Chromosomes and Genes

BiologyMolecular Basis of InheritanceFor NEET aspirants

Chromosomes and genes are built from DNA that is packed tightly with proteins. This page shows why packing is needed, since a mammalian cell holds about 2.2 metres of DNA in a nucleus about a micrometre across. It covers the nucleoid of prokaryotes, histones and the histone octamer, the 200 bp nucleosome, beads-on-string chromatin, chromatin fibres, non-histone chromosomal proteins, and euchromatin and heterochromatin. NEET often asks the nucleosome, the histone amino acids and which chromatin is active when it tests chromosomes and genes.

On this page1Packaging problem2Prokaryotes3Histones and nucleosome4Chromatin to chromosome5Euchromatin and heterochromatin6Exam essentials7Quick revision8Solved examples9Practice

Key Points at a Glance

★ Must learn Distance between two consecutive base pairs = 0.34 nm ( $0.34 \times 1 0^{- 9}$ m).
Length of DNA = number of base pairs $\times$ 0.34 nm.
★ Must learn Mammalian cell: $6.6 \times 1 0^{9}$ bp $\times$ $0.34 \times 1 0^{- 9}$ m/bp $\approx$ 2.2 m of DNA, against a nucleus of about $1 0^{- 6}$ m.
Prokaryotes have no defined nucleus; their DNA is held by positively charged proteins in the nucleoid, in large loops.
★ Must learn Histones: positively charged, basic proteins rich in lysine and arginine.
Eight histone molecules form a histone octamer.
★ Must learn Nucleosome: negatively charged DNA wrapped round the positively charged histone octamer; a typical one has 200 bp.
Nucleosomes are the repeating unit of chromatin and look like beads-on-string under the electron microscope.
Higher-level packaging needs non-histone chromosomal (NHC) proteins.
★ Must learn Euchromatin: loose, light-staining, transcriptionally active. Heterochromatin: dense, dark-staining, inactive.

1. The Packaging Problem

The distance between two consecutive base pairs is 0.34 nm ( $0.34 \times 1 0^{- 9}$ m).
The length of a DNA double helix is the total number of base pairs multiplied by this distance.

L = N_{bp} \times 0.34 \times 1 0^{- 9} m

★ Exam imp A typical mammalian cell has $6.6 \times 1 0^{9}$ bp, so its DNA is $6.6 \times 1 0^{9} \times 0.34 \times 1 0^{- 9}$ m $\approx$ 2.2 metres long.
A typical nucleus is only about $1 0^{- 6}$ m across.
So such a long polymer must be packaged very tightly to fit inside a cell.

★ Very important Length of DNA = number of bp $\times$ 0.34 nm. A mammalian cell's

6.6 \times 1 0^{9}

bp give about 2.2 m, about two million times the width of its nucleus.

Memory Trick The powers of ten cancel:

1 0^{9} \times 1 0^{- 9}

= 1, so only 6.6 $\times$ 0.34 $\approx$ 2.2 m is left to remember.

Tips and Tricks Turn both lengths into metres before dividing. For E. coli DNA of 1.36 mm:

\frac{1.36 \times 1 0 ^{- 3}}{0.34 \times 1 0 ^{- 9}}

= $4 \times 1 0^{6}$ bp.

Key idea

About 2.2 m of DNA must fit into a nucleus a millionth of a metre wide, so packaging is essential.

2. Packaging in Prokaryotes

Prokaryotes, such as E. coli, do not have a defined nucleus.
Even so, their DNA is not scattered throughout the cell.
★ Exam imp DNA, being negatively charged, is held with some positively charged proteins in a region called the nucleoid.
In the nucleoid, the DNA is organised in large loops held by proteins.

Key idea

Prokaryotic DNA stays together in the nucleoid as protein-held loops, without a nuclear membrane.

3. Packaging in Eukaryotes

3.1 Histones and the nucleosome

In eukaryotes, the organisation of DNA is much more complex.
Histones: a set of positively charged, basic proteins.
A protein gets its charge from the amino acid residues with charged side chains that it has in abundance.
★ Exam imp Histones are rich in the basic amino acid residues lysine and arginine. Both carry positive charges in their side chains.
Eight histone molecules are organised into a unit called the histone octamer.
★ Exam imp The negatively charged DNA is wrapped round the positively charged histone octamer to form a nucleosome.
A typical nucleosome contains 200 bp of DNA helix.
The nucleosome figure also shows an H1 histone, which lies outside the core on the DNA wrapped round it.

Test yourself: hide labels

Figure 1: The nucleosome. Negatively charged DNA is wrapped round a positively charged core of eight histone molecules, the histone octamer.

