CHROMOSOME STRUCTURE

For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.
Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).
As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.
These duplicated chromosomes are called dyads.

When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.
While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bonafide chromosome with a full complement of genes.
The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.
The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.

Detailed structure of chromosome

The chromosome are made up of the following part

Chromatid
Chromonema
Centromere / Primary constriction
NOR/ Secondary constriction –I
Secondary Constriction II
Telomere
Satellite

Chromatid

At metaphase Chromosomes are made up of two thread-like structures which are called chromatids.
These chromatids are joined at a place which is called centromere.
These are separated in the Anaphase stage.
They carry by spindle fiber on the opposite pole .These are two type

1- Sister chromatid

At the start of meiosis each chromosome is replicated (s-stage) and makes two identical copies of DNA
which are attached to each other at centromere.
These are called sister chromatids.

2-Non sister chromatid

Two chromatids of two homologous chromosomes are known as non sister chromatids.
In these one chromatid comes from father and one chromatid comes from mother in the progeny.

Chromonema –

During prophase and also interphase, the chromosomal material becomes visible as a very thin filament which is called chromonemata and which represents chromatid in early stages of condensation.
It is a single linear DNA molecule with its associated proteins.

Centromere/Primary constriction –

Each chromosome has a groove which is known as centromere.
In this region Kinetochore proteins are present. In which spindle fibers are attached which carry chromosomes to opposite poles.
The centromere lies within a thinner segment of the chromosome, the primary constriction.
The regions flanking the centromere frequently contain highly repetitive DNA and stain more intensely with basic dyes (heterochromatin).
Centromere contains specific DNA sequences with special protein bound to them forming a disc shaped structure called a kinetochore.

According to the no. of centromere these are the following types

1- Monocentric – Only single centromere present
2- Di-centric – Two centromere present
3- Poly centric – More than two centromeres are present.
4- Acentric – Centromere absent

According to the position of centromere chromosomes are following types

1. Telocentric –

In this centromere are present on the extreme end (head)
This chromosome contains only one long arm.
These are rod shaped and are present only in plants

2. Acrocentric

In this type of chromosome centromere is positioned sub terminally.
So this chromosome have a very small arm and other large arm

3. Sub metacentric

In this type of chromosome centromere is present sub median position of chromosome
So it have unequal arm
It looks like a “J” , or “L” shape.

4. Metacentric

In this type of chromosome the centromere is present at middle point of chromosome
So it has two equal arms.
It is have “V” like shape

4. Kinetochore –

A kinetochore is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart

5. Chromomere –

These components are beads like accumulation of chromatin material that are sometimes visible along interphase chromosomes.
Chromomeres are especially present in polytene chromosomes where they become aligned side by side, constituting the chromosome bands.
These tightly folded regions of DNA are of great interest, as they may correspond to the units of genetic functions in the chromosome.
At metaphase, chromomeres are not visible as the chromosome is tightly coiled.

6. Telomere –

This term refers to the tips of the chromosomes.
Telomeres have special properties that when chromosomes are broken, the free end without telomeres fuse with other broken chromosomes; however they do not fuse with a normal telomere.
Telomeres contain the ends of the long linear DNA molecule contain in each chromatid and therefore have an unusual DNA structure.
The end part of the chromosome called telomere.
In this region chromatin is present in the form of Loop which make heterochromatic part.
It avoid to join of chromosome.
It is formed by the help of telomerase enzyme.
In this aprx. 10kb repetitive sequence are present.
In this one end have “G” and another end have “C” as residue on it.
Protein that Bind to the repetitive sequence of telomeric DNA is Ta2/P protein.
Two type of protein Ku & Sir are help in replication of Telomeric DNA and also control the Length of this Part

7. Nucleolar Organizers –

A secondary constriction in the chromosome is a constriction other than the centromere.
It is often associated with the nucleolus during interphase and may take part in the reorganization of the nucleolus at the end of cell division.
Secondary constrictions are constant in their position and extents, and therefore useful in identifying particular chromosomes in a set.
Secondary constrictions are distinguished from the primary constriction by the absence of marked angular deviation of the chromosomal segments during anaphase.
Another constriction of the chromosome is called secondary constriction or NOR
It’s role in forming of Nucleolus
A round structure is present on it which is called satellite
In human it is present on the Chs no. 13,14,15,21 &22
These areas are certain secondary constriction that contains the gene coding for 18S and 28S ribosomal RNA and that induce the formation of nucleoli.

