Chromatin structure

Chromatin structure -Learn interactively

Dr. Rohan Gavankar

Multiple Choice

The structure in the image is called a:

• Chromosome

• Karyotype

• Gene

• Protein

Multiple Choice

A _________ is DNA tightly coiled around proteins.

• gene

• chromosome

• trait

• mutation

Multiple Choice

How many pairs of homologous chromosomes are found in each (body) cell?

• 46

• 23

• 72

• 10

Chromatin ------ know what am I made

•  A chromosome at mitotic metaphase consists of two symmetrical structures called chromatids. 

• Each chromatid contains a single DNA molecule and both chromatids are attached to each other by centromere and become separated at the beginning of anaphase.

  
  

Centromeres and Telomeres

• Centromere in a chromosome contain specific DNA sequences with special proteins bound to them, forming a disc shaped structure, called kinetochore.

• Telomere is the chromosomal ends which prevents other chromosomal segments to be fused with it.

• Besides the primary constrictions or centromeres, chromosomes also posses secondary constriction at any point of the chromosome and are constant in their position and extent.

Multiple Choice

The ___________ holds the replicated sister chromatids together.

• centriole

• chromosomes

• centromere

• cycles

Multiple Choice

The individual strands on duplicated chromosomes are called_____________

• Centromeres

• daughters

• Chromatids

• Karyotypes

DNA

­DNA is the most important chemical component of chromatin, since it plays central role of controlling heredity and is most conveniently measured in picograms.

 In addition to describing the genome of an organism by its number of chromosomes, it is also described by the amount of DNA in a haploid cell.

 This is usually expressed as the amount of DNA per haploid cell (usually expressed as picograms) or the number of kilobases per haploid cell and is called the C value. 

Proteins

 Histones: Histones are basic proteins as they are enriched with basic proteins arginine and lysine. At physiological pH they are cationic and can interact with anionic nucleic acids. They form a highly condensed structure. The histones are of five types called H1, H2A H2B, H3, and H4-which are very similar among different species of eukaryotes and have been highly conserved during evolution. H1 is the least conserved among all and is also loosely bound with DNA. H1 histone is absent in Sacharomyces cerevisiae.

Proteins

Non-histones: In addition to histones the chromatin comprise of many different types of non-histone proteins, which are involved in a range of activities, including DNA replication and gene expression. They display more diversity or are not conserved. They may also differ between different tissues of same organism.

Multiple Choice

Histones are

• proteins wound up in DNA

• critical to maintaining the shape of chromosomes

• aid in the tight packing of DNA

• all of these

Multiple Choice

When DNA is in its less coiled, "relaxed," state, between cell divisions, it is called

• autosomes
• chromatid
• chromatin
• centromere

Chromatin Packing

• Roger Kornberg in 1974 described the basic structural unit of chromatin which is called the nucleosome. 

• Winding of DNA around a protein core to produce a "bead-like" structure called a nucleosome.

• Each nucleosome has a packing ratio of six.

First level of packing

The protein core is composed of 8 histone proteins, two each of H2A, H2B, H3 and H4. Histone H1 forms the linker between to nucleosomes.

146 bp of DNA is wrapped around each nucleosome.

Second level of packing

Coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40.

This appears to be a solenoid structure with about 6 nucleosomes per turn. This gives a packing ratio of 40, which means that every 1 µm along the axis contains 40 µm of DNA. The stability of this structure requires the presence of the last member of the histone gene family, histone H1. Because experiments that strip H1 from chromatin maintain the nucleosome, but not the 30 nm structure, it was concluded that H1 is important for the stabilization of the 30 nm structure.

Final level of packing

The fiber is organized in loops, scaffolds and domains that give a final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic chromosomes.

The final level of packaging is characterized by the 700 nm structure seen in the metaphase chromosome. The condensed piece of chromatin has a characteristic scaffolding structure that can be detected in metaphase chromosomes. This appears to be the result of extensive looping of the DNA in the chromosome.                      

Multiple Select

Nucleosome core particles can be seen as a structure like beads on string under SEM Microscope.

• TRUE

• FALSE

Multiple Select

Chromatin is a highly extended nucleoprotein fibres in chromosomes.

• True

• False

• Maybe

Multiple Select

Histones is a DNA binding protein.

• True

• False

• Maybe

Multiple Choice

Write the correct sequence in the level of chromatin organization.

• 1,2,3,4

• 4,3,2,1

• 4,3,1,2

• 3,4,1,2