★ Very important Nucleosome = about 200 bp of DNA + a histone octamer. The negative DNA and the positive histones (rich in lysine and arginine) attract each other.

Memory Trick Octa = eight, as in an octopus with eight arms: a histone octamer has eight histone molecules. Lysine and Arginine are the Lifting Arms that hold the DNA, both positive.

3.2 From chromatin to chromosome

Nucleosomes form the repeating unit of chromatin, the thread-like stained (coloured) bodies seen in the nucleus.
★ Exam imp Under the electron microscope (EM), the nucleosomes in chromatin look like beads-on-string.

Test yourself: hide labels

Figure 2: Chromatin as beads-on-string. Each bead is a nucleosome, and the string is the DNA that runs from one nucleosome to the next. At metaphase, chromatin fibres coil and condense further to form chromosomes.

A mammalian cell with $6.6 \times 1 0^{9}$ bp and 200 bp per nucleosome holds, in theory, about $\frac{6.6 \times 1 0 ^{9}}{200}$ = $3.3 \times 1 0^{7}$ nucleosomes.
The beads-on-string structure is packaged into chromatin fibres.
These fibres are further coiled and condensed at the metaphase stage of cell division to form chromosomes.
★ Exam imp Packaging of chromatin at higher levels needs an additional set of proteins, the non-histone chromosomal (NHC) proteins.

DNA double helix
Nucleosome: DNA wrapped round a histone octamer
Chromatin with a beads-on-string look
Chromatin fibres (needing NHC proteins at higher levels)
Coiled and condensed chromosome at metaphase

Memory Trick DNA Needs Beads For Coiling: DNA, Nucleosome, Beads-on-string, Fibres, Coiled Chromosome.

Quick Recall: tap to check

Where is prokaryotic DNA held?

In the nucleoid, by positively charged proteins, as large loops.

In Figure 1, what is the core round which the DNA is wrapped?

The histone octamer, the core of eight histone molecules.

Which proteins are needed to pack chromatin at higher levels?

Non-histone chromosomal (NHC) proteins.

3.3 Euchromatin and heterochromatin

In a typical nucleus, some regions of chromatin are loosely packed and stain light: euchromatin.
Regions that are more densely packed and stain dark are called heterochromatin.
★ Exam imp Euchromatin is said to be transcriptionally active, whereas heterochromatin is inactive.

EuchromatinLoosely packed.
Stains light.
Transcriptionally active.

HeterochromatinDensely packed.
Stains dark.
Transcriptionally inactive.

Memory Trick Euchromatin is Easy to read: loose, light and active. Heterochromatin is Heavy: dense, dark and silent.

NEET Focus Charges are a favourite trap. DNA is negatively charged; histones are positively charged and basic, because of lysine and arginine. Also check the counts in statements: 8 histones per octamer and about 200 bp per nucleosome. Euchromatin, not heterochromatin, is transcriptionally active.

Quick Recall: tap to check

If the length of E. coli DNA is 1.36 mm, how many base pairs does it have?

1.36 \times 1 0^{- 3}

\div

0.34 \times 1 0^{- 9}

m =

4 \times 1 0^{6}

bp.

About how many nucleosomes are present in a mammalian cell?

6.6 \times 1 0^{9}

\div

200 bp =

3.3 \times 1 0^{7}

nucleosomes.

Which two amino acids make histones positively charged?

Lysine and arginine, both basic, with positive side chains.

Key idea

DNA wraps on histone octamers into nucleosomes, which fold into fibres and then metaphase chromosomes; loose euchromatin is active.

4. Exam Essentials

Pairs to Match

List I	List II
Nucleoid	Region holding DNA in prokaryotes
Histones	Positively charged, basic proteins
Lysine and arginine	Basic residues that make histones positive
Histone octamer	Unit of eight histone molecules
Nucleosome	About 200 bp of DNA wrapped round a histone octamer
Chromatin	Thread-like stained bodies in the nucleus
Beads-on-string	Chromatin seen under the electron microscope
NHC proteins	Packaging of chromatin at higher levels
Metaphase	Stage at which chromatin fibres are most coiled and condensed
Euchromatin	Loosely packed, light-staining, transcriptionally active
Heterochromatin	Densely packed, dark-staining, inactive

Exceptions

Prokaryotes have no defined nucleus, yet their DNA is not scattered; it stays in the nucleoid.
Histones are basic, not acidic.
Higher-level packaging uses non-histone chromosomal proteins, not histones.
Only euchromatin is transcriptionally active; heterochromatin is inactive.

