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RNA Transcription

Class - XII

RNA TRANSCRIPTION

Very short questions - One mark Question: -

1.What is a cistron?

ANS: - Cistron is a segment of DNA consisting of a stretch of base sequences that codes for one polypeptide chain, one transfer RNA (tRNA), ribosomal RNA (rRNA) molecule or performs any other specific function in connection with transcription, including controlling the functioning of other cistrons.

2.Write the function of RNA polymerase II.

ANS: - In eukaryotes, RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).

3.Name the enzyme that transcribes hnRNA in eukaryotes.

ANS: - In eukaryotes, RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).

4.Di­fferentiate between exons and introns.

ANS: - Exons are the segments in genes which contain coding nucleotide sequences. These sequences are ultimately translated into polypeptide. ‑us, exons carry genetic information. Introns are the segments in genes which contain non-coding nucleotide sequences. These do not form part of mRNA and are removed during the processing of hnRNA.

5.Which one out of Rho factor and Sigma factor act as initiation factor during transcription in a prokaryote?

ANS: - Sigma (ε) factor acts as initiation factor during transcription in a prokaryote.

6.Which one of an intron and an exon is the reminiscent of antiquity?

ANS: - Intron is considered to be as the reminiscent of antiquity.

7.Why hnRNA is required to undergo splicing?

ANS: - hnRNA undergo splicing in order to remove introns, which are intervening or non-coding

Sequences and exons are joined in a defined order to form functional mRNA.

8.Mention the two additional processing which hnRNA needs to undergo after splicing so as to become functional.

ANS: - Capping and tailing

9.When and at what end does the ‘tailing’ of hnRNA take place?

ANS: - ‘Tailing’ of hnRNA takes place during modification of hnRNA into functional mRNA. It takes place at 3′-end.

10.At which ends do ‘capping’ and ‘tailing of hnRNA occur respectively?

ANS: - Capping and tailing of hnRNA occur respectively at 5′ end and 3′ end.

11.Name the parts ‘A’ and ‘B’ of the transcription given below:

ANS: - A – Promoter

B – Coding strand

Short questions - Two mark Question:-

12.Di­fferentiate between a template strand and coding strand of DNA.

ANS:-

S.No

Template Strand

Coding Strand

01

Strand of DNA having 3′ → 5′ polarity.

Strand of DNA having 5′ → 3′

polarity.

02

Participates in transcription.

Do not take part in transcription

13.A template strand is given below. Write down the corresponding coding strand and the mRNA strand that can be formed, along with their polarity.

3′ ATGCATGCATGCATGCATGCATGC 5′

ANS: - The corresponding coding strand is:

5′ TACGTACGTACGTACGTACGTACG 3′

The corresponding mRNA strand is

5′ UACGUACGUACGUACGUACGUACG 3′

14.State the di­fference between the structural genes in a transcription unit of prokaryotes and eukaryotes

ANS:-

S.No

Structural gene in prokaryotes

Structural gene in eukaryotes

01

Consists of functional coding

sequences.

Consists of both exons and introns.

02

Information is continuous as only exons are present.

Information is split due to presence of introns in between exons.

03

Splicing does not take place.

Splicing occur to make functional mRNA.

15.State the functions of the following in a prokaryote:

(a) tRNA

(b) rRNA

ANS: -

(a) tRNA helps in transferring amino acids to ribosome for synthesis of polypeptide chain. tRNA reads the genetic codes, carries amino acids to the site of protein synthesis and acts as an adapter molecule.

(b) rRNA is the most abundant RNA. Prokaryotic ribosomes are of three types 23S, 16S and 5S. 23S

and 5S occur in large subunit of ribosome while 16S is found in smaller subunit. It plays structural and

catalytic role during translation

16. Differentiate between a cistron and an exon.

ANS: - Cistron is segment of DNA consisting of a stretch of base sequences those codes for one polypeptide chain, one transfer RNA (tRNA), ribosomal RNA (rRNA) molecule or performs any other specific function in connection with transcription, including controlling the functioning of other cistrons.Exons are the regions of a gene, which become part of mRNA and code for the di­fferent proteins.

17.Differentiate between exons and introns.

ANS:-

S.No

Exons

Introns

01

Regions of a gene which become part of mRNA.

Regions of a gene which do not form

part of mRNA.

02

Code for the di­fferent proteins.

Removed during the processing of

mRNA.

Short questions - Three mark Question:-

18. Write the help of a schematic diagram; explain the location and the role of the following in a transcription unit: Promoter, Structural gene, Terminator.

ANS: - Schematic representation of transcription unit is as follows:

(i) Promoter - ‑e promoter is located towards 5′-end of the coding strand. It is a DNA sequence

that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that defines the template and coding strands.

