Structure Of Dna And Rna

A single human cell contains approximately 2 meters of DNA when fully stretched out. This extensive length is compacted and organized within the cell nucleus through a complex structure involving histones and higher-order chromatin folding. Despite the long length, DNA is efficiently packaged to fit inside the microscopic nucleus of the cell. DNA Packaging: Conclusion: The DNA in a single human cell, if stretched out, measures approximately 2 meters in length. % Quicktip

Concept:
Transcription is the biological process in which genetic information stored in DNA is copied into an RNA molecule, usually messenger RNA (mRNA). Although DNA consists of two complementary strands, only one strand of DNA serves as the template for RNA synthesis. The two strands of DNA involved in transcription are named based on their relationship with the produced mRNA:

• Template strand (Antisense strand): This strand is read by RNA polymerase in the direction to synthesize RNA in the direction.
• Coding strand (Sense strand): This strand has the same nucleotide sequence as the mRNA (except that thymine (T) in DNA is replaced by uracil (U) in RNA).

Step 1: Understanding the template strand.
During transcription, RNA polymerase binds to DNA and reads the template (antisense) strand. The sequence of mRNA is formed by pairing complementary bases with this strand. Therefore, the mRNA sequence is complementary to the template strand. Hence, the statement that the template strand is also called the antisense strand is correct.
Step 2: Understanding the coding strand.
The other DNA strand is known as the coding (sense) strand. This strand is not directly used by RNA polymerase during transcription. However, its nucleotide sequence is almost identical to that of the mRNA produced, except that: Because its sequence resembles the final genetic code carried by mRNA, it is referred to as the sense strand. Therefore, the second statement is also correct. Final Answer:
Both Statement I and Statement II are correct.

Concept: Colour blindness is a sex-linked recessive disorder controlled by a gene located on the X-chromosome.
• = Normal vision allele
• = Colour blindness allele Important points:
• Males have one X and one Y chromosome ( ).
• Females have two X chromosomes ( ).
• A recessive trait like colour blindness appears in males if their single X chromosome carries the defective gene.
• A female must have two defective alleles to be colour blind, otherwise she becomes a carrier.

Step 1: Identify the parental genotypes. The given cross is: Gametes produced:
• Female gametes: ,
• Male gametes: ,

Step 2: Construct the Punnett square.

Step 3: Interpret the offspring genotypes.
• → Normal daughter
• → Carrier daughter
• → Normal son
• → Colour blind son Thus, out of the four possible offspring combinations, only one represents a colour blind son.

Step 4: Calculate the probability. Therefore, the probability of a colour blind son is .

Concept: DNA replication is the process by which a DNA molecule produces two identical copies of itself. During this process, the double-stranded DNA must first be separated so that each strand can act as a template for synthesis of a new strand. Several enzymes participate in DNA replication, each performing a specific function.
• Helicase – Unwinds the DNA double helix by breaking hydrogen bonds between base pairs.
• DNA polymerase – Synthesizes the new DNA strand by adding nucleotides.
• Primase – Synthesizes short RNA primers needed to start DNA synthesis.
• Ligase – Joins fragments of DNA (especially Okazaki fragments) by forming phosphodiester bonds.

Step 1: Understand the first step of DNA replication.
During replication, the DNA double helix must first open so that the two strands can separate. This separation occurs at a region called the replication fork.

Step 2: Identify the enzyme responsible for unwinding DNA.
The enzyme helicase moves along the DNA molecule and breaks the hydrogen bonds between complementary base pairs. This action unwinds the double helix and separates the two strands.

Step 3: Eliminate the incorrect options.
• DNA polymerase: Responsible for synthesizing new DNA strands.
• Ligase: Joins DNA fragments during replication.
• Primase: Produces RNA primers needed to initiate DNA synthesis. None of these enzymes unwind the DNA helix.

Step 4: Select the correct answer. Thus, the enzyme responsible for unwinding DNA during replication is:

Concept: In genetic coding, codons are sequences of three nucleotides present in messenger RNA (mRNA) that specify particular amino acids during protein synthesis. Each codon corresponds to a specific amino acid or signals the start or termination of translation.

Step 1: Understanding start codons.
Protein synthesis begins at a specific codon called the start codon. This codon signals the ribosome to begin translation and establishes the correct reading frame.

Step 2: Role of AUG.
The codon AUG performs two important roles:
• It acts as the initiator codon that signals the beginning of protein synthesis.
• It codes for the amino acid Methionine.

Step 3: Comparison with other codons.

• , , and are stop codons that terminate protein synthesis.
• Only functions as both a start codon and a codon for methionine.

Step 4: Conclusion.
Therefore, the codon that has the dual function of initiation and coding for methionine is AUG.

Step 1: Understanding the Concept:
Transcription is the process of copying genetic information from one strand of the DNA into RNA.
A transcription unit in DNA primarily consists of a promoter, the structural gene, and a terminator.

Step 2: Key Formula or Approach:
Analyze the standard nomenclature of DNA strands during transcription based on their polarity ( or ) and function.

Step 3: Detailed Explanation:
Statement I: Only one strand functions as a template and is called the antisense strand. This is correct because the DNA-dependent RNA polymerase catalyzes polymerization in only the direction, so the strand with polarity acts as the template (antisense strand).
Statement II: The coding strand of the transcription unit is called the sense strand. This is also correct. The other strand with polarity has the exact same sequence as the RNA (except thymine is replaced by uracil) and is appropriately called the coding or sense strand.

Step 4: Final Answer:
Both statements accurately describe the components of a transcription unit, so option (A) is the correct choice.