Question

1. Describe in your own words the structure of DNA.

2. Describe in your own words how DNA makes copies of itself. (I.e. Describe DNA replication.).
Protein synthesis involves two processes, transcription and translation.

Describe in your own words how each process occurs.

Transcription

Translation

Answer 1

1. STRUCTURE OF DNA

DNA is made of nucleotides. Each nucleotide made up of deoxyribose, a phosphate group and a nitrogenous base. In DNA, there are 4 bases –Adenine, Guanine, cytosine, and thymine.

DNA is a double helical structure. In the DNA duplex there are two strands one in 3’-5’ direction while other in the opposite direction. The DNA duplex is stabilized by hydrogen bonding.

Watson and Crick discovered the structure of DNA. There are 3 different type of DNA Watson and Crick discovered B form of DNA.

B-DNA

Two long polypeptide strands coiled around central axis. Strand are wrapped around a central axis. As I mentioned earlier strands are anti-parallel and the interaction in the helix is hydrogen-bonding in-between the complementary base pairs. G forms three hydrogen bonds with C and A forms two hydrogen bond with T. The angle of interaction between base pairs results in a major and minor grooves. Helix diameter is 20 Aº. 10.4 base pair per helical turn.

2. REPLICATION

DNA synthesis

Replication is the process of synthesis of new DNA. It is different in both prokaryotes and Eukaryotes. DNA polymerase is the enzyme requires for the replication.

First, we can look into prokaryotic replication. The genome of prokaryote is usually circle and continues so replication is bidirectional. In addition, the polymerize starts to do polymerization from Ori site or Origin of replication site. This site is rich with A;T base pairs because it is easy to break comparatively G:C (3 bonds). Helicase help to unwind strand. DNA C will recruit helicase. Then polymerize synthesize strand in both direction one is called lagging strand and other is called leading strand. The continues synthesis occurring strand is called leading strand and the other is called lagging strand. The polymerase will synthesize in 5’-3’ direction always. So in case of lagging strand because of the opposite direction it is difficult so the parent strand will form loop and then only the daughter strand in the 5’-3’ direction this is called trombone model (figure provided.) Finally after synthesize the two fork in opposite direction will meet at termination site and this leads to termination of the replication.

In case of eukaryotes the polymerase is littlie different. In addition the genome is heavy and linear. So there are so many ori site and replication starts from more than one ORI site simultaneously. In addition, there is no specific ter site. So termination occur where fork meet. Here in the initiation step, ORC protein will bind and cdc6 and cd1 proteins bind to the ori site. Then it recruit MCM helicase and in the cell cycle during S phase CDKs and DDKs will phosphorylate both CDcs and MCM helicase and as result of interaction of CDC45 and Gins complex MCM helicase will activate and starts to unwind the strand. Then polymeraze will starts to polymerase. All other steps are similar.

3. TRANSCRIPTION

RNA synthesis from DNA.

RNA polymerase is the enzyme required for the transcription. Promoter sequence of the DNA will recognized by the alpha subunit of RNA polymerase. Omega will stabilize the enzyme. Alpha subunit is for the assembly. So first recognition will happen then the sigma subunit is release from the enzyme and then continues transcription will occur. According to scrunching model the tension generated in the DNA is the force for releasing alpha subunit. The key event in the transcription is the conversion of closed promoter complex to open promoter complex. This is by melting from promoter region. There is no proof reading activity for RNA polymerase hence some editing are important. Termination is by either Rho independent or Rho dependent. In rho independent form anti termination loop will form as result of hoogstean base pairing and it is followed by continues stretch of Uracil because of weak bond and heavy loop RNA will slip off from DNA. In case of eukaryotes it is little different.

4. TRANSLATION.

Translation is the synthesis of protein from amino acid. It is occurring in cytoplasm Ribosome synthesize proteins based on the mRNA. tRNA will gather amino acids and peptide bond will form in-between the amino acids. 61 codons code for amino acid and there are 3 different stop and one start codon.

1. STRUCTURE OF DNA

DNA is made of nucleotides. Each nucleotide made up of deoxyribose, a phosphate group and a nitrogenous base. In DNA, there are 4 bases –Adenine, Guanine, cytosine, and thymine.

DNA is a double helical structure. In the DNA duplex there are two strands one in 3’-5’ direction while other in the opposite direction. The DNA duplex is stabilized by hydrogen bonding.

Watson and Crick discovered the structure of DNA. There are 3 different type of DNA Watson and Crick discovered B form of DNA.

B-DNA

Two long polypeptide strands coiled around central axis. Strand are wrapped around a central axis. As I mentioned earlier strands are anti-parallel and the interaction in the helix is hydrogen-bonding in-between the complementary base pairs. G forms three hydrogen bonds with C and A forms two hydrogen bond with T. The angle of interaction between base pairs results in a major and minor grooves. Helix diameter is 20 Aº. 10.4 base pair per helical turn.

2. REPLICATION

DNA synthesis

Replication is the process of synthesis of new DNA. It is different in both prokaryotes and Eukaryotes. DNA polymerase is the enzyme requires for the replication.

First, we can look into prokaryotic replication. The genome of prokaryote is usually circle and continues so replication is bidirectional. In addition, the polymerize starts to do polymerization from Ori site or Origin of replication site. This site is rich with A;T base pairs because it is easy to break comparatively G:C (3 bonds). Helicase help to unwind strand. DNA C will recruit helicase. Then polymerize synthesize strand in both direction one is called lagging strand and other is called leading strand. The continues synthesis occurring strand is called leading strand and the other is called lagging strand. The polymerase will synthesize in 5’-3’ direction always. So in case of lagging strand because of the opposite direction it is difficult so the parent strand will form loop and then only the daughter strand in the 5’-3’ direction this is called trombone model (figure provided.) Finally after synthesize the two fork in opposite direction will meet at termination site and this leads to termination of the replication.

In case of eukaryotes the polymerase is littlie different. In addition the genome is heavy and linear. So there are so many ori site and replication starts from more than one ORI site simultaneously. In addition, there is no specific ter site. So termination occur where fork meet. Here in the initiation step, ORC protein will bind and cdc6 and cd1 proteins bind to the ori site. Then it recruit MCM helicase and in the cell cycle during S phase CDKs and DDKs will phosphorylate both CDcs and MCM helicase and as result of interaction of CDC45 and Gins complex MCM helicase will activate and starts to unwind the strand. Then polymeraze will starts to polymerase. All other steps are similar.

3. TRANSCRIPTION

RNA synthesis from DNA.

RNA polymerase is the enzyme required for the transcription. Promoter sequence of the DNA will recognized by the alpha subunit of RNA polymerase. Omega will stabilize the enzyme. Alpha subunit is for the assembly. So first recognition will happen then the sigma subunit is release from the enzyme and then continues transcription will occur. According to scrunching model the tension generated in the DNA is the force for releasing alpha subunit. The key event in the transcription is the conversion of closed promoter complex to open promoter complex. This is by melting from promoter region. There is no proof reading activity for RNA polymerase hence some editing are important. Termination is by either Rho independent or Rho dependent. In rho independent form anti termination loop will form as result of hoogstean base pairing and it is followed by continues stretch of Uracil because of weak bond and heavy loop RNA will slip off from DNA. In case of eukaryotes it is little different.

4. TRANSLATION.

Translation is the synthesis of protein from amino acid. It is occurring in cytoplasm Ribosome synthesize proteins based on the mRNA. tRNA will gather amino acids and peptide bond will form in-between the amino acids. 61 codons code for amino acid and there are 3 different stop and one start codon.

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