CLASS 12TH BIOLOGY CHAPTER- 6 MOLECULAR BASIS OF INHERITANCE NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM

CHAPTER 6: MOLECULAR BASIS OF INHERITANCE

Search for genetic material and DNA as genetic material; Structure of DNA and RNA; DNA packaging: DNA replication; Central dogma; transcription, genetic code, translation; gene expression and regulation lac operon; genome and human and rice genome projects; DNA fingerprinting.

Anticodon: A sequence of three nitrogenous bases on tRNA which is com plementary to the codon on mRNA.

Genome : Sum total of genes in haploid set of chromosomes.

DNA Polymorphism: The variations at genetic level, where an inheritable mutation is observed, in a population at high frequency.

Satellite DNA: The repetitive DNA sequences which form a large portion of genome and have high degree of polymorphism but do not code for any proteins. Operon: A group of genes which control a metabolic pathway.

Exons: The regions of a gene which become part of mRNA and code for different regions of proteins.

Introns: The regions of a gene which are removed during the processing of mRNA.

Euchromatin: The region of chromatin which is loosely packed and transcriptionally active, it stains lighter.

Heterochromatin: The chromatin that is more densely packed, stains dark and is transcriptionally inactive.

Splicing: The process in eukaryotic genes in which introns are removed and the exons are joined together to form mRNA. 

Bioinformatics : Science of use of techniques including statistics, storing as data bases, analysing, modelling and providing access to various aspects of biological information usually on the molecular level.

Central Dogma:

Replication fork: The Y shaped structure formed when double stranded DNA is unwound upto a point during its replication.

VNTR : Variable Number of Tandem Repeats Glycosidic bond (N-gylcosidic linkage)-A linkage between a nitrogenous base and a pentose sugar to form a nucleoside. Phosphodiester bond - The bond between two adjacent nucleutides to two adjacent sugar modecules at 3' and 5' positions with phosphate group. Tandem Repeat-(One behind the other)-A DNA segment in which a nucleotide sequence is repeated one after another two or more times eg ATTCCGATTCCG ATTCCG is a tandem repeat in which the sequence ATTCCG is repeated threetimes. KB-Kilobase-A unit for length for nucleic acids consisting of 1000 nucleotides abbreviated kb or kbp (kilobase pairs) DNA. Oncogene-A gene that induces uncontrolled cell proliferation.

YAC : Yeast Artificial Chromosome

BAC : Bacterial Artificial Chromosome

SNPS: Single Nucleotide polymorphism

HGP: Human Genome Project

hnRNA: Heterogenous nuclear RNA. It is precursor of mRNA.

Chemical Structure of Polynucleotide Chain (DNA/RNA): A nucleotide has three components

1. Nitrogen base

(i) Purines: Adenine and Guanine

(ii) Pyrimidines: Cytosine, Thymine and Uracil (Thymine in DNA and Uracil in RNA.)

2. Pentose Sugar: Ribose (in RNA) or Deoxyribose (in DNA).

3. Phosphate Group

• Nitrogen base is linked to pentose sugar through N-Glycosidic linkage. 

• Nitrogen base + Sugar - Nucleoside

• Phosphate group is linked to 5'-OH of a nucleoside through phosphoester linkage.

• Nucleoside + Phosphate group = Nucleotide

• Two nucleotides are linked through 3'-5 phosphodiester linkage to form a dinucleotide 

• A polynucleotide chain has free phosphate group at 5' end of ribose sugar and a free 3'-OH group at other end.

RNA is highly reactive than DNA: In RNA nucleotide has an additional OH group at 2' positions in the ribose; RNA is also catalytic.

Double-helix Structure of DNA: Proposed by Watson and Crick in 1953.

(i) DNA is made up of two polynucleotide chains.

(ii) The backbone is made up of sugar and phosphate and the bases project inside.

(iii) Both polynucleotide chains are antiparallel i.e. one chain has polarity 5'-3' and other chain has 3'-5'..

(iv) These two strands of chains are held together by hydrogen bonds i.e. A=T, C=G.

