DNA

Question 1

What are okazaki fragments?

Answer 1

Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging strand during DNA replication.

The synthesis of the lagging strand occurs discontinuously because DNA polymerase can only synthesize DNA in the 5’ to 3’ direction. Since the two strands of DNA are antiparallel, with one running in the 3’ to 5’ direction and the other in the 5’ to 3’ direction, the lagging strand is synthesized away from the replication fork in short, discrete stretches.

The steps involved in Okazaki fragment synthesis are as follows:

1. Primase Synthesis: The enzyme primase synthesizes a short RNA primer on the lagging strand to provide a starting point for DNA synthesis.
2. DNA Polymerase Extension: DNA polymerase extends the RNA primer by adding DNA nucleotides in the 5’ to 3’ direction, synthesizing a short DNA fragment called an Okazaki fragment. This synthesis occurs in the direction away from the replication fork.
3. RNA Primer Removal: After the Okazaki fragment is synthesized, the RNA primer is removed by the enzyme RNase H or by the combined action of DNA polymerase and exonuclease.
4. DNA Fragment Joining: DNA polymerase fills in the gap left by the removed RNA primer by adding DNA nucleotides. Finally, DNA ligase seals the nick between adjacent Okazaki fragments by forming phosphodiester bonds, resulting in a continuous lagging strand.

Okazaki fragments typically range in size from 100 to 200 nucleotides in prokaryotes and are shorter in eukaryotes. These fragments are later joined together by DNA ligase to form a complete, continuous lagging strand during DNA replication.

Question 2

Adenine is present in which coenzymes?

Answer 2

Coenzymes containing adenine include:

1. Adenosine triphosphate (ATP): ATP is a primary energy carrier molecule in cells. It consists of adenine, ribose (a sugar), and three phosphate groups. ATP stores and releases energy for cellular processes such as metabolism, muscle contraction, and active transport.
2. Nicotinamide adenine dinucleotide (NAD+): NAD+ is a coenzyme involved in redox reactions, particularly in the transfer of electrons during cellular respiration and other metabolic pathways. It functions as an electron carrier, accepting electrons (along with a hydrogen ion) to form NADH.
3. Flavin adenine dinucleotide (FAD): FAD is another coenzyme involved in redox reactions. It accepts and donates electrons during various metabolic processes, serving as a cofactor for enzymes involved in oxidation-reduction reactions.

These coenzymes containing adenine play essential roles in cellular metabolism, energy production, and various biochemical processes.

Question 3

What is Replication fork?

Answer 3

The replication fork is a structure that forms during DNA replication, where the two strands of the DNA double helix separate and serve as templates for the synthesis of new DNA strands. It is called a “fork” because the replication process resembles the splitting of a fork into two separate tines.

During DNA replication, the enzyme DNA helicase unwinds the double helix by breaking the hydrogen bonds between the complementary base pairs, separating the two DNA strands and forming a replication bubble. The region where the strands separate and DNA synthesis occurs is called the replication fork.

At the replication fork, DNA polymerase enzymes bind to each template strand and synthesize new DNA strands by adding complementary nucleotides. The leading strand is synthesized continuously in the 5’ to 3’ direction, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments.

As DNA replication proceeds, the replication fork moves along the DNA molecule, unwinding and synthesizing new strands until the entire DNA molecule has been replicated. The replication fork is a dynamic structure that is essential for the accurate and efficient replication of the genome during cell division.

Question 4

Why fattyacid metabolism doesn’t occur in brain?

Answer 4

Fatty acid metabolism does occur in the brain, but it is different from fatty acid metabolism in other tissues. While most tissues can utilize fatty acids for energy through β-oxidation, the brain has unique metabolic requirements and relies predominantly on glucose as its primary energy source under normal physiological conditions.

