Nature of the Gene and Protein Synthesis
Gene This is a segment of DNA that tell the body how to produce specific proteins – contain the genetic information that is passed from parents to their offspring. These genes are found in structures called chromosomes, which are passed to the embryo during conception. Because the DNA passed from one human to another helps determine gender and physical characteristics, this nucleic acid is necessary for the survival of the species.
Evidences that DNA is the hereditary material
1. DNA is a stable molecule
2. The amount of DNA in a given species is constant for all cells.
3. Mutation or changes in the composition of DNA alter the organisms‟ characteristics
4. Purified DNA rather than proteins from dead virulent bacterium Pneumococcus strain which causes pneumonia was shown to transform the non-virulent form into a virulent form. The ability of DNA to transform nonvirulent form was stopped by addition of an enzyme that breaks DNA in the dead virulent form before DNA was purified.
5. DNA rather than proteins of bacteriophage (T2 phage) that infect E. coli was shown to be the hereditary material because it enabled the E. coli to synthesize the new T2 phages viruses.
Nucleic acids This is a form of genetic material in all living organisms including the simplest viruses. Nucleic acids are polymers made of subunits called nucleotides.
1. Deoxyribonucleic acid (DNA) is found in the nucleus
2. Ribonucleic acid (RNA) is found in both nucleus and cytoplasm. A nucleotide is made up of three molecules:
(i) Phosphate group
(ii) Pentose sugar – either Deoxyribose (in DNA) or Ribose (in RNA)
(iii) Nitrogen base – any purine (Adenine, Guanine) or pyrimidine (Cytosine and either Thymine in DNA or Uracil in RNA)
Structure of nucleotide It is made of phosphoric acid, sugar and an organic base.
(b) Sugar: the pentose sugar in RNA is ribose while that of DNA is deoxyribose sugar. Deoxyribose sugar lacks an oxygen atom on the second carbon atom
(c) Organic Bases: DNA contains four different organic bases; adenine (A), guanine (G) cytosine (C) and thymine (T). RNA also contains adenine (A), guanine (G), cytosine (C) and Uracil (U). all these bases are ring compounds, made of carbon and nitrogen. (i) Pyrimidines (cytosine, uracil and thymine; CUT) have a six-membered ring.
Nucleoside forms when a pentose sugar joins an organic base by condensation reaction (a water molecule is lost).
Nucleotide forms when a nucleoside (pentose sugar + organic base) joins a phosphate by loss of second water molecule.
The sugar-phosphate-sugar backbone is formed when the 3‟ carbon on one sugar joins to the 5‟ carbon on the next sugar by phosphodiester bonds repeatedly to form a polynucleotide (long chain of nucleotides) with organic bases protruding sideways from sugars. Nitrogen base
DNA structure according to Watson and Crick
1. DNA consist of two polynucleotide strands.
2. The polynucleotide stands are antiparallel (face in opposite directions) i.e. one runs from 3‟ to 5‟ direction while another one runs from 5‟ to 3‟ direction.
3. A DNA nucleotide is made up of three molecules:
(i) Phosphate group
(ii) Deoxyribose sugar
(iii) Nitrogen base – either adenine (A), guanine (G), thymine (T) or cytosine (C).
4. Untwisted DNA is ladder-like, in which the sugar-phosphate backbones represent the handrails while the nitrogen base pairs represent the rungs.
5. Twisted DNA forms a double helix of major and minor grooves.
6. The sugar-phosphate-sugar backbone is held by covalent phosphodiester bonds, while the nitrogen bases from the two strands form weak hydrogen bonds by complimentary base pairing i.e. A with T, C with G.
DNA STRUCTURE IMAGE
Population growth and growth curves
Populations grow and decline in characteristic ways. The size of population increase will be determined by the reproductive potential of the organism concerned and by environmental resistance. A Plot of the number of individuals against time form a growth curve.
Two basic forms of growth curves can be identified by the J-shaped growth curve and the S- shaped or sigmoid growth curves.
ADAPTATIONS OF DNA
(i) Sugar-phosphate backbone is held together by strong covalent phosphodiester bonds to provide stability. (ii) The two sugar-phosphate backbones are antiparallel which enables purine and pyrimidine nitrogen bases to project towards each other for complimentary pairing.
(iii) Sugar-phosphate backbones are two / it is double stranded to provide stability.
(iv) The two sugar-phosphate backbones form a double helix to protect bases/hydrogen bonds. (v) Long/large molecule for storage of much information.
(vi) Double helical structure makes the molecule compact to fit in the nucleus.
(vii) Base sequence allows information to be stored.
(viii) Double stranded for replication to occur semi- conservatively/ strands can act as templates.
(ix) There are many hydrogen bonds which increase stability of DNA molecule
(x) There is complementary base pairing / A-T and G-C for accurate replication/identical copies can be made;
(xi) Weak hydrogen bonds enable unzipping /separation of strands to occur readily.
Theories of DNA replication
DNA replication is the process by which the parent DNA molecule makes another copy of itself.
1. Fragmentation hypothesis (Dispersive hypothesis) The parent DNA molecule breaks into segments and new nucleotides fill in the gaps precisely.
2. Conservative hypothesis This suggests that the DNA strands remain intact but in some way initiate the synthesis of new but exact copies of DNA to the parent DNA.
3. Semi-conservative hypothesis The parent DNA molecule separates into its two component strands, each of which acts as a template for the formation of a new complementary strand. The two daughter molecules therefore contain half the parent DNA and half new DNA. The semi conservative hypothesis was shown to be the true mechanism by the work of Meselsohn and Stahl (1958) in their experiment on bacterium E.coli using radioactive 15N.
Necessities of DNA replication
1. Free nucleotides to bond with complementary bases on the separated DNA strands.
2. Energy source in form of ATP
3. Complementary DNA strand
4. Enzymes such as DNA polymerase, DNA helicase and DNA lingaze.
Steps of DNA Replication
1. DNA unwinds and then split open by a Helicase enzyme to expose the bases on either strand. The initiation point is a place rich in A-T probably because these are held by two hydrogen bonds as oppose to the three hydrogen bonds between C and G. The structure that is created is known as “Replication Fork”.
2. An enzyme RNA Primase bind to DNA at the initiation point and attracts the first nucleotide of the 3‟-5‟ strand
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