(ii) Structural gene - ‑e structural genes code for the enzymes and proteins. It transcribe the mRNAfor the same.

(iii) Terminator - ‑e terminator is located towards 3′-end of the coding strand and it defines the end of the process of transcription.

19.(a) What are the transcriptional products of RNA polymerase III?

(b) Di­fferentiate between ‘Capping’ and ‘Tailing’.

(c) Expand hnRNA.

ANS: -

(a) The transcriptional products of RNA polymerase III are tRNA, 5SrRNA and snRNA.

(b) In capping, additional nucleotides (methyl guanosine triphosphate) are added to the 5′-end of

hnRNA. In tailing, adenylate residues (200 - 300) are added at the 3′-end in a template independent manner.

(c) Heterogeneous nuclear RNA.

20.(a)Construct a complete transcription unit with promotor and terminator on the basis of the hypothetical template strand given below:

(b) Write the RNA strand transcribed from the above transcription unit along with its polarity.

Ans:-

21.Describe with the help of a schematic representation the structure of a transcription unit.

22.(a) Name the enzyme that catalyses the transcription of hnRNA.

(b) Why does the hnRNA need to undergo changes? List the changes hnRNA undergoes and where in the cell such changes take place.

ANS: -

(a) RNA polymerase II transcribes hnRNA.

(b) Post transcription processing of hnRNA is required to convert primary transcript of all types of RNA into functional RNAs. ‑e eukaryotic transcription involves certain complexity, one of the complexity is that the primary transcript contain both the exons and the introns (which are non-functional). Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. the transcribed eukaryotic RNA or heterogenous nuclear RNA (hnRNA), undergoes additional processing called as capping and tailing. In capping, an unusual nucleotide (methyl guanosine triphosphate) is added to the 5’ - end of hnRNA. In tailing, adenylate residues (200 - 300) are added at the 3′-end in a template independent manner. It is the fully processed hnRNA, now called mRNA. This process takes place in the nucleus of a cell and then mRNA is transported out of the nucleus for translation.

23.Monocistronic structural genes in eukaryotes have interrupted coding sequences. Explain.How are they di­fferent in prokaryotes?

ANS: - In eukaryotes, monocistronic structural genes could be said as interrupted, as both introns and exons are present.

The corresponding coding strand is :5′ TACGTACGTACGTACGTACGTACG 3′

The corresponding mRNA strand is :5′ UACGUACGUACGUACGUACGUACG 3′

24. Di­fferentiate between the following:

(a) Promoter and terminator in a transcription unit.

(b) Exon and intron in an unprocessed eukaryotic mRNA.

ANS: - (a) Di­fferences between promoter and terminator.

S.No

Promoter

Terminator

01

Located upstream of structural gene.

Located downstream of structural gene.

02

Has RNA polymerase binding and recognition site.

Rho factor required for termination.

03

In many cases, promoter has AT

rich region.

It has stop signal and also possess 4-8 A-nucleotides.

25.Describe the initiation process of transcription in bacteria.

ANS: - A DNA transcription unit has a promoter region, initiation site, coding region and a terminator region. Transcription begins at the initiation site and ends at the terminator region. A promoter region has RNA polymerase recognition site and RNA polymerase binding site. Chain opening occurs in the region occupied by TATAATG nucleotides (TATA box) in most prokaryotes. Enzymes required for chain separation are unwindases, gyrases and single stranded binding proteins. During initiation of transcription, RNA polymerase (common in prokaryotes and specific in eukaryotes) binds itself to the promoter region. ‑e two strands of DNA uncoil progressively from the site of polymerase binding. One of the two strands of DNA (3′ → 5′) functions as template for transcription of RNA (template strand). Transcript formation occurs in 5′ → 3′ direction. Ribonucleoside triphosphates present in the surrounding medium come to lie opposite the nitrogen bases of the DNA template (anti-sense strand). They form complementary pairs; U opposite A, A opposite T, C opposite G and G opposite C. A pyrophosphate is released from each ribonucleoside triphosphate to produce ribonucleotide.

26.Describe the elongation process of transcription in bacteria.

ANS: - With the help of RNA polymerase the adjacent ribonucleotides held over DNA template join to form RNA chain. As the RNA chain formation initiates, the sigma (s) factor of the RNA polymerase separates. RNA polymerase (core enzyme) moves along the DNA template causing elongation of RNA chain at the rate of some 30 nucleotides per second.RNA synthesis stops as soon as polymerase reaches the terminator region.

27.Explain the role of DNA-dependent RNA polymerase in transcription.

