Define DNA.
Deoxyribonucleic acid is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. It carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids.
DNA is present in all cells except plant virus. In eukaryotic cells, DNA is present in the chromosomes of nucleus. Mitochondria and plastids also contain DNA
Who did discovery of DNA?
Rosalind Franklin invented the spiral shape of DNA. James Watson and Francis Crick used the information from Franklin and other scientists to build a 3-D model of DNA
Q 1. What is Chargaff’s Rule?
- Chargaff’s Rule:
- Erwin Chargaff, a biochemist, discovered that the number of nitrogenous bases in the DNA was present in equal quantities. The amount of A is equal to T, whereas the amount of C is equal to G.
- A=T; C=G
- However amount of A+T is not equal to G+C
- The base ratio A+T / G+C may vary from one species to another
- The deoxyribose sugar and phosphate components occur in equal properties
Q 2. Write the name of nitrogenous bases that are found in DNA.
- A 2. There are two types of nitrogenous bases found in DNA
- (1) Purines: Adenine, Thymine
- (2) Pyrimidines: Cytosine, Guanine
Q 3. Describe the components that make up the nucleic acid (DNA).
Components of nucleic acid (DNA):
- Sugar: deoxyribose sugar which is pentose sugar that contains five carbon atoms (C5H10O4), one oxygen atom less than the ribose sugar. The second carbon of deoxyribose bonds with two hydrogen atoms.
- Phosphoric acid (H3PO4): joins with pentose sugar by phosphate diester bond
- Nitrogenous bases: Adenine, Thymine, Cytosine, Guanine
- Theses nitrogenous bases bind with pentose sugar by glycosidic bond
Q 4. Write structure of DNA molecule Or Describe Watson and Crick model of DNA structure
- Watson and Crick in 1953 designed the structure of DNA. According to them, DNA is present in the form of double helix.
- DNA is a nucleic acid. It is a macromolecule. It is made up of two polynucleotide chains. Each polynucleotide is made up of many small units of nucleotides.
- Nucleosides: •In each nucleosides, C-1 of pentose sugar is attached with nitrogen atom (in position 1 of purine or in position 9 of pyrimidine) of the nitrogen base by a glycosidic bond.
- Nucleotides: •A phosphoric acid molecule is linked with the sugar of nucleoside to form a nucleotide. Thus a nucleotide is made up of three components, namely a nitrogen base, a pentose sugar and a phosphoric acid. Many nucleotides are linked together to form a polynucleotide chain. Two nucleotides are joined by a phosphodiester bond. It is formed between the sugar of one nucleotide and the phosphate component of another nucleotide.
- Each DNA molecule has two polynucleotide chains. The nucleotides of adjacent chains are linked. Adenine is always linked with thymine (A-T). Similarly guanine of one chain is linked with cytosine (G-C). •The linking between purines and pyrimidines is brought about by weak hydrogen bonds. There are two hydrogen bonds between A and T (A=T) and three hydrogen bonds between G and C (G C)
- The amount of Adenine is equivalent to the amount of Thymine and the amount of Guanine is equivalent to the amount of Cytosine (Chargaff’s Rule)
- At one end of the polynucleotide chain, the 3rd carbon of the sugar is free and it is not linked to any nucleotide. This end is called 3 prime (3’) end. At the other end, the 5th carbon of the sugar is free and this end is called 5 prime (5’) end.
- The two strands of a DNA run anti-parallel to each other i.e. one chain is upside down to the other.
- In DNA the two polynucleotides chains are coiled around each other to form a double helix
- The width (diameter) of the DNA helix is 20 A0
- The DNA helix has two external grooves, namely Major groove and Minor groove.
- The Major groove is wide and deep. The Minor groove is shallow and narrow.
- The distance between sugar and phosphate is 7 A0
Functions of DNA:
- DNA plays an important role in all bio-synthetic and hereditary functions of all living organisms.
- DNA acts as the carrier of genetic information from generation to generation.
- DNA controls all developmental processes of an organism and all life activities.
- DNA has the genetic information for protein synthesis
Q 6. Explain the mechanism of DNA Replication
- DNA Replication is the duplication process by which a DNA molecule produces exact copies of its own structure
- DNA Replication occurs during interphase. Replication occurs inside the chromosomes. During interphase the amount of DNA doubles and during anaphase of mitosis it is equally distributed between the two daughter cells.
- Watson and Crick suggested mechanism of DNA Replication on the basis of its double helical structure
- During replication the weak hydrogen bonds between the nitrogenous bases of the nucleotides separate, so that the two polynucleotide chains of DNA also separate and uncoil.
- Each chain of the double helix of DNA serves as a template or model on which its complementary chain is built
- The method of DNA replication is described as semi-conservative by Meselson and Stahl. They proved through the experiment that each daughter DNA molecule is a hybrid conserving one parental polynucleotide chain and the other one newly synthesized strand
- DNA replication occurs in all living organisms eg. Prokaryotes, Eukaryotes
- In E. coli (Prokaryotes) the DNA replication involves following steps:
- (1) Initiation (2) Elongation (3) Termination
- Initiation:
- Recognition of the initiation point:
- In E. coli replication starts from a unique single point called orogin of replication otr ‘ori’ or initiation point, where replication fork begins.
- In Prokaryotes only one initiation point or origin per chromosome present.
- Separation:
- The enzyme helicase unwinds the DNA strands. The separation of DNA strands produces an ‘eye’ like structure on the origin called replication fork. The replication fork is theta shaped.
- The replication is bidirectional, the replication fork moves on both the directions from the origin
- Unwinding of DNA:
- The unwinding creates twisting on the double helix. The enzyme DNA gyrase binds to double helix near the replication fork. It removes the twists caused by the unwinding action of helicase.
- Single stranded binding proteins (SSBP) binds to separated DNA strands. They hold the DNA strands and prevent from them from folding.
- RNA Primer:
- The separated DNA strands act as templates. DNA synthesis requires a RNA primer. It is a short strand of RNA polynucleotide chain. It is synthesized by the RNA polymerase and is complementary to DNA stand.
- Formation of DNA :
- The new strands of DNA are formed in the 5’-3’ direction by addition of deoxyribonulceotides to the 3’ end of the primer RNA. The reaction catalysed by the enzyme DNA polymerase III
- Elongation:
- Addition of nucleotides leads to the elongation of the primer nucleotide in the 5’ 3’ direction.
- Leading Strand:
- On the parent DNA 3’ 5’ strand, the daughter strand is synthesized as a continuous strand. This strand is called leading strand because it is synthesized first.
- Lagging Strand:
- On the second DNA strand having 5’ 3’ end, the daughter strand synthesis begins slightly later. Hence the strand is called lagging strand. The lagging strand is synthesized in short polynucleotide fragments called Okazaki fragments. (after R. Okazaki, who first identified them)
- Excision of RNA Primer:
- RNA primer are removed by the enzyme polymerase I.
- In lagging strand, once a small segment of an Okazaki fragment has been formed, the RNA primers removed from 5’ end one by one by the action of 5’ – 3’ exonuclease activity of DNA polymerase I.
- Joining of Okazaki fragments:
- The gaps left between Okazaki fragments are filled with complementary deoxyribonucleotide residues by DNA polymerase I. Finally, the adjacent 5’ and 3’ ends are joined by DNA ligase
- Polymerisation:
- During replication, deoxyribonucleotides are added one by one resulting in the formation of a polynucleotide a chain. This process is called polymerisation.
- Termination:
- The replication stops when the replication fork of the two sides meet at a site called terminus.
- Terminus is situated exactly opposite to origin.
Mendel Genetics
Q. Who is the father of Genetics?
Gregor Johann Mendel is the father of Genetics
Q. What is the phenotypic ratio of Mendel’s dihybrid cross?
Phenotypic ratio of Mendel’s dihybrid cross is 9:3:3:1
Q. Write the Mendel’s Law of Dominance
Law of Dominance:
If the two alleles at a locus differ, then one – the dominant allele determines the organism’s appearance AND the other – the recessive allele has no noticeable effect on the organism’s appearance
Q. Mendel’s Laws of Segregation
Law of segregation states that two alleles for a heritable trait separate during gamete formation randomly and end up in different gametes
Q. Law of independent Assortment
Each pair of alleles segregates independently of other pairs of alleles during gamete formation
Q. Prepare a chart of monohybrid cross
Q. Explain the dihybrid cross of experiment of Mendal
- Dihybrid cross: Two character analysed at a time
- Law of independent assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation
References:
- Jadeja B.A. and Odedra N.K. Botany=II (Angiosperms, Tools and Techniques in Botany, Biochemistry and Genetics: First Ed. 2017, Nirav Prakashan
- Joshi J, Dave R, Sharma M and Bhatt P (2017-18) College Botany (B.Sc. Semester II) Ekta Prakashan & Co.
Britannica: https://www.britannica.com/science/DNA
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