8. Secondary constrictions II

Some time other than secondary constriction I another constriction is present on the chromosome which is called secondary constriction II (SCII)
It is present on the specific place which help in recognization of chromosome .
In human it is present on the Chs No. 1,10,13,16 &Y

9. Satellites –

A rounded button like structure is present ahead to the secondary constriction which is known as satellite.
It attached to the chromosome by a thin thread like structure
The chromosome on which it attaches is called SAT chromosme
The satellite and constriction are constant in shape and size for each particular chromosome.

PACKAGING OF DNA

In living organisms, the length of DNA always greatly exceeds the dimensions of the nucleus which contains it.
The DNA must be compressed very tightly to fit into the available space.
When comparing prokaryotic cells to eukaryotic cells, prokaryotes are much simpler than eukaryotes in many of their features.
Most prokaryotes contain a single, circular chromosome that is found in an area of the cytoplasm called the nucleoid.
The size of the genome in one of the most well-studied prokaryotes,
E.coli, is 4.6 million base pairs (approximately 1.1 mm, if cut and stretched out). So how does this fit inside a small bacterial cell?
The DNA is twisted by what is known as supercoiling. Supercoiling means that DNA is either under-wound (less than one turn of the helix per 10 base pairs) or over-wound (more than 1 turn per 10 base pairs) from its normal relaxed state.
Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as DNA gyrase help in maintaining the supercoiled structure.
The overall compression of the DNA has also been described as packaging ratio, means the length of DNA divided by the length of unit that contain it.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus.
The ratio of salivary gland chromosome of Drosophila at metaphase is 100:1.
Each chromatid, therefore, consists of a single unbroken molecule of DNA which is coiled, supercoiled and folded to form the chromatid.
Compaction is aided by the associated nucleoprotein

Q1. Chromosomes are best visible during which phase of the cell cycle?
A. Interphase
B. Anaphase
✅ C. Metaphase
D. G1
PYQ: NEET 2021

Q2. Duplicated chromosomes visible during mitosis are called:
A. Chromatin
✅ B. Dyads
C. Chromonemata
D. Chromomeres

Q3. The duplicated chromosomes are held together by:
A. Telomere
✅ B. Centromere
C. Chromonema
D. Spindle fibers

Q4. The shorter arm of a chromosome is called:
A. L arm
✅ B. p arm
C. q arm
D. H arm

Q5. The longer arm of a chromosome is known as:
A. L arm
B. p arm
✅ C. q arm
D. S arm

Q6. What forms the point of spindle fiber attachment during cell division?
✅ A. Kinetochore
B. Telomere
C. Centriole
D. Centrosome

Q7. Sister chromatids are formed during:
A. G1 phase
✅ B. S phase
C. Metaphase
D. Cytokinesis

Q8. Non-sister chromatids are present in:
A. Haploid cells
✅ B. Homologous chromosomes
C. Autosomes only
D. Only X-chromosomes

Q9. Chromonema appears during:
✅ A. Prophase
B. Telophase
C. Anaphase
D. Interkinesis

Q10. Chromonema represents:
✅ A. Early chromatid
B. RNA
C. Microtubules
D. Kinetochores

Q11. A chromosome with only one centromere is:
✅ A. Monocentric
B. Acentric
C. Dicentric
D. Polycentric

Q12. A chromosome without centromere is:
A. Telocentric
✅ B. Acentric
C. Submetacentric
D. Acrocentric

Q13. A chromosome with centromere on terminal end is called:
✅ A. Telocentric
B. Metacentric
C. Submetacentric
D. Acrocentric

Q14. A chromosome with subterminal centromere is:
A. Metacentric
✅ B. Acrocentric
C. Telocentric
D. Polycentric

Q15. Metacentric chromosomes have:
✅ A. Equal arms
B. No arms
C. One long arm
D. One centromere only

Q16. The terminal part of chromosome is:
✅ A. Telomere
B. Chromonema
C. Satellite
D. Chromocenter

Q17. Telomeres are made up of:
A. Centrioles
✅ B. Repetitive sequences & heterochromatin
C. Lipids
D. Sugars

Q18. Which enzyme maintains telomere length?
A. DNA polymerase
✅ B. Telomerase
C. RNA polymerase
D. Ligase

Q19. NORs are related to:
A. Spindle formation
✅ B. Nucleolus formation
C. DNA replication
D. Chromatin unwinding
PYQ: NEET 2020

Q20. NOR is found on which human chromosomes?
✅ A. 13,14,15,21,22
B. 1,3,7,16
C. X and Y
D. 10 and 12

Q21. DNA of E. coli is packaged using:
✅ A. Supercoiling
B. Telomere looping
C. Telomerase
D. Exon folding

