRIMIDINE.
When A bonds with T, there are two hydrogen bonds holding them together.
When C bonds with G, there are three hydrogen bonds holding them together.
The importance of DNA
OK, so that’s all well and good…..but……WHO CARES?!
-
ALL chemical reactions in a cell are controlled by enzymes
-
Enzymes are proteins
-
DNA codes for amino acids (which make up proteins)
-
Therefore, DNA controls the activities of a cell
The way in which DNA codes for amino acids is complex. However, we MUST look at this
in detail (yay!)
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Biology AS
The Triplet Code
-
the sequence of bases in a DNA molecule code for a sequence of amino acids
-
the code for each amino acid is three letters or a triplet code
-
each sequence of three bases code for one amino acid
-
three letters code for one amino acid
-
three bases together code for one amino acid
-
that’s right…I’ve said it enough times…hopefully you have it in your head now!
-
the three bases together are called “codons”
-
a codon codes for an amino acid:
o For example:
C-A-A (on the sense strand) = VALINE
T-T-T (on the sense strand) = LYSINE
The genetic code is non overlapping and degenerate
Non-overlapping
-
there is a specific codon that indicates the start of the code
-
this is always AUG
-
AUG codes for methionine
-
by having this start codon, it ensures that the code is always read correctly and “in
frame”
E.g.
AUCGUAUGTTTCAA….
will code for methionine and then lysine and valine
Degenerate
-
some amino acids are coded for by more than one codon
-
e.g. GUC, GUG, GUA and GUU all code for Valine
which one of these is the code from the valine sense strand shown above?
-
there are 20 different amino acids even though there are 64 different
arrangements of A, U, C and G
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Biology AS
Protein Synthesis
In the nucleus
DNA is split by DNA helicase
-
as it is split, free nucleotides bind to the exposed bases of the NON-CODING
STRAND (this is because it is complementary – it is the opposite of what will be
on the tRNA – which is the actual amino acid)
-
this process is helped by RNA polymerase
-
the nucleotides join together to form mRNA (messenger RNA)
-
the DNA closes back together
-
the mRNA formed is then spliced:
o in this process, the introns (which are NON-CODING) are removed
o the exons (which are CODING) are kept
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In the cytoplasm
the spliced mRNA then moves out of the nucleus via a nuclear pore
-
the mRNA goes to a ribosome
-
the first triplet code to be read is AUG. This codes for the start codon
(methionine)
-
the codes on the mRNA are called codons
-
the ribosome can hold 2 triplet codes together…
-
the mRNA enters the ribosome and as it does so, tRNA (transfer RNA) comes
and joins complementary bases to it. The complementary triplet bases on the tRNA
are called anti-codons.
-
a tRNA molecule is made up of three bases at one end and an amino acid at the
other end
-
many different tRNA molecules are present in the cytoplasm for this to occur
-
this continues and as it does, the amino acids line up (2 at a time – as each
ribosome can hold 2 triple codes) and bond together by peptide bonds catalysed
by the enzyme peptidyl transferase.
-
This continues until a stop codon is reached. The stop codons do not code for any
amino acid, but stop the translation.
-
many amino acids make up a protein
-
the mRNA that moves along can simply be re-used again and again
Translation animation
Highly recommended….very nice way to put it all together
http://www.maxanim.com/genetics/Protein%20Synthesis/Protein%20Synthesis.htm
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Biology AS
A
T
G
C
T
A
base pairs
A
T
G
C
T
A
sugar phosphate backbone
A
A
T
C
G
C
G
The enzyme DNA helicase breaks the
hydrogen bonds between the cases causing
the strands to separate (to unzip) from one
end.
One original molecule of DNA
A
T
T
A
G
T
C
T
A
G
C
T
A
T
C
G
A
A
A
T
C
T
G
G
C
C
G
G
T
C
A
G
C
Complementary bases join with hydrogen
bonds and free nucleotides link up to for a
new strand
The exposed bases attract the
complementary bases of free DNA
nucleotides. (e.g. C will only attract G)
A
T
G
C
T
A
original (old) DNA strand
A
T
G
C
T
A
A
T
C
G
new strand
Two exact copies of the
original DNA molecule have
been made
A
T
C
G
The sequence of the bases is
kept exactly the same due to
specific base pairing
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Biology AS
The Human Genome Project
Ethical issues
-
A part of a DNA molecule which codes for a single polypeptide is called a
gene.
In humans, it is estimated that there are about 140 000 genes
The total set of genes is called the genome
The genome represents the genetic code for a particular organism
The human genome project was a project to find out the FULL base
sequence of the human genome
There are about 3 billion bases in the human genome. The aim is to
identify each human gene
This was completed in 2003
gene testing:
o can improve lives but has problems such as abortion issues
possibility of cloning
SYLLABUS CHECKLIST
Unit 1.1 – Molecules
Nucleic Acids
• understand that ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are
composed of mononucleotides;
• recall the basic structure of a mononucleotide; thymine, uracil and cytosine as
pyrimidines; adenine and guanine as purines;
• understand that condensation reactions are involved in the formation of
mononucleotides and polynucleotides (DNA and RNA);
• recall the structure and understand the roles of messenger and transfer RNA;
• recall the structure of DNA; understand base pairing; understand the double helix;
• understand the mechanism of replication of DNA (semiconservative); understand the
nature of the genetic code;
• understand that a gene is a sequence of bases on the DNA molecule which codes for
a sequence of amino acids in a polypeptide chain;
• understand the processes of transcription and translation in the synthesis of proteins;
understand that amino acid sequences are specified by DNA, and know the function
of the ribosomes;
• understand codons and anticodons in relation to messenger and transfer RNA;
• appreciate the Human Genome Project in the light of the structure and roles of
nucleic acids; consider the spiritual, moral, ethical, social and cultural issues of this
project.
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