ANS: - Transcription requires DNA-dependent RNA polymerase. RNA polymerase binds to promoter and initiates transcription. With the help of RNA polymerase the adjacent ribonucleotides held over DNA templates join to form RNA chain. Once the polymerases reaches the terminator region, the nascent RNA falls off, so also the RNA polymerase. This results in termination of transcription. In eukaryotes there are at least three DNA dependent RNA polymerases in the nucleus. The RNA polymerase I transcribes rRNAs (28S, 18S and 5.8S), whereas the RNA polymerase II transcribes precursor of mRNA, the heterogenous nuclear RNA (hnRNA). RNA polymerase III is responsible for transcription of tRNA, 5S rRNA and some snRNAs. Whereas prokaryotes have only single DNA dependent RNA polymerase.

Long Question - Five mark Question( Any Three):-

28 .(a) Describe the process of transcription in bacteria.

(b) Explain the processing the hnRNA needs to undergo before becoming functional mRNA of eukaryotes.

ANS: - (a) Transcription is the process of copying genetic information from one strands of DNA into

RNA. A transcription unit of a DNA has three regions a promoter, a structural gene and a terminator.

Bacterial structural gene in a transcription unit is polycistronic. Transcription requires a DNA dependent RNA polymerase. Prokaryotes have only one DNA dependent RNA polymerase which synthesises all types of RNA. Three major types of RNAs in bacteria are mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs are needed to synthesise a protein in a cell. The mRNA provides the template, tRNA brings amino acids and reads the genetic code, and rRNAs play structural and catalytic role during translation. In bacteria/prokaryotes, transcription occurs in contact with cytoplasm as their DNA lies in the cytoplasm. RNA polymerase binds to promoter and initiates transcription (Initiation). It uses nucleoside triphosphates as substrate and polymerises the mRNA strand in a template depended fashion following the rule of complementarity. It also facilitates opening of the helix and continues elongation. Only a short stretch of RNA remains bound to the enzyme. Once the polymerases reaches the terminator region, the nascent RNA falls o­f, so also the RNA polymerase. ‑is results in termination of transcription.

(b) In eukaryotes, the primary transcript which is often larger than the functional RNA is called heterogeneous nuclear RNA or hnRNA. Post transcription processing is required to convert

primary transcript of all types of RNAs into functional RNAs. It is of four types.

(i) Cleavage : Larger RNA precursors are cleaved to form smaller RNAs.

(ii) Splicing : Eukaryotic transcripts possess extra segments called introns or intervening sequences or noncoding sequences. They do not appear in mature or processed RNA. The functional coding sequences are called exons. Splicing is removal of introns and fusion of exons to form functional RNAs.

(iii) Terminal additions (capping and tailing) :

Additional nucleotides are added to the ends of RNAs for specific functions, e.g., CCA segment in tRNA, cap nucleotides at 5′ end of mRNA or poly-A segments (200-300 residues) at 3′ end of mRNA. Cap is formed by modifications of GTP into 7-methyl guanosine or 7 mG.

(iv) Nucleotide modifications They are most common in tRNA-methylation (e.g., methyl cytosine,methyl guanosine), deamination (e.g., inosine from adenine), dihydrouracil, pseudouracil, etc.

29. Explain the process of transcription in prokaryotes. How is the process di­fferent in eukaryotes?

ANS: - Mechanism of transcription in prokaryotes:

In bacteria/prokaryotes, transcription occurs in contact with cytoplasm as their DNA lies in the cytoplasm.

(a) Activation of ribonucleotides – The four types of ribonucleotides are adenosine monophosphate (AMP), guanosine monophosphate (GMP), uridine monophosphate (UMP) and cytidine monophosphate (CMP). They occur freely in the nucleoplasm. Prior to transcription the nucleotides are activated through phosphorylation. Enzyme phosphorylase is required along with energy. The activated or phosphorylated Ribonucleotides are adenosine triphosphate (ATP), guanosine triphosphate (GTP), uridine triphosphat (UTP) and cytidine triphosphate (CTP).

(b) Binding of RNA polymerase to DNA duplex – On a signal from the cytoplasm, DNA segment become ready to transcribe. The RNA polymerase enzyme binds to a specific site, called promoter, in the DNA double helix. Prokaryotes have only one RNA polymerase that synthesise all types of RNA. The promoter also determines which DNA strand is to be transcribed. Thus, a promotor region has RNA polymerase recognition site and RNA polymerase binding site.

(c) Base pairing – Ribonucleoside triphosphates present in the surrounding medium come to lie opposite the nitrogen bases of the DNA template (anti-sense strand). They form complementary pairs- U opposite A, A opposite T, C opposite G and G opposite C. A pyrophosphate is releasedfrom each ribonucleoside triphosphate to produce ribonucleotide.