Q22. Supercoiling helps in:
✅ A. Fitting long DNA into small space
B. DNA unwinding
C. Spindle fiber formation
D. Protein synthesis

Q23. Enzyme that maintains DNA supercoiling:
A. DNA ligase
✅ B. DNA gyrase
C. Telomerase
D. Helicase

Q24. DNA packaging ratio is:
A. Length of chromosome ÷ DNA
✅ B. DNA length ÷ unit length
C. Genome size × protein
D. Base pair per turn

Q25. Eukaryotic DNA is packaged using:
✅ A. Histones
B. Gyrase
C. RNA
D. Centrosome

Q26. DNA supercoiling in prokaryotes is stabilized by:
A. Telomerase
✅ B. DNA-binding proteins
C. Ribosomal RNA
D. Spindle fibers

Q27. The shape of prokaryotic chromosome is:
A. Linear
✅ B. Circular
C. Spiral
D. Zig-zag
PYQ: NEET 2022

Q28. Eukaryotic DNA is wrapped around histone octamers to form:
A. Telomeres
✅ B. Nucleosomes
C. Centrosomes
D. Exons

Q29. DNA packing ratio in salivary gland chromosomes of Drosophila is:
A. 10:1
B. 20:1
✅ C. 100:1
D. 500:1

Q30. Which histone protein helps in nucleosome compaction?
A. H2B
B. H4
✅ C. H1
D. H3

Q31. Chromomeres are most visible in:
✅ A. Polytene chromosomes
B. Telomeres
C. S-phase
D. Metaphase

Q32. Chromomeres represent:
A. Active genes
B. RNA molecules
✅ C. Tightly folded chromatin
D. Loosely packed DNA

Q33. Kinetochore is found on:
A. Telomere
✅ B. Centromere
C. Satellite
D. Chromomere

Q34. The disc-shaped protein at centromere is called:
A. Chromatid
✅ B. Kinetochore
C. Histone
D. Centriole

Q35. Which helps in movement of chromosomes during mitosis?
✅ A. Spindle fibers + kinetochore
B. Telomere loops
C. RNA polymerase
D. Histone

Q36. Which of the following is not a type of centromere classification?
A. Monocentric
B. Dicentric
✅ C. Bi-axial
D. Polycentric

Q37. Secondary constriction is also known as:
✅ A. NOR
B. Telomere
C. Satellite
D. Heterochromatin
PYQ: CUET 2023

Q38. A chromosome that has a satellite is called:
✅ A. SAT chromosome
B. NOR chromosome
C. Loop chromosome
D. Telomeric chromosome

Q39. Secondary constriction II (SCII) helps in:
✅ A. Chromosome recognition
B. Gene regulation
C. DNA packaging
D. Histone formation

Q40. SCII is found on which chromosomes in humans?
A. 1, 2, 4, 5
✅ B. 1, 10, 13, 16, Y
C. 6, 7, 11, 12
D. 21, 22

Q41. Telomeres are made of:
A. Non-repetitive DNA
✅ B. Repetitive G-C rich sequences
C. Satellite RNA
D. mRNA strands

Q42. The “G” and “C” rich ends in telomeres serve what function?
✅ A. Prevent chromosome fusion
B. Activate mitosis
C. Inhibit RNA synthesis
D. Form spindle fibers

Q43. The protein that binds to telomeric DNA is:
A. Tubulin
B. Histone H1
✅ C. Ta2/P protein
D. Ku protein

Q44. Proteins Ku & Sir help in:
A. Translation
✅ B. Replication & telomere length regulation
C. Mutation repair
D. Spindle elongation

Q45. Telomerase is active in which cells?
A. Mature RBCs
✅ B. Embryonic cells and cancer cells
C. Neurons
D. Pancreatic cells

Q46. Which part of the chromosome prevents end-to-end fusion?
A. Centromere
B. NOR
✅ C. Telomere
D. Chromatid

Q47. What are satellites in chromosomes?
A. RNA regions
✅ B. Round structure attached to secondary constriction
C. Plastid DNA
D. Nucleosomes

Q48. Which protein complex helps chromatids move during anaphase?
✅ A. Kinetochore
B. Ribosome
C. Telomerase
D. Histone core

Q49. Chromatid separation occurs during:
A. Metaphase
B. Interphase
✅ C. Anaphase
D. Cytokinesis

Q50. Which part of DNA helps determine genetic identity during crossing over?
A. Centromere
B. NOR
✅ C. Chromatid arms
D. Telomeres