All questions of Genetic Recombination and Gene Linkage for Grade 9 Exam

RNA polymerase moves along the template strand, synthesising an mRNA molecule. In prokaryotes RNA polymerase is a holoenzyme consisting of a number of subunits, including a sigma factor (transcription factor) that recognises the promoter. In eukaryotes there are three RNA polymerases: I, II and III. The process includes a proofreading mechanism.

Splicing is the process of removing introns and joining exons in a defined order in a transcription unit. This process takes place in the nucleus of eukaryotic cells and is mediated by a complex called the spliceosome.

Explanation:

• Transcription is the process of synthesizing RNA from a DNA template. During transcription, the entire DNA sequence is transcribed into RNA, including both the introns and exons.

• Introns are non-coding sequences within a gene, while exons are the coding sequences that specify the amino acid sequence of a protein. Introns need to be removed from the RNA sequence before translation can occur.

• Splicing is the process of removing introns and joining exons in a defined order, resulting in a mature messenger RNA (mRNA) molecule that can be translated into a protein.

• The splicing process is mediated by the spliceosome, a complex that consists of small nuclear RNAs (snRNAs) and proteins. The snRNAs base-pair with the intron sequences, while the proteins catalyze the chemical reactions that remove the introns and join the exons.

• Splicing is essential for the proper expression of genes, as it allows for the creation of multiple protein variants from a single gene by alternative splicing. Mutations that affect splicing can lead to a variety of diseases, including cancer and genetic disorders.

In conclusion, splicing is the process of removing introns and joining exons in a defined order in a transcription unit, and it is essential for the proper expression of genes.

TRNA with an attached amino acid is said to be "charged". The enzyme that attaches the amino acid to the 3'-OH is called an aminoacyl tRNA synthetase (aaRS). There is a specific tRNA for each amino acid, 20 in all. Similarly, there is a specific aaRS for each tRNA.

The site at which RNA polymerase binds during transcription is called the promoter. It is a specific region of DNA that is recognized by RNA polymerase, which then initiates the process of transcription. The promoter is located upstream of the transcriptional start site and contains specific DNA sequences that are recognized by RNA polymerase and other transcription factors.

The promoter plays a crucial role in regulating gene expression, as different promoters can activate or repress transcription depending on the cellular context. The strength of the promoter can also affect the rate of transcription, with stronger promoters resulting in higher levels of mRNA production.

There are different types of promoters, including constitutive promoters, which are active in all cells, and inducible promoters, which are activated in response to specific signals or conditions. The sequence and structure of the promoter can vary depending on the gene being transcribed and the organism in which it is expressed.

In summary, the promoter is a critical element in the process of transcription, serving as the site at which RNA polymerase binds and initiating the production of mRNA from DNA. It plays a crucial role in regulating gene expression and can vary in strength and specificity depending on the cellular context and the gene being transcribed.

The genetic code is degenerate: Some amino acids are encoded by more than one codon, inasmuch as there are 64 possible base triplets and only 20 amino acids. In fact, 61 of the 64 possible triplets specify particular amino acids and 3 triplets (called stop codons) designate the termination of translation. Thus, for most amino acids, there is more than one code word.

The Role of the Anticodon Loop in tRNA
The anticodon loop is a crucial component of transfer RNA (tRNA), playing a significant role in the process of translation during protein synthesis.
Function of the Anticodon Loop
- The anticodon loop contains a sequence of three nucleotides, known as the anticodon.
- These three nucleotides are complementary to a specific codon on the messenger RNA (mRNA).
Complementarity to mRNA Codon
- During translation, the mRNA codon is read by the ribosome.
- The anticodon pairs with the corresponding mRNA codon through base pairing (A-U and G-C).
- This ensures that the correct amino acid, carried by the tRNA, is added to the growing polypeptide chain.
Importance of Accurate Pairing
- Accurate pairing between the anticodon and the mRNA codon is essential for the fidelity of protein synthesis.
- Mistakes in pairing can lead to the incorporation of incorrect amino acids, potentially resulting in malfunctioning proteins.
Summary
In summary, the anticodon loop of tRNA is vital for the translation process because it has bases that are complementary to the mRNA codon. This complementary interaction ensures that the correct amino acid is brought to the ribosome, facilitating the accurate synthesis of proteins. Thus, option 'B' correctly describes the role of the anticodon loop in tRNA.