(d) Formation of RNA chain – With the help of RNA polymerase the adjacent ribonucleotides held over DNA template join to form RNA chain. A single RNA polymerase recognise promoter and initiation region is prokaryotes. As the RNA chain formation initiates, the sigma (s) factor of the RNA polymerase separates. RNA polymerase (core enzyme) moves along the DNA template causing elongation of RNA chain at the rate of some 30 nucleotides per second. RNA synthesis stops as soon as polymerase reaches the terminator region. Rho factor (r) has ATP-ase activity and also possesses 4-8 adenine ribonucleotides.

(e) Separation of RNA chain – With the help of rho factor, the fully formed RNA chain is now released. One gene forms several molecules of RNA, which are released from the DNA template one after the other on completion. The released RNA is called primary transcript.

(f) Duplex formation – As the RNA chain is released, the transcribed region of the DNA moleculegets hydrogen bonded to the sense strand and the two are spirally coiled to assume the original doubl helical form. The protective protein coat is added again to the DNA duplex. Gyrases, helicases and helix stabilizing proteins are released.

Di­fferences between prokaryotic and eukaryotic transcription

S.No

Prokaryotic transcription

Eukaryotic transcription

01

It occurs in contact with cytoplasm.

It occurs inside the nucleus.

02

There is no specific period for

its occurrence.

Major part of transcription occurs in G1 and G2 phases.

03

It is coupled to translation.

 

Transcription and translation are

Specially separated.

04

Products of transcription

become e­ffective in situ.

 

Products of transcription come

out of the nucleus for functioning in

cytoplasm.

05

There is only one RNA polymerase.

There are three types of RNA polymerases.

06

RNA polymerase does not

have separate transcription

factors.

Transcription factors are involved

in recognition of promotor site.

07

mRNA is generally polycistronic.

 

mRNA is generally monocistronic.

08

Splicing is generally not required.

 

In most of the cases splicing is required for removing intervening

sequences.

30. Explain the process of transcription in eukaryotes.

ANS: - In eukaryotes, transcription occurs throughout

I-phase in di­fferentiated cells but more so in G1 and G2 phases of cell cycle inside the nucleus. Depending upon the requirement, a structural gene may transcribe one to numerous RNA molecules. The transcription products move out into cytoplasm for translation. Transcription requires a DNA dependent RNA polymerase. Eukaryotes have three RNA polymerase, Pol I (Pol A) (for ribosomal or rRNAs except 5S rRNA). Pol II (for mRNA, snRNAs) and Pol III (for transfer or tRNA, 5S rRNA. and some snRNAs). Eukaryotic RNA polymerases also require transcription factors for initiation. Prior to transcription, the nucleotides are activated through phosphorylation. Enzyme phosphorylase is required alongwith energy. Each DNA transcription

segment has a promoter region, initiation site, coding region and a termintor region. RNA polymerase (common in prokaryotes and specic in eukaryotes) binds itself to the promoter region. The two strands of DNA uncoil progressively from the site of polymerase binding. One of the two strands of DNA (3′ → 5′) functions as a template for transcription of RNA. Transcript formation occurs in 5′ → 3′ direction. Ribonucleoside triphosphate present in the surrounding medium form complementary pairs. With the help of RNA polymerase the adjacent

ribonucleotides held over DNA template join to form RNA chain. In eukaryotes, there are separate transcription factor and RNA polymerase for activation of transcription. RNA polymerase (core enzyme) moves along the DNA template causing elongation of RNA chain at the rate of some 30 nucleotides per second. RNA synthesis stops as soon as polymerase reaches the terminator region. In eukaryotes, the transcription unit yields a monocistronic mRNA. Diagrammatic representation of transcription in eukaryotes is as follows:

31.Draw a labeled schematic structure of a transcription unit. Explain the function of each component of the unit in the process of transcription.

ANS: - A transcription unit in DNA is defined by these regions in DNA :

i) Promoter :-The promoter is located towards 5′-end of the coding strand. It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that defines the template and coding strands.

(ii) Structural gene:- The structural genes code for the enzymes and proteins. It transcribe the mRNA for the same.

(iii) Terminator:-The terminator is located towards 3′-end of the coding strand and defines the end of the process of transcription.

(iv) Coding strand :- The strand of DNA with 5′ → 3′ polarity is the coding strand, it does not code for RNA.

(v) Template strand - ‑e strand of DNA with 3′ → 5′ polarity acts as the template for transcriptionof mRNA.

32. (a) Describe the process of transcription in prokaryotes. (b) Mention how is the process di­fferent in eukaryotes.

ANS: - (a) Refer to answer 28. (b) Refer to answer 28.

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