(v) Both chains are coiled in right handed fashion. The pitch of helix is 3.4 nm with 10 base pairs in each turn.

PACKAGING OF DNA HELIX 

¶ The average distance between the two adjacent base pairs is 0.34 nm (0.34 x 10 m or 3.4 Å)

¶ The number of base pairs in Escherichia coli is 4.6 × 10%,

¶ DNA Packaging in Prokaryotes: DNA is not scattered throughout the cell, DNA (negatively Charged) is held by some proteins (has positive charges) in a region termed as nucleoid. The DNA in nucleoid is organised in large loops held by proteins.

¶ DNA packaging in Eukaryotes: There is a set of positively charged basic proteins called histones. Eight histone molecules combine together to form histone octamer.

¶ The negatively charged DNA is wrapped around positively charged histone octamer to form as structure called nucleosome.

¶ Histone HI is situated outside of nucleosomal DNA in linker region.

¶ Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin.

¶ The beads-on-string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to form chromosomes.

¶ The packaging of chromatin at higher level requires additional set of protein that collectively are referred to as Non-histone chromosomal (NHC) proteins. At some places chromatin is density packed to form darkly staining heterochromatin. At other places chromatin is loosely packed to form. euchromatin.

¶ Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive.

TRANSFORMING PRINCIPLE:

Frederick Griffith (1928) performed experiments with Streptococcus peumoniae and mice. This bacterium has two strains.

1. S-strain (Virulent)-which possess a mucilage coat and has ability to cause pneumonia. 

2. R-strain (Nonvirulent) which do not possess mucilage coat and is unable to cause pneumonia.

¶ Griffth injected R-strain bacteria into mice.

→ No disease noticed and mice remain live.

¶ On injecting S-strain bacteria into mice.

→ Mice died due to pneumonia.

¶ When heat-killed S-strain bacteria were injected into mice No pneumonia symptoms noticed and mice remain alive.

¶ He than injected a mixture of R-strain bacteria (Non virulent) and heat killed S-strain bacteria (virulent) into mice → mice died due to pneumonia.

¶ Moreover Griffith recovered living S-strain (virulent) bacteria from the dead mice.

Conclusion: He concluded that presence of heat-killed S-strain bacteria caused transformation of some R-strain bacteria into virulent by a chemical substance, called 'transforming principle'. But biochemical nature of the genetic material was not defined by him.

CHEMICAL NATURE OF TRANSFORMING PRINCIPLE

In 1944, Avery, MacLeod and McCarty worked to determine the chemical nature of 'transforming principle".

Hershey and Chase Experiment: In 1952, Hershey and Chase performed an experiment on bacteriophages (Virsues that infect bacteria) and proved that

DNA is the genetic material.

MESSELSON AND STAHL'S EXPERIMENT:

¶ Messelson and Stahl performed the experiment in 1958 on E. coli to prove that DNA replication is semiconservative

¶ E. coli was grown in 15NH4Cl for many generations.

¶ N15 was incorporated into newly synthesised DNA.

¶ This heavy DNA could be differentiated from normal DNA by centrifugation in cesium chloride (CSCI) density gradient.

¶ Then they transferred these E.coli into medium with normal 14NH4CI.

¶ After 20 minutes, it was found that all the DNA molecules of daughter cells were hybrid-First generation.

¶ After 40 minutes, it was found that 50% DNA molecules were hybrid and 50% were normal-second generation.

DNA REPLICATION

DNA strands start separating from ori (origin of replication). This unwinding is catalysed by many enzymes. Y-shaped structure is formed at ori called replication fork.

                                       ↓

DNA polymerase attaches to the replication fork and add nucleotides complementary to the parental DNA strand. The direction of polymerisation is 5'-3'.

                                      ↓ 

DNA polymerase cannot initiate the polymerisation itself, so a small segment of RNA called primer is attached at replication start point.

                                     ↓

DNA polymerase adds nucleotides on one of the template strand, called as leading strand (the template with polarity 3'-5'). In this strand nucleotides are added continuously therefore called as continuous replication.

