Molecular Basis of Inheritance Important Questions

Molecular Basis of Inheritance can be explained as the study that includes the information of genes and genetic variations along with the concept of heredity. Two types of nucleic acids are present in living organisms: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). During the early years, the pattern of factors that regulate inheritance was not clear.

Over a hundred years of investigation, DNA was considered as the genetic material in most living beings. Some viruses have RNA as their genetic material though it mainly works as a messenger, structural, adapter and catalytic molecule. Thus, the term "molecular basis of inheritance" describes the molecular processes by which genetic information is expressed, stored, and replicated in organisms.

Molecular Basis of Inheritance

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Very short answer type questions (1 Mark Questions)

Q1. Name the genetic material of viruses giving one example?

Ans - RNA or ribonucleic acid is considered as the genetic material of viruses. The common example of virus having RNA as genetic material are Retroviruses.

Q2. How is the length of DNA defined?

Ans - The length of DNA is defined as the number of nucleotides or base pairs present in it.

Q3. What are the primary functions of RNA?

Ans - Following are some of the primary functions of RNA: 

• It acts as a messenger.
• RNA is considered as a genetic material like DNA.
• It also acts as a structural, adapter and catalytic molecule.

Q4. List the three components of a nucleotide?

Ans - A nucleotide has a nitrogenous base, pentose sugar and a phosphate group. 

Q5. Which nitrogenous base is present in both DNA and RNA and which is present only in RNA?

Ans - Cytosine is present in both DNA and RNA. Uracil is exclusive only to RNA. 

Q6. In a DNA helix, the distance between two base pairs is ______?

Ans - The distance between two base pairs is 0.34 nm.

Q7. Explain DNA polymorphism?

Ans - DNA polymorphism is a phenomenon that explains the existence of two or more different DNA sequence variations that can be in any individual or a population. Mutation at non-coding sequences is the reason for its occurrence.

Q8. What are histones?

Ans - Histones are composed of positively charged proteins that helps in DNA helix packaging. 

Q9. How many base pairs of DNA helix are present in a nucleosome?

Ans - 200 base pairs of DNA helix are present in a nucleosome.

Q10. Scientists that prove DNA is the genetic material are _______?

Ans - Alfred Hershey and Martha Chase (1952) are together credited for giving proof for DNA as genetic material/

Q11. Name the enzyme that uses a DNA template to catalyze the polymerization reaction of deoxynucleotides? 

Ans - DNA polymerase is the enzyme that uses a DNA template to catalyze the polymerization reaction of deoxynucleotides. 

Q12. What is the origin of replication?

Ans - The region in E.coli where the replication starts is called the origin of replication.

Q13. What is the difference between DNAse and DNA polymerase?

Ans - DNAase enzyme digests the DNA strand, while DNA polymerase is an important enzyme that plays significant role in the polymerization reaction. 

Q14. Define replication fork?

Ans - The region of DNA where replication takes place is called a replication fork. DNA strands uncoil to form a Y-shaped structure. 

Q15. What is the function of the Poly-A tail?

Ans - Poly-A tail attaches to mRNA to provide stability to the molecule. It also increases its length.

Q16. What are the three regions of the transcription unit?

Ans - The three regions of the transcription unit are: a promoter, a structural gene and a terminator.

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Short answer type questions (2 Marks Questions)

Q1. Give the reason why DNA is more stable than RNA?

Ans - DNA’s deoxyribose sugar contains one less oxygen hydroxyl group which makes it stable. RNA contains ribose sugar which has a hydroxyl group at 2 prime locations on the pentose ring. RNA is less stable and more vulnerable to hydrolysis because of this hydroxyl group.

Q2. What is the central dogma theory in molecular theory of inheritance?

Ans - Francis Crick gave proposed the famous central dogma theory. According to the theory of Central Dogma, “genetic information flows in one direction. From DNA to DNA (replication), DNA to mRNA(transcription) and mRNA to protein (translation)”.

