Explanation
DNA replication is the bidirectional, semiconservative process of synthesis of daughter DNA strand by polymerisation of deoxyribonuclotides using parental DNA strand as template. The process is carried out by DNA polymerase. DNA polymerase cannot synthesize daughter strand de novo but can only add monomers to already present DNA strand. This requirement is fulfilled by a primer which is a strand segment (complementary to the template). It has a free 3-hydroxyl group to which a nucleotide can be added. Most primers are oligonucleotides of RNA rather than DNA. A RNA polymerase, known as primase, synthesizes the primer using DNA strand as template. Thus, RNA polymerase with primase activity control DNA replication.
Two deoxyribonucleotides are joined together by phosphoryl group transfer. The 3’ hydroxyl group of the nucleotide at the 3’ end of the growing strand serve as nucleophile. Nucleophilic attack occurs at the $$\alpha$$-phosphorus of the incoming deoxynucleotide with release of inorganic pyrophosphate. Nucleotides have phosphate group at their 5’ carbon, the 3’ carbon has hydroxyl group; thus, the phosphoester bond cannot be formed between 5’ hydroxyl and 3’ phosphate group, option B is incorrect. Correct answer is A.
A transcription unit is a sequence of nucleotides in DNA that carries the information for encoding a single RNA molecule as well as the sequences required for its transcription namely a promoter, an RNA-coding sequence, and a terminator. The promoter is a DNA segment that serves to initiate transcription of a particular gene. They are present upstream near the transcription start sites of genes. The gene that codes for any RNA / protein product which does not serve as regulatory protein is known as structural genes. The lac z, lac y and lac genes in lac operon are the structural genes. The inducer is a molecule that binds to the repressor and causes conformational changes in repressor which in turn can no longer bind to the operator. Thus, inducer induces the expression of the structural gene. It is not part of an operon. Termination sites are present downstream the structural genes which serve to terminate the transcription. The correct option is A.
Correct Option: D
Explanation:
In DNA there are two strands.
They have many base pairs which form bonding and form a stair-like structure.
Both the strands in DNA run opposite to each other.
They are called antiparallel.
The polarity is 5’→3’ always.
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