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Topic 14 - Molecular Genetics
1. DNA and chromosomes
2. Protein synthesis
3. Genetic engineering & biotechnology
Protein synthesis
14-2
Term
Gene
Genetic code
DNA template
Transcription
Translation
Definition
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Sequence of nucleotides
Part or length of DNA / part of chromosome / on a chromosome
A unit of inheritance
Codes for a protein or polypeptide or enzyme / controls a characteristic
Set of rules
Determines how a polypeptide is translated from the information encoded in
DNA
The non-coding strand
Sequence of nucleotides or bases, that
Specify the sequence of amino acids that make up a polypeptide
The process
Genetic information in DNA
Transferred onto a mRNA (messenger RNA)
The process
Genetic information in mRNA
Used to synthesise a polypeptide
Statements (syllabus):
• DNA is used to carry the genetic code, which is used to synthesise specific polypeptides.
• Each gene is a sequence of nucleotides, as part of a DNA molecule.
1. The basic unit of inheritance is called gene.
2. A gene is made up of a short length or segment of a DNA molecule. So a DNA molecule may contain
thousands of genes along its length.
Figure 15-4: Chromosome and Gene
3. One gene contains the genetic code which directs the synthesis of a polypeptide.
4. The type of polypeptide formed is determined by the nucleotide sequence. Each set of three bases forms
a codon, which codes for one amino acid.
5. In DNA molecule, there is no fixed order between the nitrogenous bases within a strand. Thus there is a
total of 43 ways of arrangement in a codon.
6. Different arrangements of nitrogenous bases eg. AAA and ACA code for different amino acids.
7. The code, which specifies which amino acid each codon codes for, has been figured out and it is known
as ‘genetic code’.
8. A sequence of amino acids gives rise to a polypeptide and the folding of this polypeptide produces a
functional enzyme or protein. In turn, this enzyme or protein contributes to the “building up” of all the
characteristics of the organism.
9. Hence, a gene is responsible for the expression of a specific character.
Transcription
10. Since DNA always remains inside the nucleus of the cell while the synthesis of proteins takes place in
the cytoplasm, an intermediary is used to “copy” and then carry the information from DNA to the
cytoplasm.
11. This intermediary is the messenger ribonucleic acid, or mRNA which is made up of RNA.
12. This process of “copying” is also known as transcription.
13. During transcription, the non-coding strand will be the DNA template where complementary base pairing
takes place.
14. Each polynucleotide strand has different names and this depends on which strand is the coding strand.
DNA
Non-coding
Non-sense
Strand 1
template
strand
strand
Non-template
Sense
Coding strand
Strand 2
strand
strand
Coding strand: the DNA strand which has the same base sequence as the mRNA (codons).
Non-coding strand: the DNA strand which has the same base sequence as the tRNA (anti-codons).
15. Deoxyribonucleic Acid (DNA) has the word “deoxy” in front because it lacks an oxygen atom compared
to Ribonucleic Acid (RNA).
16. Ribonucleic Acid is RNA. RNA copies the DNA message from the nucleus and carries it out to the
cytoplasm where the protein molecules are to be synthesized.
17. RNA is similar to DNA but differs structurally in 4 ways shown in the table below.
DNA
RNA
Number of strand(s)
2
1
Structural make-up
1 less oxygen atom; hence the
name “deoxyribose” in front
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
1 more oxygen atom; hence
the name “ribose” in front
Adenine (A)
Uracil (U)
Cytosine (C)
Guanine (G)
Nucleotide Bases
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DNA
RNA
Uracil replaces Thymine
Pairing of nucleotide bases
A-T
C-G
A-U
C-G
Single Strand
NH²
Cytosine
H
(C)
C
Guanine
(G)
H H
Deoxy
Ribose
C
N
C
C
H
N
H
O
||
C
N C
O
N
H
NH²
Cytosine
H
(C)
C
C
N
C
C
H
Ribose
H
N C N C N
H
H
Adenine
(A)
Double Strands
Nitrogenous
bases
N
H
Guanine
O
||
(G)
H
N C C
N
H H
H
C
C
N
N
N
H
H
Adenine
NH²
||
(A)
NC
NC C
N
N
H C
H C
C
C
N C
N C
H
N
H
N
H
H
Sugar
Phosphate
Thymine
Uracil
O
O
bases
(T)
(U)
H
C
H
C
C
³
H
H
C
C
N
N
C
C
C
C
H
O RNA
N
H
N
O
DNA
Ribonucleic acid Deoxyribonucleic acid
H
H
NH²
||
C
Base
Figure 14-3: Comparing RNA and DNA
Translation
18. The mRNA leaves the nucleus after the completion of transcription. It then travels to the cytoplasm
where protein synthesis occurs.
