Chromosomes: Viral, Prokaryotic, and Eukaryotic Overview

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Questions and Answers

What is the primary role of telomeres at the ends of chromosomes?

To assist in DNA replication

To increase chromosomal length

To provide stability and protection (correct)

To bind non-histone proteins

Which histone is loosely bound to the nucleosome core particle?

H2A

H1 (correct)

H2B

What happens to telomeres every time chromosomes reproduce?

They shorten (correct)

They become longer

They remain constant in length

They replicate independently of the chromosomes

Which of the following statements about non-histone proteins is true?

They are responsible for specific gene expression Signup and view all the answers

What is the first level of DNA packaging in chromatin?

Nucleosomes Signup and view all the answers

What is the diploid chromosome number (2n) for humans?

46 Signup and view all the answers

Which organism has the highest known chromosome number?

Fern (Ophioglossus reticulatum) Signup and view all the answers

What structure do chromosomes condense into during metaphase or prophase?

Distinct chromosomes Signup and view all the answers

What part of the chromosome contains the kinetochore?

Centromere Signup and view all the answers

Which of the following best describes a karyotype?

The number and appearance of chromosomes Signup and view all the answers

What is the process by which the chromosome number is reduced to half in gametes called?

Meiosis Signup and view all the answers

Which of the following statements about chromosomes is NOT correct?

All organisms have the same number of chromosomes. Signup and view all the answers

Why do medical specialists use karyotypes?

To diagnose chromosomal abnormalities in patients Signup and view all the answers

What is the main chemical constituent of chromosomes that serves as the hereditary material?

DNA Signup and view all the answers

What type of chromosome do prokaryotic organisms typically have?

A single circular chromosome Signup and view all the answers

Which of the following statements correctly describes viral chromosomes?

They can consist of RNA or DNA, single-stranded or double-stranded. Signup and view all the answers

How does the organization of DNA in eukaryotic chromosomes differ from that of prokaryotic chromosomes?

Eukaryotic chromosomes are located within the nucleus. Signup and view all the answers

In eukaryotic cells, what term refers to the number of chromosomes found in somatic cells?

Diploid number Signup and view all the answers

What is the significance of supercoiling in prokaryotic chromosomes?

It helps in the packaging of the DNA into a compact form. Signup and view all the answers

What distinguishes the haploid cells from diploid cells in eukaryotes?

Haploid cells have half the number of chromosomes compared to diploid cells. Signup and view all the answers

Which ions assist in packaging the viral genome inside the virus?

Sodium, Magnesium, and Potassium Signup and view all the answers

What is the shape of a metacentric chromosome?

V-shaped Signup and view all the answers

Which type of chromatin remains condensed throughout the cell cycle?

Constitutive heterochromatin Signup and view all the answers

What is a characteristic of sat chromosomes?

They have a secondary constriction. Signup and view all the answers

What sequence is repeated at the ends of human chromosomes as telomeres?

TTAGGG Signup and view all the answers

How is dosage compensation achieved in female mammals regarding the X chromosome?

One X chromosome is inactivated at random. Signup and view all the answers

What type of chromosome has a centromere located very close to one end?

Acrocentric Signup and view all the answers

What describes facultative heterochromatin in mammals?

Condensed in some cells, euchromatic in others. Signup and view all the answers

Which of the following is NOT a classification of chromosomes based on their centromere number?

Multicentric Signup and view all the answers

Study Notes

Viral Chromosomes

Single stranded or double stranded DNA or RNA
Linear or circular molecules
Packaged in the capsids of mature virus particles or virons, freely inside the host cell or incorporated in the host DNA
Small positively charged ions and nucleic acid binding proteins help in packaging the genome inside the virus
Genome size varies: A few thousand to a hundred thousand nucleotides

Prokaryotic Chromosomes

Single circular chromosome in the nucleoid
Extensively supercoiled
DNA binding proteins help in packaging bacterial DNA
Two types of supercoiling:
- Negative supercoiling: DNA twisted in the opposite direction of the clockwise turn of the right-handed double helix
- Positive supercoiling: DNA supercoiled in the same direction as the clockwise turn of the helix