                                    ↓

On the other strand the replication is discontinuous, small fragment of DNA are formed called okazaki fragments which are later joined by DNA ligase. This strand is called as lagging strand.

                                   ↓

Accuracy of polymerisation is maintained by Proof reading and any wrong base added is removed.

Transcription in Prokaryotes: In prokaryotes the process of transcription is completed in three steps:

1. Initiation: RNA polymerase binds with initation factor (sigma factor) and then binds to promotor site. 

2. Elongation: RNA polymerase separates from sigma factor and adds nucleoside triphosphate as substrate. RNA is formed during the process following the rule of complementary and remains bound to enzyme RNA

polymerase. 

3. Termination: On reaching terminator region, RNA polymerase binds with rho factor (terminator factor) as a result nascent RNA separates.

TRANSCRIPTION IN EUKARYOTES:

¶ In eukaryotes three types of RNA polymerases are found in the nucleus. (In addition to the RNA polymerase found in the organelles) are involved in transcription.

RNA Polymerase I: Transribes rRNAS.

RNA Polymerase II: Transcribes hnRNA (which is precursor of mRNA).

¶ RNA Polymerase III: Transcribes tRNA, 5 srRNA and sn RNA.

¶ The primary transcription has both exon and intron regions.

¶ Introns which are non-coding regions removed by a process called splicing.

¶ hnRNA undergoes two additional process:

(a) Capping: An unusual nucleotide (methylguanosine triphosphate) is added to 5-end of hnRNA.

(b) Tailling: Adenylate residues (200-300) are added at 3-end. It is fully processed hnRNA. (now called mRNA) is transported out of the nucleus.

GENETIC CODE

(i) The codon is triplet 61 codons code for amino acids and 3 codons function as stop codons (UAG, UGA, UAA)

(ii) Onecodoncodes for only one amino acid, hence the codon is unambiguous 

(iii) Some amino acids are coded by more than one codon, hence called as degenerate. 

(iv) The codon is read in mRNA in a contiguous fashion. There are no punctuations.

(v) The code is nearly universal.

(vi) AUG has dual functions. It codes for Methionine (met) and it also acts as initiator codon.

¶ tRNA has an anticodon loop that has bases complementary to the code, and also has an amino acid acceptor and through which it binds to amino acids.

TRANSLATION

¶ Translation refers to the process of polymerization of amino acids to form a polypeptide. The order and sequence of amino acids are defined by the sequence of bases in the mRNA. 20 amino acids participate in naturally occuring protein synthesis.

¶ First step is charging of t-RNA or aminoacylation of t-RNA-here amino acids are activated in the presence of ATP and linked to specific t-RNA.

¶ Initiation: Ribosome binds to mRNA at the start codon (AUG) that is recognized by the initiator t-RNA.

¶ Elongation phase: Here complexes composed of an amino acid linked totRNA.Sequentially bind to the appropriatecodoninmRNAby forming complementary base pairs on t-RNA as anticodon. The ribosomes move from codon to codon along with mRNA. Amino acids are added one by one, translated into polypeptide sequences.

¶ Termination : Release factors binds to the stop codon (UAA, UAG, UGA) translation and releasing the complete polypeptide from the ribosome.

LAC OPERON

¶ The concept of operon was proposed by Jacob and Monod. Operon is a unit of prokaryotic gene expression.

¶ The lac operon consists of one regulatory gene (the i-gene) and three structural genes (z, y and a)..

¶ The i-gene codes for repressor of lac operon.

¶ Promoter - It is the site where RNA-polymerase binds for transcription. • Operator acts as switch for operon.

¶ Lactose is an inducer.

¶ Operator: Act as switch for operon.

¶ Gene z-Codes for b-galactosidase

Gene y-Codes for permease

Gene a-Codes for transacetylase.

IN THE ABSENCE OF INDUCER (LACTOSE)

Repressor (i-gene) binds with operator (O)

                    ↓

Operator (O) turns off

                    ↓

RNA polymerase stops the transcription

                    ↓

structural genes (z, y and a ) do not produce lac mRNA and enzymes

IN THE PRESENCE OF INDUCER (LACTOSE)

Repressor binds to inducer (lactose).

