Ch 6. Molecular Basis of Inheritance (stack 1)

Question 1

Name the pentose sugar present in DNA

Answer 1

Deoxyribose sugar

Question 2

Name the pentose sugar present in RNA

Answer 2

Ribose sugar

Question 3

Mention an example for purine

Answer 3

Adenine /Guanine ·

Question 4

Mention an example for pyrimidine

Answer 4

Cytosine/Thymine

Question 5

Name the bond present between pentose sugar and nitrogen base in a nucleoside

Answer 5

N - glycosidic bond

Question 6

Name the bond present between a nucleoside and the phosphate group

Answer 6

Phosphoester bond

Question 7

Name the bonds present between the two strands of a DNA molecule

Answer 7

Hydrogen bond

Question 8

Name the bond formed between sugar and phosphate back bone of single stranded DNA

Answer 8

Phosphodiester bond

Question 9

What is the alternative chemical name of thymine?

Answer 9

5-methyl uracil

Question 10

Name the nitrogenous base present in DNA but not in RNA

Answer 10

Thymine

Question 11

Name the nitrogenous base present in RNA but not in DNA

Answer 11

Uracil

Question 12

Name the nucleoside present only in DNA

Answer 12

Deoxythymidine

Question 13

Name the nucleoside present only in RNA

Answer 13

Uridine

Question 14

What was the observation of Erwin Chargaff for a double stranded DNA?

Answer 14

For the double stranded DNA the ratios between Adenine and Thymine and Guanine and Cystisineare constant and equals to one.

Question 15

Which nitrogenous base provides additional stability to DNA?

Answer 15

Thymine

Question 16

State the central dogma of molecular biology

Answer 16

It states that genetic information flows from DNA –> RNA –> Proteins

Question 17

What is reverse dogma of molecular biology?

Answer 17

The flow of genetic information from RNA –> DNA –> RNA –> Proteins is called reverse dogma

Question 18

What is nucleosome?

Answer 18

A nucleosome is the basic subunit of chromatin in eukaryotes in which two turns of DNA are wrapped around a group of eight positively charged proteins called histones or the histone octamer.

Question 19

Name the proteins which form the core of nucleosomes

Answer 19

Histone proteins.

Question 20

Mention any one basic amino acid present in large quantities in the histone proteins of nucleosomes

Answer 20

Lysine/Arginine

Question 21

How many base pairs (bp) of a DNA helix are found in a typical nucleosome?

Answer 21

200 bp

Question 22

What is chromatin?

Answer 22

The repeating units of nucleosomes with the DNA strand showing beads-on-string structure inside the nucleus is called chromatin

Question 23

What is euchromatin

Answer 23

The loosely packed, lightly stained and transcriptionally active chromat in is called euchromatin

Question 24

Define heterocbromatin

Answer 24

The more densely packed, darkly stained and transcriptionally inactive chromatin is called heterochromatin.

Question 25

Replication in the region of euchromatin would be faster. Justify.

Answer 25

Because it is loosely packed, separation and unwinding of DNA strands becomes easy.

Question 26

Replication in the region of heterochromatin would be slow. Justify

Answer 26

Because it is densely packed, separation and unwinding of DNA strands becomes relatively difficult

Question 27

Heterochromatin is transcriptionally inactive when compared to euchromatin. Give reason?

Answer 27

It is because, the heterochromatin is not easily accessible by the enzymes as it is more densely packed than the euchromatin

Question 28

Mention the four nitrogen bases present in DNA

Answer 28

Adenine, guanine, cytosine and thymine

Question 29

Mention the four nitrogen bases present in RNA

Answer 29

Adenine, guanine, cytosine and uracil

Question 30

List the nucleosides of DNA

Answer 30

Adenosine, guanosine, cytidine and thyrnidine

Question 31

Mention the nucleosides of RNA

Answer 31

Adenosine, guanosine, cytidine and uridine

Question 32

If a double-stranded DNA has 20% of cytosine, calculate the percent of adenine in the DNA

Answer 32

According to Chargaffs mle - No. of purines= No. of pyrimidines

Therefore, 20% C - G 20% = 40%

30% A-T 30% =60%

Question 33

Explain the central dogma of molecular biology with a schematic representation.

Answer 33

Francis Crick proposed the Central dogma in molecular biology, which states that the genetic information flows from DNA –> RNA –> Protein

Question 34

Draw a labelled diagram of nucleosome

Answer 34

Question 35

How many base pairs of DNA are present in a typical nucleosome?

Mention the types of proteins present in a nucleosome

Answer 35

200bp per nucleosome. It consists of histone octamer (assembly of 8 histone subunits) and a Hl histone.

Question 36

The base sequence in one of the DNA strands is TAGCATGAT. Based on this answer the following

1. Give the base sequences of its complementa1y strand.
2. How are these base pairs held together in a DNA molecule?
3. What was the observation of Chargaff on the base ratio in double- stranded DNA?

Answer 36

1. ATCGTACTA
2. Through hydrogen bonds
3. For a double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.

Question 37

Explain packaging of DNA in prokaryotes

Answer 37

• In prokaryotes, such as, E. coli, though they do not have a defined nucleus, the DNA is not scattered throughout the cell.
• DNA (being negatively charged) is held with some proteins (that have positive charges) in a region termed as ‘ nucleoid ‘ .
• The DNA in nucleoid is organised in large loops held by proteins.

Question 38

The length of a DNA molecule in a typical mammalian cell is calculated to be approximately 2.2 meters. How is the packaging of this long molecule done to accommodate it within the nucleus having a size of approximately 6µ?

Answer 38

• In eukaryotes, the packaging of DNA helix is a complex process.
• There is a set of positively charged basic proteins called histones.
• Histones are organized to form a unit of eight molecules called histone octamer.
• The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called a nucleosome.
• Nucleosomes constitute the repeating unit of a structure in the nucleus called chromatin, thread-like stained (coloured) bodies seen in the nucleus. The nucleosomes in chromatin are seen as ‘beads-on­ string ‘ structures when viewed under an electron microscope.
• The beads-on-string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at the metaphase stage of cell division to form chromosomes.