Eukaryotic Chromosomes

Diploid: Two sets of chromosomes (one from each parent)
Diploid number (2n) represents the number of chromosomes in a dual set of a diploid somatic cell
Haploid (n) cells: Sex cells contain half the number of chromosomes
Fertilization restores the diploid number (2n)
Each species has a characteristic and constant number of chromosomes:
- Human: 46
- Ant (Myrmecia pilosula): 2
- Chimpanzees: 48
- Potato: 48
- Fern (Ophioglossus reticulatum): 1260
Genome: The complete set of DNA
Human genome: More than 3 billion nucleotide base pairs
Karyotype: The number and appearance (size, shape) of chromosomes in the nucleus of a eukaryotic cell
Karyogram: A photograph of the chromosomes of a cell, arranged in homologous pairs and in a numbered sequence

Morphology of Eukaryotic Chromosome

During interphase: Chromosomes appear as long, coiled, elastic, contractile thread-like structure called chromatin thread
During metaphase of mitosis or prophase of meiosis: Chromatin thread becomes highly condensed and seen as distinct chromosomes
Each chromosome consists of two strands called chromatids and a constricted region called the centromere
Kinetochore region is where spindle fibers attach during cell division

Centromere Classification

Based on number:
- Monocentric: One centromere
- Dicentric: Two centromeres
- Polycentric: More than two centromeres
Based on position:
- Telocentric: Centromere is truly terminal; chromosomes are rod shaped
- Acrocentric: Centromere is located closer to one end of chromatid, resulting in long chromatids on the opposite side
- Submetacentric: Centromere is subterminal; chromosome is J-shaped
- Metacentric: Centromere is in the center; chromosome is V-shaped

SAT Chromosomes

Some chromosomes have a secondary constriction in addition to the centromere
A short segment of a chromosome is attached to this constriction
Examples: Chromosomes 13, 14, 15, 21, 22 in humans
Associated with the formation of the nucleolus

Molecular Structure of Chromosomes

Chromosomes are made up of linear, double-stranded DNA molecules associated with histone and non-histone proteins
Chromatin: The mixture of DNA and protein

Euchromatin and Heterochromatin

Euchromatin: Condensed during division and extended or uncoiled during interphase
Heterochromatin: Remains condensed throughout the cell cycle; genetically inactive and contains no genes or repressed genes
Heterochromatin: Two types
- Facultative heterochromatin: Some chromosomes become condensed in one sex or cell types while remaining euchromatic in others
- Constitutive heterochromatin: Remains heterochromatin at all times

Barr Body (Example of Facultative Heterochromatization)

In mammals: Males are heterogametic (XY) and females are homogametic (XX)
Dosage compensation between male and female sex with respect to X chromosomes is achieved by inactivating one of the two X chromosomes in females
The heterochromatized X chromosome appears as a darkly-staining body attached to the nuclear membrane called a Barr body
The discovery of X inactivation is attributed to British geneticist Mary Lyon - “Lyonization"

Telomeres

Ends of chromosomes
Consist of many tandem repeats of short sequences (generally hexamers)
In humans, the telomeric sequence is TTAGGG (repeated over 50 times)
Added to the ends of chromosomes by an enzyme called telomerase
Provide stability to chromosomal ends
As cells age, telomeres shorten, contributing to senescence (aging)

Chromatin Structure

Histones: Small basic, positively charged proteins that bind to negatively charged DNA
5 basic histones in eukaryotes: H1, H2A, H2B, H3, and H4
Chromatin contains an equal amount of histone and DNA
Non-histone proteins: Acidic proteins, found in smaller proportions; vary from tissue to tissue and between species; responsible for specific gene expression

Nucleosomes

First level of packaging DNA within the nucleus
DNA is wound around a core of histones
Nucleosome core particle: Two each of H2A, H2B, H3, and H4 (histone octamer)
H1 histone loosely bound to the nucleosome core particle
146 base pairs wrap around the core particle

Higher Levels of Packaging

30 nm chromatin fiber: Nucleosomes folded in a zigzag manner
300 nm fiber: 30 nm fiber packed
700 nm fiber: Further folding of the 300 nm fiber
1400 nm fiber: Highest level of compaction seen in metaphase chromosomes

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Description

Explore the fascinating world of chromosomes with this quiz covering viral, prokaryotic, and eukaryotic structures. Understand the differences in packaging, structure, and function among these varying types of chromosomes. Test your knowledge on genome sizes, supercoiling, and diploid numbers.