• The concepts of transcription and translation were first coined by Francis Crick.
• The genetic information can also directly change from RNA to protein.
• In some viruses, it occurs in the opposite direction i.e, from RNA to DNA.

Q3. What is semi-conservative DNA replication?

Ans - According to the semi-conservative DNA replication, the two DNA strands separate and act as the base for the creation of new complementary strands. Each newly formed DNA strand will have one new synthesized strand while the other one will be the parental one. 

Q4. How nucleosomes are formed?

Ans - Lysine and arginine are abundant in histones. Both amino acids carry a positive charge on their side chains. Eight histone molecules come together to form a histone octamer whose nature is positively charged because of the amino acids present. Negatively charged DNA wraps around it to form a nucleosome. 

Q5. What is chromatin? Explain euchromatin and heterochromatin?

Ans - Chromatin is a complex of DNA and protein. Its function is to pack long DNA strands into complex structures. They appear as stained bodies inside the nucleus. The chromatin region which stains light and is loosely packed is called euchromatin.

• On the other hand, heterochromatin is densely packed and stains darker.
• Euchromatin is transcriptionally active.
• Euchromatin was found to be affected by temperature, age and sex.
• Heterochromatin remains unchanged by such variables. 

Q6. What are the conditions that a genetic material must fulfill?

Ans - A genetic material must fulfill the following conditions - 

1. It should be able to retain and communicate information in order to pass on the heritable traits.
2. It should be able to undergo mutations to produce variations.

iii. It must create its own copy. 

Q7. Give the major differences between polycistronic mRNA and monocistronic mRNA?

Ans - Polycistronic - it is an mRNA with a single polypeptide that it can code. It has only one cistron. It is generally present in eukaryotes.

Whereas, Monocistronic - it is an mRNA with more than one polypeptide it can code for. It has more than one cistron. It is present in prokaryotic cells. 

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Short answer type questions (3 Marks Questions)

Q1. Explain the function of three major types of RNAs?

Ans - Messenger RNA (mRNA) - functions as a transport molecule and carries information from the nucleus to the cytoplasm where its sequence is scanned to convert into appropriate amino acids.

Transfer RNA (tRNA) - Aids in translating mRNA sequences into a protein. tRNA works at specific locations in the ribosome.

Ribosomal RNA (rRNA) - binds to mRNA and tRNA to initiate translation. It has structural and catalytic roles. 

Q2. At what level the regulation of gene expression can be exerted in eukaryotes?

Ans - The gene expression can be regulated in eukaryotes at the following levels - 

• Transcription level (primary transcript formation).
• Processing level (splicing regulation).

ii. mRNA transport to the cytoplasm from the nucleus.

• Translation level.

Q3. What are the characteristics of Watson and Crick’s DNA double helix structure?

Ans - i. It is made up of two polynucleotide chains with a sugar phosphate backbone.

• The two chains are anti-parallel.

ii. Bases of two strands are formed between two strands by hydrogen bonds. It results in the Formation of base pairs. A forms two hydrogen bonds with thymine. Cytosine forms three hydrogen bonds with guanine.

• The strands are coiled in a right-handed fashion with 10 base pairs in each turn.
• The plane of one base pair stacks on the other, providing stability to the helical structure.

Q4. What are the functions of histones?

Ans - Histones are a group of positively charged proteins found in chromatin. Their functions are as follows - 

• Helps DNA to condense into chromatin.
• Gives chromosomes their shape.

ii. Helps in controlling the activity of the gene.

• Dense packaging makes it less prone to protein factors.
• Protects the integrity of chromosomes.

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Long answer type questions (5 Marks questions)

Q1. What is DNA fingerprinting?

Ans - There are differences in DNA sequences that make everyone unique in their phenotypic appearance. If the difference among a population or between individuals has to be found out, DNA sequencing becomes a time taking and expensive process given the many bases in the human genome. 

