Heterochromatin And Euchromatin

Heterochormatin - It is a region of chromosome whose properties are as follows-

• It is highly condensed.
• It is found in centromere and telomere region .
• It contains long stretches of repetitive sequences called satellite DNA.
• Replication occours in late s- phase .
• Meiotic recombination is absent .
• Heterochomatin Dna is not very active.
• Transcriptionally silent and silences adjecent genes .
• Types of heterochromatin -

1.Constitutive Heterochromatin - It remains always as heterochromatin untill no change due to mutation hence also called as obligate heterochromatin. It is usually repetitive and forms structural functions such as centromere or telomers.

e.gCentromeric heterochromatin,telomeric heterochromatin, Retrotransposons

2.Facultative Heterochromatin -It may convert to euchromatin depending upon the requirement .It is the result of genes that are silenced through a mechanisms such as Histone methylation or sirna by RNAi.

e.g Barr body in Mammalian female and female Drosophila 

                                    Image source credit -http://www.discoveryandinnovation.com/BIOL202/notes/lecture18.html

Euchromatin -

• It is less condensed ,loosely packed regions of chromatin .
• It is lightly stained.
• Replication occours in throughout  s- phase .
• Meiotic recombination is present.
• Euchomatin Dna is very active.
• Transcriptionally  not silent, very active .

MCQs.
1.In eukaryotes, transcription is generally associated with 
A) euchromatin only. 
B) heterochromatin only. 
C) very tightly packed DNA only. 
D) both euchromatin and histone acetylation

Ans. d

2.Some of the heterochromatin region can convert into euchromatic regions ,in different cell cycle stages. They are called -
a.Constitutive heterochromatin
b.Facultative heterochromatin
c.Euchromatin 
d.None of the above 

Ans. b

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