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Everything you need to know about molecular biology. Some of the most frequently asked questions are as follows:-
Q.1. What is transcription?
Ans: The transcription is the process in which the genetic information stored in the DNA is used to code the synthesis of RNA. The strand of DNA responsible for doing it or working as a template for the synthesis of RNA is known as sense strand.
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Q.2. What is mRNA or messenger RNA?
Ans: The mRNA has the code which is transcribed from DNA genetic information and is used to specify a sequence of amino acids; in protein synthesis.
Q.3. How does the longevity of bacterial and eukaryotic mRNA differ?
Ans: The mRNA of bacterial cells generally last for only a few minutes while in eukaryotic cells it remains functional for hours and days.
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Q.4. What is tRNA or transfer RNA?
Ans: The tRNA decodes the mRNA sequence or translates it into a correct amino acid sequence. The tRNA carries a specific amino acid to the ribosome for the process of protein synthesis.
Q.5. What is rRNA or ribosomal RNA?
Ans: The rRNA molecules constitute the major part of the ribosome while the remaining part of the ribosome is the ribosomal protein.
Q.6. How does the synthesis of RNA take place in the process of transcription?
Ans: It takes place in the following stages:
1. Unwinding the double helical DNA molecule for a short sequence of nucleotide.
2. Alignment of complimentary ribonucleotides by base pairing opposite the nucleotides of the DNA strand being transcribed and linkage of these nucleotides with phosphodiester bonds by DNA dependent RNA polymerase.
3. In transcription RNA polymerase moves along a DNA template adding ribonucleotides to synthesize RNA.
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4. The process moves from an upstream region where RNA polymerase binds till termination occurs.
5. The downstream movement is towards the 3 ‘-OH end of the synthesized RNA molecule.
Q.7. Which are the enzymes to synthesize RNA from ribonucleotides?
Ans: These are DNA dependent RNA polymerases.
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Q.8. What is open reading frame?
Ans: The ORF or open reading frame is a region of nucleotide sequence coding for a polypeptide. The ORF is equivalent to a gene and contains codons which code for amino acids.
Q.9. What is promoter region? Give its function.
Ans: It is a specific region of nucleotides in DNA. Its function is to initiate the process of transcription.
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Q.10. What is meant by “upstream” in DNA undergoing transcription task?
Ans: It indicates that it is transcribed before downstream nucleotides.
Q.11. What is Pribnow sequence?
Ans: It is a conserved DNA nucleotide sequence in bacterial genes that take part in initiation of transcription. It is easily recognized by RNA polymerase and is same or almost same as TATAAT for many of the bacterial promoters.
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Q.12. What is a consensus sequence?
Ans: It means region of general agreement that is high nucleotide sequence homology. It is a conserved sequence of nucleotides serving same function in diverse cells.
Q.13. What do you mean by TATA box?
Ans: It is a conserved A-T rich DNA sequence in eukaryotic cells that is recognized by TFIID and helps RNA polymerase II in initiating transcription. It is represented as 5′-TATA (T/A) A (T/ A).
Q.14. What is sigma (σ) factor?
Ans: It is the factor which ensures that RNA synthesis begins at a correct site. A sigma unit is, however, a subunit of RNA polymerase which helps to recognize the promoter site.
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Q.15. What are strong and weak promoters in the process of transcription?
Ans: A promoter with nucleotide sequence which favours transcription by RNA efficiently, is strong promoters while the one which favours inefficient transcription, is called a weak promoter.
Q.16. How does elongation of RNA strand take place in the process of transcription?
Ans: On establishment of initiation of transcription the enzyme releases its sigma factor. A ternary complex of RNA poly merase-DN A-newly synthesized RNA strand results. The RNA polymerase after it moves along the DNA template and elongates the growing RNA strands. In this way new ribonucleotides are added at the rate of about 40 nucleotides per second at 37°C which is very slow in comparison to the rate of replication of DNA in which 1000 deoxyribonucleotides per second are added.
Q.17. How does termination of transcription occur? Give its types in bacteria.
Ans: There are certain sites that are called termination sites which act as a signal to stop the transcription. The termination sites may be located either in the RNA sequences which have already been transcribed or in the DNA template.
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In bacteria the termination of transcription is of two types, which are:
1. Simple or rho (p) independent termination that does not need any additional factor.
2. Rho (p) dependent termination which requires an additional protein called rho (p) factor.
Q.18. Which are the major post-transcriptional modifications of RNA?
Ans: It involves removal of specific nucleotide sequences known as introns which are intervening sequences and splicing (joining) together of remaining sequences known as exons.
Q.19. What do you mean by cap in post-transcriptional RNA modification?
Ans: The cap in it is of 7-methyl guanosine (mG), in eukaryotic: RNA molecules. It also has a poly A tail, and a leader sequence besides a cap.
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Q.20. What do you mean by translation of genetic code?
Ans: The assembly of polypeptide chains in which mRNA serves as a template, occurring in ribosomes is the translation of genetic code. In simple words it may be regarded as the process in which information in RNA obtained from DNA in the process of transcription, is directed to the sequence of amino acids in a protein.
Q.21. Define the genetic code.
Ans: The genetic code may be defined as the code for specific amino acids formed by three sequential nucleotides in mRNA. The genetic code is also known as triplet code. The 64 codons (each triplet nucleotide sequence) formed by sequences of 3 nucleotides specify the genetic information of all organisms.
Q.22. Why are termination codons or nonsense codons named so?
Ans: As they do not code for any amino acid.
Q.23. What is the function of termination codons?
Ans: They work as punctuators that signal the termination of the synthesis of a polypeptide chain.
Q.24. Which is the first amino acid of polypeptide sequence in Archaeal and eukaryotic microorganisms?
Ans: It is methionine coded by the codon AUG.
Q.25. What is the role of transfer RNA in protein synthesis? How does it work with its unique structure?
Ans: The transfer RNA brings the amino acids to ribosomes and properly aligns them during the process of translation. The attachment of an amino acid to its specific tRNA molecule is known as charging and the tRNA molecule which is attached to its amino acid is referred to as charged and the process as charging of tRNA.
There are at least 20 different types of amino acids and each of the 20 amino acids that occur in proteins binds to a different RNA. The unique structure of tRNA (as clover-leaf model) plays a very important role in establishing proper alignment of molecules during translation.
One of the four lobes is attached to amino acid, contains a nucleotide sequence which interacts with rRNA for proper orientation to ribosomes, third reacts with aminoacyl synthesis and fourth has a region called anticodons which interacts with mRNA.