19. The mRNA is used to give instructions for the construction of protein molecules.
20. Different arrangements of nitrogenous bases eg. AAA and ACA code for different amino acids
21. Different amino acids in a chain would eventually lead to production of different polypeptides or proteins
which then give rise to variation in characteristics we see in human.
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22. Although genes get a lot of attention, it is the proteins that perform most life functions and make up the
majority of cellular structures.
23. Proteins:
• is the more complex form of polypeptides. Poly (many) peptides (smaller segment of a polypeptide).
A peptide can be broken down into even smaller subunits known as amino acids as illustrated below.
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protect and provide the structure to our body because they form our skin, hair, and muscles. Thus
proteins determine your physical traits.
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protect our body chemistry because our enzymes (digestive enzymes), hormones and antibodies are
all made of proteins.
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help in the transportation of oxygen and other important substances in our body.
24. When a gene encodes a protein, the gene is said to be expressed. Our physical traits are a result of the
expression of different genes in our cells.
Figure 14-1: Protein
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25. (Extra information/ out of syllabus) Synthesis of protein is a result of DNA, RNA and amino acids
involving the 2 stages: Transcription and Translation.
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There are three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal
RNA (rRNA)
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Transcription is a process whereby the genetic information stored in the DNA of a specific gene
is “copied” to a strand of newly produced mRNA. In other words, the DNA is to be transcribed.
This process takes place in the nucleus of a cell. Transcription takes place in 3 steps:
(1) The 2 strands of DNA unwind.
(2) RNA nucleotides pair with the DNA nucleotides of one DNA strand.
(3) When the information of a whole gene has been “copied” finished, transcription ends. The 2
strands of DNA winds up again while the single strand of mRNA leaves the nucleus for the
cytoplasm carrying the genetic sequence for the synthesis of protein.
•
The diagrams below show the process of transcription.
DNA to be transcribed unwinds
Transcribed DNA winds up
Pairing up of DNA – RNA Nucleotide Bases
Transcription ends and mRNA leaves nucleus
Figure 14-8: Transcription
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Translation is the process whereby proteins are synthesized or produced. During this process,
mRNA works together with the tRNA, ribosomes (contain rRNA) and amino acids in the
cytoplasm to produce a specific chain of polypeptides or protein. The diagram below shows
the process of translation.
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Protein
Amino Acid
tRNA
A strand of mRNA
Ribosome contains
rRNA
Figure 14-9: Translation
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To give an illustration of transcription and translation, suppose a specific gene contains the
information for having sharp nose. This information is stored in the DNA of the gene. During
transcription, the information is copied to mRNA in the nucleus. Then the mRNA leaves the
nucleus for the cytoplasm and combines with the tRNA, ribosome and amino acids to produce
the specific protein that will give you the trait of having a sharp nose. Now the specific gene is said
to be expressed.
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A codon is a triplet nucleotide bases that codes for a particular amino acid, and allows the
establishment of bonding with a specified tRNA carrying a specified amino acid to be bonded to
the polypeptide being developed.
http://www.youtube.com/watch?v=41_Ne5mS2ls&feature=related
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In summary, the flow of genetic information is shown in the diagram below:
Nucleus
DNA
Transcription
RNA
Translation
Cytoplasm
Protein
Figure 14-10: Protein Synthesis in a Cell
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