• Repetitive DNA sequences in which a small portion of DNA is present many times. In DNA fingerprinting, differences are identified in these repetitive DNA regions. 
• The bulk genomic DNA, which produces a major peak, is separated from repetitive DNA using density gradient centrifugation. 
• Smaller peaks are referred to as satellite DNA. On the basis of factors like length, base composition and repetitive unit number, satellite DNA can be categorized as micro and mini satellites. 
• These sequences which exhibit a significant level of variability, serve as the basis of DNA fingerprinting. 
• These sequences take up a large chunk of the human genome but do not code for any protein. 
• The basis of DNA fingerprinting is polymorphism in DNA sequences. It is used in forensics as an identification tool and in paternity testing as polymorphism is passed down from parents to offspring. 

Alec Jeffreys developed the technique of DNA fingerprinting, in which he used satellite DNA with high polymorphism as a probe. He called them VNTR or Variable Number of Tandem Repeats. Radiolabelled VNTR as a probe was used in Southern blotting. VNTR belongs to mini satellites and their size varies from 0.1 to 20 kb as they have high polymorphism. With the advent of PCR, the efficiency of DNA fingerprinting has increased. 

Q2. What is the Human Genome Project? What are its goals and features?

Ans - The Human Genome Project (HGP) is a research initiative to find the base pairs that constitute the human DNA as well as of identifying, mapping and sequencing every gene of the human genome. It was launched in 1990 and is known as a mega project. High-speed computing devices are required for the storage and analysis of data due to the massive volume that was anticipated to be generated. HGP is one of the major factors for the sudden and continuous growth of the bioinformatics field. 

 The goals of the HGP are as follows -

•  To identify all genes of the human DNA.
•  To identify the 3 billion base pairs that make up the human DNA.
• Improvement of data analysis tools.
• Creating and maintaining databases to store the information.
• To address ELSI issues that may arise from the project.

HGP has the following features - 

• 3164.7 bp makes up the human genome.
• The size of the human gene varies constantly with the average size being 3000 bases.
• 99.9 % of bases are completely similar in all people. 30,000 is the total number of genes estimated.
• For 50 per cent of genes that have been found, the functions are unknown. 
• Less than 2% of the genome is used to code for proteins.
• Large chunk of the genome is taken up by repeated sequences.
• Chromosome with the most genes is 1 (2968) and Y has the least (231).
• SNPs occurring at over 1.4 million locations have been identified. 

Q3. Explain genetic code and list its properties?

Ans - Genetic code is the mechanism by which DNA and RNA molecules transport genetic data in living cells. DNA and mRNA have complementary code systems. The genetic code is the arrangement of nucleotides in DNA and RNA that controls the order of amino acids in proteins. 

• More than one codon specifies some amino acids. The term anticodon refers to the nucleotide sequences on the tRNA molecule that complements the codon. 
• For instance, leucine has a codon of CUG and anticodon of GAC. Similarly, AAA is anticodon for phenylalanine and UUU is codon. 

• The genetic code being degenerate, non-overlapping, triplet, non-ambiguous and commaless are some of its properties. Genetic code is also universal and shows collinearity.
• The nucleotides A, T, G and C in DNA and A, U, G and C in RNA are referred to as code letters. A nonsense codon, like UGA, is a sequence of three that does not code for any amino acid. 

Q4. Explain operon? 

Ans - An operon is a group of connected genes that controls genes involved in protein synthesis. A functional unit of DNA, it is regulated by a single promoter. It has controllers and structural genes which control lactose or lac operon catabolism. A lac operon has the following components - 

• The DNA strand responsible for transcribing mRNA.
• Repressor protein encoding regulator gene.
• Inducer that prevents repressor binding to the operator.
• Operator, a DNA region where the repressor protein interacts that is close to the promoter. 
• Promoter which is the starting site of transcription. RNA polymerase binds at the promoter.

A quantifiable increase in the expression in response to an enhancer, inducer or positive regulator is known as an inducible operon. An example is the lac operon of E. coli.

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