                    ↓

Operator (O) turns ON

                    ↓

RNA polymerase starts the transcription.

Structural genes (z, y and a) produce mRNA and enzymest (B-galactosidase, permease and transacetylase respectively)

Human Genome Project was a 13 year project coordinated by the U.S. Department of Energy and National institute of Health, it was completed in 2003.

IMPORTANT GOALS OF HGP

(i) Identify all the approximately 20,000-25,000 genes in human DNA. 

(ii) Determinate the sequence of the 3 millon chemical base pairs that make up human DNA.

(iii) Store this information in database.

(iv) Transfer the related technologies to other sectors, such as industries.

(v) Address the ethical, legal and social issues (ELSI) that may arise from the project.

STEPS FOR SEQUENCING:

• DNA isolated from cell and converted into fragments.

• DNA is cloned for amplification is suitable host using specialised vectors.

• Commonly used hosts Bacteria, Yeast

• Commonly used Vectors-BAC (Bacterial Artifical Chromosomes) YAC (Yeast Artificial Chromosomes)

INTERNATIONAL RICE GENOME SEQUENCING PROJECT (IRGSP) 

¶ Rice benefits from having the smallest genome of the major cereals, dense genetic maps.

¶ The IRGSP, formally established in 1998, pooled the resources of sequencing groups in 10 nations (Japan, Korea, UK, Taiwan, China, Thailand, India, United States, Canada and France)

¶ Estimated Cost-Rs. 200 million.

¶ India joined in June 2000 and chosed to sequence a part of chromosome 11.

¶ Tools used in sequencing were:

BAC (Bacterial Artificial chromosomes).

PAC (P1-Phase derived artificial chromosomes)

¶ How Sequenced

Shotgun sequencing involved-generation of short DNA fragments that are then sequenced and linearly arranged. It enables full coverage of the genome in a fraction of time required for the atternative BAC sequence approach.

¶ Salient Features of Rice Genome

Rice is monocarpic annual plant, wind pollinated. It is with only 389 base pairs.

The world's first genome of a crop plant that was completely sequenced. 2,859 genes seem to be unique to rice & other cereals. Repetitive DNA is estimated to constitute at least 505 of rice genome. The transposon content of rice genome is at least 35%.

APPLICATIONS

• To improve efficiency of Rice breeding.

• To improve nutritional value of rice, enhance crop yield by improving seed quality resistance to pests and diseases and plant hardiness.

DNA FINGERPRINTING :

It is a technique to determine nucleotide sequence of certain areas of DNA which are unique to each individual.

Principle of DNA Fingerprinting : Short nucleotide repeats in the DNA are very specific in each individual and vary in number from person to person but are inherited, these are Variable Number Tandem Repeats (VNTRs.). Each individual inherits these repeats from his/her parents which is used as genetic markers. One half of VNTR alleles of the child resembles that of mother and other half of the father.

Steps/Procedure in DNA Fingerprinting

¶ Extraction of DNA-using high speed refrigerated centrifuge. 

¶ Amplification many copies are made using PCR

¶ Restriction Digestion using restriction enzymes DNA is cut into fragments.

¶ Separation of DNA fragments using electrophoresis agarose polymer gel

¶ Southern Blotting: Separated DNA sequences are transferred on to nitrocellulose or nylon membranes.

¶ Hybridization: The nylon membranes exposed to radio active probes.

¶ Autoradiography: The dark bands develop at the probe site.

APPLICATIONS OF DNA FINGERPRINTING

(i) identify criminals if their DNA from blood, hair follicle, skin, bone, saliva, sperm etc is available in forensic labs.

(ii) determine paternity

(iii) verify whether a hopeful immigrant is really close relative of an already established resident.

(iv) identify racial groups to rewrite biological evolution.

CLASS 12TH BIOLOGY CHAPTER- 7 EVOLUTION NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM