Distinguish between heterochromatin and euchromatin. Which of the two is transcriptionally active?
Densely packed and dark stained chromatin regions are called hetorochromation, while, loosely packed light stained regions are called euchromatin.
Euchromation is transcriptionally active and is transcribed into mRNA. Due to very tight coiling heterochromatin can not be transcribed and is inert/inactive form.
The enzyme DNA polymerase in E.coli is a DNA dependent polymerase and also has the ability to proofread the DNA strand being synthesised Explain. Discuss the dual polymerase.
In bacteria, three types of DNA polymerases are there. All of them can add nucleotides in $5^{\prime} \rightarrow 3^{\prime}$ direction. They process exonuclease activity as well. DNA polymerase III can proofread the newly synthesised strand and senses the wrong base insertions.
It deletes wrong bases and helps correct the mistake by putting in the right one, DNA polymerase. The only mistake it cannot corrects substitution of uracil in place of thymin.
It can repair any damages done to DNA by UV exposure, etc., or the left over proofreading mistakes. It detects mutation caused by UV, removes mismatched pairs and puts back the right ones.
What is the cause of discontinuous synthesis of DNA on one of the parental strands of DNA? What happens to these short stretches of synthesised DNA?
Synthesis of DNA always takes place in $5^{\prime} \rightarrow 3^{\prime}$ direction. In a double stranded DNA both strands are anti parallel and complementary. During DNA synthesis as both strands act as templates, only one strand, i.e., $3^{\prime} \rightarrow 5^{\prime}$ can synthesis complementary strand in $5^{\prime} \rightarrow 3^{\prime}$ direction.
The other strand, i.e., $5^{\prime} \rightarrow 3^{\prime}$ has to be synthesised in small stretches in opposite direction as replication fork moves to right. That is why DNA synthesis is discontinuous on one of the parental strands of DNA. These small stretches called Okazaki fragments are joined together by DNA ligase enzyme that closes the nicks.
Given below is the sequence of coding strand of DNA in a transcription unit 3' AATGCAGCTAT TAGG-5' write the sequence of
(a) its complementary strand
(b) the $m R N A$
According to base complementary rules,
(a) 5'TTACGTCGATAATCC-3'
(b) 5'CGAUUAUCGACGUAA-3'
RNA uses the base uracil ( U ) rather than thymine ( T ). So, in RNA the base pairs are
Adenine (A) pairs with uracil (U)
Guanine (G) pairs with cytosine (C).
What is DNA polymorphism? What is it important to study it?
DNA polymorphism refers to the variation in DNA arising through mutation at non-coding sequences.
A special type of polymorphism, called VNTR (Variable Number of Tandem Repeats), is composed of repeated copies of a DNA sequence that lie adjacent to one another on the chromosome. Since, polymorphism is the basis of genetic mapping of human genome, therefore, it forms the basis of DNA fingerprinting too.
The single nucleotide polymorphisms are used in locating diseases and tracing of human history as well as in case of paternity testing.