There are several reasons why fatty acid metabolism is less prominent in the brain:

1. Blood-Brain Barrier: The blood-brain barrier tightly regulates the entry of nutrients into the brain, including fatty acids. While some fatty acids can cross the blood-brain barrier, they are less readily available compared to glucose.
2. Glucose Preference: The brain has a high demand for energy and relies heavily on glucose metabolism to meet its metabolic needs. Glucose is the preferred fuel for neurons due to its efficiency in producing ATP and its role in neurotransmitter synthesis.
3. Ketone Bodies: During periods of fasting or low carbohydrate intake, the liver synthesizes ketone bodies (e.g., β-hydroxybutyrate, acetoacetate) from fatty acids as an alternative fuel source. Ketone bodies can cross the blood-brain barrier and provide an additional source of energy for the brain when glucose availability is limited.
4. Oxidative Stress: Fatty acid metabolism generates reactive oxygen species (ROS) as byproducts, which can contribute to oxidative stress and damage cellular components. Neurons are particularly vulnerable to oxidative damage, and excessive fatty acid metabolism could potentially exacerbate oxidative stress in the brain.

Overall, while fatty acid metabolism does occur in the brain, glucose is the primary energy substrate for normal brain function. The brain’s reliance on glucose reflects its unique metabolic requirements and the importance of maintaining stable energy levels to support neuronal function and neurotransmission.

Question 5

Steps of ribosome synthesis?

Answer 5

Sure, here’s a simplified flowchart outlining the main steps involved in the formation of ribosomes:

1. Transcription of rRNA Genes:
   
   • RNA polymerase I transcribes rRNA genes in the nucleolus.
2. Processing of Pre-rRNA Transcripts:
   
   • Pre-rRNA undergoes extensive processing to remove non-coding sequences.
3. Assembly of Ribosomal Proteins:
   
   • Ribosomal proteins are synthesized in the cytoplasm and transported into the nucleolus.
   • Ribosomal proteins assemble into ribosomal subunits with the help of assembly factors and chaperone proteins.
4. Formation of Ribosomal Subunits:
   
   • Mature rRNA molecules combine with assembled ribosomal proteins to form small (40S) and large (60S) ribosomal subunits.
5. Export of Ribosomal Subunits:
   
   • Ribosomal subunits are exported from the nucleolus to the cytoplasm.
6. Assembly into Functional Ribosomes:
   
   • Small and large ribosomal subunits join together with mRNA and other factors to form functional ribosomes in the cytoplasm.
7. Protein Synthesis:
   
   • Ribosomes translate the genetic code carried by mRNA into specific amino acid sequences, leading to protein synthesis.

This flowchart provides an overview of the main steps involved in the formation of ribosomes, from transcription and processing of rRNA to the assembly of ribosomal subunits and protein synthesis.

Question 6

Steps of polymerase chain reaction?

Answer 6

The Polymerase Chain Reaction (PCR) is a laboratory technique used to amplify a specific segment of DNA, generating millions to billions of copies of the target sequence. The PCR process typically consists of the following steps:

1. Denaturation: The double-stranded DNA template containing the target sequence is heated to a high temperature (usually around 95°C). This causes the two strands of the DNA molecule to separate or denature, resulting in single-stranded DNA molecules.
2. Annealing: The reaction temperature is lowered to allow specific DNA primers to anneal or bind to complementary sequences flanking the target region on the single-stranded DNA template. Primers are short, synthetic DNA sequences that serve as starting points for DNA synthesis.
3. Extension: The reaction temperature is raised to an optimal temperature (typically around 72°C), allowing a DNA polymerase enzyme to extend the primers by synthesizing new DNA strands complementary to the template DNA. The polymerase adds nucleotides to the primers, creating new DNA strands.
4. Repeated Cycles: The denaturation, annealing, and extension steps are repeated for multiple cycles (usually 20-40 cycles). Each cycle doubles the amount of DNA present, resulting in exponential amplification of the target sequence.

By repeating these steps, the target DNA sequence is amplified, and millions to billions of copies of the target DNA are generated within a relatively short period of time. PCR is a powerful and widely used technique in molecular biology, genetics, medical diagnostics, forensics, and other fields.