Solved Exercise, Bio-12, Ch-20


(i) Particular tRNA molecules become attached to specific amino acids through the action of activating enzymes called _______. (aminoacyl-tRNA synthetase)

(ii) _______ is the transfer of genetic material from one cell to another and can alter the genetic make up of the recipient cell. (transformation)

(iii) In bacteria, a subunit of RNA polymerase called _______ recognizes -10 sequence in the promoter and binds RNA polymerase there. (sigma factor)

(iv) A typical human chromosome contains about ______ nucleotides in its DNA. (140 million)

(v) Miescher extracted a white substance from the nuclei of human cells and fish sperm and called this substance _______. (nuclein)


(i) The strand of DNA that is not transcribed is called the coding strand. (TRUE)

(ii) TATAAT sequence called -35 sequence is part of promoter, where transcription actually starts. (FALSE)

CORRECT: TATAAT sequence called -10 sequence is part of promoter, where transcription actually starts.

(iii) Rosalind Franklin carried out an X-ray diffraction analysis of DNA. (TRUE)

(iv) The base pairs in DNA helix are planar and stack 34 nm apart as a result of hydrophobic interactions. (FALSE)

CORRECT: The base pairs in DNA helix are planar and stack 0.34 nm apart as a result of hydrophobic interactions.


(i) mRNA is synthesized by.

(a)  DNA polymerase

(b)  RNA polymerase

(c)  RNA ligase

(d) None of these

EXPLANATION: During transcription, RNA polymerase reads the DNA template and synthesizes a complementary mRNA strand, which carries the genetic information from the DNA to the ribosomes for protein synthesis.

(ii) Which of the following are nonsense codons?

(a)  AUG

(b)  UAA

(c)  CUA

(d) All of above

EXPLANATION: Out of 64 codons, three codons UAA, UAG and UGA do not code for any amino acid and hence are known as nonsense codons. These codons are usually present at the end of the gene and hence are also called stop codons.

(iii) Enzyme are responsible for assembly of.

(a)  Nucleic acid

(b)  Protein

(c)  Carbohydrates

(d) All a, b, c

EXPLANATION: Genes produce their effects by specifying the structure of enzymes which then facilitate the biochemical reactions involved in the synthesis of these macromolecules.

(iv) In bacteria, the newly synthesized mRNA is released in.

(a)  Nucleus

(b)  Cytoplasm

(c)  Mitochondria

(d) b and c

EXPLANATION: In bacteria, unlike eukaryotic cells, there is no nucleus, and transcription and translation occur in the same cellular compartment. Therefore, the newly synthesized mRNA is released directly into the cytoplasm where it can undergo translation to produce proteins.



(1) Ribosomal RNA:

“The class of RNA found in ribosome is called ribosomal RNA (rRNA).”

During translation, rRNA provides the site where polypeptides are assembled.

(2) Transfer RNA:

“Transfer RNA molecules transport the amino acids to the ribosomes for use in building the polypeptides and also position each amino acid at the correct place on the elongating polypeptide chain.”

Human cells contain about 45 different kinds of tRNA molecules.

(3) Messenger RNA:  

Messenger RNA are long strands of RNA that are transcribed from DNA and that travel to the ribosomes to direct precisely which amino acids are assembled into polypeptides

Function of RNA Polymerase in Transcription:

There are different types of RNA polymerases (I, II, & III) in eukaryotes which are responsible for transcribing different types of RNA (rRNA, mRNA, and tRNA). The functions of RNA polymerase in transcription are:

(1) Initiation:

RNA polymerase binds to the promoter on the DNA template strand to start the transcription process.

(2) Elongation:

RNA polymerase synthesizes an RNA copy of the DNA sequence in the 5′ to 3′ direction.

(3) Termination:

RNA polymerase recognizes the termination signal and releases the newly synthesized RNA molecule, completing the transcription of the gene.

After Crick’s initial experiments, Marshall Nirenberg, Philip Leader and Har Gobind Khorana tested all the 64 codons by making artificial mRNAs and triplet codons and using them to synthesize a protein or aminoacyl-tRNA complexes in cell free systems.


“An anticodon is a sequence of three nucleotides on a transfer RNA (tRNA) molecule that is complementary to a specific codon on mRNA.”

The anticodon pairs with the corresponding codon during protein synthesis, ensuring the accurate placement of amino acids in the growing polypeptide chain.


Consult textbook at page 128.

Consult textbook at page 129 — 132.

Consult textbook at page 133 — 134.

Consult textbook at page 135 — 136.

Consult textbook at page 138 — 139.

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