Numbers to Remember

0.34 nm between consecutive base pairs.
$6.6 \times 1 0^{9}$ bp in a mammalian cell, giving about 2.2 m of DNA.
Nucleus: about $1 0^{- 6}$ m.
E. coli DNA of 1.36 mm = $4 \times 1 0^{6}$ bp.
8 histone molecules per octamer; 200 bp per typical nucleosome; about $3.3 \times 1 0^{7}$ nucleosomes per mammalian cell.

5. Quick Revision

Base pairs are 0.34 nm apart; length = bp $\times$ 0.34 nm.
Mammalian DNA: $6.6 \times 1 0^{9}$ bp, about 2.2 m; nucleus about $1 0^{- 6}$ m.
Prokaryotes: negative DNA held by positive proteins in the nucleoid, in large loops.
Histones: positive, basic, rich in lysine and arginine.
Histone octamer: eight histone molecules.
Nucleosome: DNA wrapped round the histone octamer; about 200 bp.
Nucleosomes are the repeating unit of chromatin (thread-like stained bodies).
Electron microscope: chromatin looks like beads-on-string.
Beads-on-string $\to$ chromatin fibres $\to$ coiled, condensed chromosomes at metaphase.
NHC proteins handle higher-level packaging.
Euchromatin: loose, light, active; heterochromatin: dense, dark, inactive.

6. Solved Examples

Solved Example 1

A DNA molecule has

2 \times 1 0^{9}

base pairs. What is its length?
(A) 0.068 m
(B) 0.68 m
(C) 6.8 m
(D) 68 m

Solution:

Answer: (B). Length = $2 \times 1 0^{9} \times 0.34 \times 1 0^{- 9}$ m = 0.68 m.

Solved Example 2

Match List I with List II.
List I: A. Histones, B. NHC proteins, C. Nucleoid, D. Euchromatin
List II: I. Higher-level packaging of chromatin, II. Transcriptionally active chromatin, III. Basic proteins of the nucleosome, IV. DNA region of a prokaryote
Choose the correct answer:
(A) A-III, B-I, C-IV, D-II
(B) A-I, B-III, C-IV, D-II
(C) A-III, B-IV, C-I, D-II
(D) A-III, B-I, C-II, D-IV

Solution:

Answer: (A). Histones form the nucleosome core (III); NHC proteins pack chromatin at higher levels (I); the nucleoid holds prokaryotic DNA (IV); euchromatin is active (II).

Solved Example 3

Read the statements about the nucleosome.
A. It has a core of eight histone molecules.
B. A typical nucleosome contains 200 bp of DNA.
C. The DNA in it is positively charged.
D. Nucleosomes are the repeating unit of chromatin.
Choose the correct answer:
(A) A, B and D only
(B) A, B and C only
(C) B, C and D only
(D) A and C only

Solution:

Answer: (A). C is wrong: DNA is negatively charged; the histone octamer is positively charged.

Solved Example 4

Arrange the levels of DNA packaging from the simplest to the most condensed.
A. Chromatin fibre
B. Nucleosome
C. Metaphase chromosome
D. Beads-on-string chromatin
Choose the correct answer:
(A) B, D, A, C
(B) D, B, A, C
(C) B, A, D, C
(D) B, D, C, A

Solution:

Answer: (A). DNA wraps into nucleosomes (B), which appear as beads-on-string (D), pack into fibres (A), and condense into chromosomes at metaphase (C).

Solved Example 5

Which of the following is NOT true of histones?
(A) They are basic proteins
(B) They are rich in lysine and arginine
(C) They are negatively charged
(D) Eight of them form an octamer

Solution:

Answer: (C). Histones are positively charged; that is why the negatively charged DNA wraps round them.