Control of gene expression takes place at the level of transcription, which is the process of synthesizing RNA from DNA template. Transcription is a highly regulated process that involves the binding of transcription factors and other regulatory proteins to specific DNA sequences, known as cis-acting elements, located in the promoter and enhancer regions of genes. The control of gene expression at the level of transcription involves various mechanisms that can enhance or repress transcription, such as:

1. Gene regulatory proteins: These proteins bind to specific DNA sequences and either activate or repress transcription. Examples of gene regulatory proteins include transcription factors, coactivators, and corepressors.

2. Epigenetic modifications: These modifications can alter the accessibility of DNA to transcriptional machinery, either by loosening or tightening the chromatin structure. Examples of epigenetic modifications include DNA methylation, histone modifications, and chromatin remodeling.

3. RNA processing: Alternative splicing, polyadenylation, and RNA editing can generate multiple mRNA isoforms from a single gene, which may have different stability, localization, and translation efficiency.

4. Regulatory RNAs: Non-coding RNAs, such as microRNAs and long non-coding RNAs, can regulate gene expression by binding to mRNAs and either repressing their translation or promoting their degradation.

In summary, the control of gene expression at the level of transcription is a complex process that involves the interplay of various regulatory mechanisms. Understanding these mechanisms is essential for developing treatments for diseases that result from dysregulated gene expression.

Incorrectly Matched Pair: Anticodons - RNA
The correct answer to the question about incorrectly matched pairs is option 'D', which states "Anticodons - RNA". Let's delve into why this is not accurate.
Understanding Anticodons
- Anticodons are indeed sequences of three nucleotides that are complementary to codons on mRNA.
- However, they are specifically found on tRNA (transfer RNA), not just any RNA.
Role of Anticodons
- Each anticodon pairs with a corresponding codon on the mRNA during the process of translation.
- This pairing ensures that the correct amino acid is brought to the growing polypeptide chain.
Other Options Explained
- Option A: Initiation codons - AUG, GUG
- Correct: AUG is the primary start codon, and GUG can also function as a start codon in some contexts.
- Option B: Stop codons - UAA, UAG, UGA
- Correct: These are the three stop codons that signal the termination of protein synthesis.
- Option C: Methionine - AUG
- Correct: Methionine is encoded by the start codon AUG, which is the first amino acid in protein synthesis.
Conclusion
In summary, while anticodons are related to RNA, they specifically refer to the tRNA molecules that carry them. Therefore, option 'D' is incorrectly matched, as it lacks the specificity of tRNA. Understanding these distinctions is crucial for grasping the nuances of genetic translation in molecular biology.

Explanation:

Recognition sequences are specific DNA sequences that are recognized and cleaved by Type II restriction enzymes. These enzymes typically recognize palindromic sequences, which read the same forward and backward on both strands of DNA. The recognition sequence is usually 4-8 base pairs in length.

Let's analyze each option to determine if it can be a recognition sequence for a Type II restriction enzyme:

a) GAATTC: This sequence is the recognition sequence for the restriction enzyme EcoRI. It is a palindromic sequence, reading the same forward and backward on both strands: 5'-GAATTC-3' on one strand and 3'-CTTAAG-5' on the complementary strand.

b) AGCT: This sequence is not palindromic and therefore cannot be a recognition sequence for a Type II restriction enzyme. It reads 5'-AGCT-3' on one strand and 3'-TCGA-5' on the complementary strand.

c) GCGGCCGC: This sequence is the recognition sequence for the restriction enzyme NotI. It is a palindromic sequence: 5'-GCGGCCGC-3' on one strand and 3'-CGCCGGCG-5' on the complementary strand.

d) ATGCCT: This sequence is not palindromic and therefore cannot be a recognition sequence for a Type II restriction enzyme. It reads 5'-ATGCCT-3' on one strand and 3'-TACGGA-5' on the complementary strand.

Therefore, option D (ATGCCT) cannot be a recognition sequence for a Type II restriction enzyme because it is not palindromic. The correct answer is D.