Question

The structure of chromatin at the centromeres is passed on to daughter cells through epigenetic inheritance, meaning without any input from the DNA sequence. Explain how this occurs.  

Answer 1

Chromatin: It is a mass of genetic material and chromatin is composed of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.

Centromeres: The centromere is that point on a chromosome where mitotic spindle fibers are attached to drag sister chromatids during cell division.

Epigenetics: It consists of a record of the chemical changes to the DNA and histone proteins of an organism; these changes can be passed down to an organism’s offspring via transgenerational epigenetic inheritence.

Now, the question is structure of chromatin at the centromere is passed on to daughter cells through epigenetic inheritance, without any input from the DNA sequence.

Answer: Epigenetic is passing on modified gene functions to subsequent generations without changing the DNA sequence.

• There are four mechanisms mentioned below:
• DNA Methylation
• Covalent Histone Modification
• Chromatin remodelling or non covalent modifications
• Non Coding RNA

• DNA Methylation: DNA methylation can lead to gene silencing by either preventing or promoting the recruitment of regulatory proteins to DNA.
• Methylation of DNA is associated with suppression of gene transcription.
• In most higher eukaryotes, a proportion of the cytosine bases are modified by the addition of methyl ( CH3 ) group to the number- 5 carbon atom by the enzyme called DNA methylase.
• In mammals, cytosines are modified preferentially in 5’- CG- 3’ dinucleotides.

• Covalent Histone Modification: Histone modifications can lead to either activation or repression depending upon which residues are modified and type of modifications present. For instance: Lysine acetylation correlates with transcriptional activation whereas lysine methylation leads to transcriptional activation and repression depending upon which residue is modified and degree of methylation. These modifications serve as epigenetic tags or marks.

• Chromatin remodelling:

• FIRST IS NUCLEOSOME SLIDING
• THE SECOND IS EJECT HISTONE OCTAMERS
• THE THIRD IS REMOVE H2A-H2B DIMERS
• FOURTH IS REPLACE DIMERS

• Chromatin remodelling factors use energy from ATP hydrolysis to rearrange the packing of nucleosomes in higher order chromatin structures.

• Non coding RNA: Regulatory ncRNA includes short interfering RNA’s, micro RNA’s and non coding RNA’s.
• It play important roles in gene expression regulation at post transcriptional level by mRNA degradation during splicing.