7. Practice Questions

Practice Questions

If the length of E. coli DNA is 1.36 mm, calculate the number of base pairs in it.Answer: $4 \times 1 0^{6}$ bp, from $1.36 \times 1 0^{- 3}$ m $\div$ $0.34 \times 1 0^{- 9}$ m.
Theoretically, how many nucleosomes are present in a mammalian cell?Answer: About $3.3 \times 1 0^{7}$ , from $6.6 \times 1 0^{9}$ bp $\div$ 200 bp per nucleosome.
Match List I with List II.
List I: A. Euchromatin, B. Heterochromatin, C. Histone octamer, D. Beads-on-string
List II: I. Eight histone molecules, II. Dense and dark-staining, III. Chromatin under the electron microscope, IV. Loose and light-staining
(A) A-IV, B-II, C-I, D-III (B) A-II, B-IV, C-I, D-III (C) A-IV, B-II, C-III, D-I (D) A-I, B-II, C-IV, D-IIIAnswer: (A). Euchromatin is loose and light, heterochromatin dense and dark, the octamer has eight histones, and EM shows beads-on-string.
Statement I: Histones are rich in lysine and arginine.
Statement II: Histones carry a net negative charge.
(A) Both Statement I and Statement II are correct (B) Statement I is correct, Statement II is incorrect (C) Statement I is incorrect, Statement II is correct (D) Both are incorrectAnswer: (B). Lysine and arginine have positive side chains, so histones are positively charged.
Which statement about prokaryotic DNA is NOT correct? (A) It lies in the nucleoid (B) It is held by positively charged proteins (C) It is scattered all over the cell (D) It is organised in large loopsAnswer: (C). Although there is no defined nucleus, the DNA is not scattered; it is held in the nucleoid.
Read the statements.
A. A mammalian cell has about 2.2 m of DNA.
B. Chromatin fibres condense most at metaphase.
C. NHC proteins form the histone octamer.
D. Heterochromatin is transcriptionally active.
Choose the correct answer: (A) A and B only (B) A, B and C only (C) B and D only (D) A, C and D onlyAnswer: (A). C is wrong: the octamer is made of histones; NHC proteins act at higher levels. D is wrong: heterochromatin is inactive.
What is the length of DNA, in metres, in a cell with $3.3 \times 1 0^{9}$ bp (haploid human content)?Answer: About 1.1 m, from $3.3 \times 1 0^{9} \times 0.34 \times 1 0^{- 9}$ m.

Common Mistakes to Avoid

Watch out

Calling histones acidic. Correct: histones are basic and positively charged.
Saying the octamer has eight DNA turns. Correct: it has eight histone molecules.
Giving the nucleosome a negative core. Correct: the core (histone octamer) is positive and the DNA wrapped round it is negative.
Calling heterochromatin the active form. Correct: euchromatin is transcriptionally active; heterochromatin is inactive.
Saying euchromatin stains dark. Correct: euchromatin stains light; heterochromatin stains dark.
Saying prokaryotic DNA is scattered in the cytoplasm. Correct: it is held in the nucleoid by positive proteins.
Crediting histones with all levels of packaging. Correct: higher-level packaging needs non-histone chromosomal proteins.
Leaving 0.34 nm unconverted in length problems. Correct: use $0.34 \times 1 0^{- 9}$ m, so $6.6 \times 1 0^{9}$ bp gives 2.2 m.

Frequently Asked Questions

How long is the DNA in a mammalian cell?

Multiply the number of base pairs by the distance between them. A typical mammalian cell has 6.6 billion base pairs, each 0.34 nanometre apart, which gives about 2.2 metres of DNA. This is far longer than the nucleus, which is only about a millionth of a metre across.

How is DNA packaged in prokaryotes such as E. coli?

Prokaryotes have no defined nucleus, but their DNA is not scattered. The negatively charged DNA is held with positively charged proteins in a region called the nucleoid. There the DNA is organised in large loops held by proteins.

What are histones and why are they positively charged?

Histones are basic proteins found with DNA in eukaryotes. A protein's charge depends on its charged amino acid residues, and histones are rich in lysine and arginine, which carry positive charges in their side chains. So histones attract the negatively charged DNA.

What is a nucleosome?

A nucleosome is the unit formed when negatively charged DNA is wrapped round a positively charged histone octamer, a group of eight histone molecules. A typical nucleosome contains about 200 base pairs of DNA. Nucleosomes are the repeating units of chromatin.

What is the beads-on-string structure of chromatin?

Under the electron microscope, chromatin looks like beads on a string. Each bead is a nucleosome, a histone octamer with DNA wrapped round it, and the string is the DNA running between nucleosomes. This structure is then packed into chromatin fibres.

What do non-histone chromosomal proteins do?

Non-histone chromosomal (NHC) proteins are needed for packaging chromatin at higher levels. They help pack the beads-on-string chromatin into chromatin fibres, which coil and condense further at metaphase to form chromosomes.

What is the difference between euchromatin and heterochromatin?

Euchromatin is the loosely packed region of chromatin that stains light and is transcriptionally active. Heterochromatin is densely packed, stains dark and is transcriptionally inactive. Both are found in a typical nucleus.

How many nucleosomes does a mammalian cell contain?

A mammalian cell has about 6.6 billion base pairs, and a typical nucleosome holds about 200 base pairs. Dividing gives about 33 million (3.3 x 10^7) nucleosomes, in theory, in each cell.

Previous year questions on Chromosomes and Genes

11 questions from past papers, each with a step-by-step solution.

Show all 11 questions

NEET 2018, Biology Q71

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