In context of DNA methylation, which one of the following statements is FALSE?
Generally, methylation occurs at the 3rd carbon position of cytosine and converts it to 3- methylcytosine
DNA Methylation Fundamentals
DNA methylation is an epigenetic mechanism that plays a crucial role in gene expression, cell differentiation, and development. In vertebrates, DNA methylation typically occurs at the carbon-5 position of cytosine residues, primarily within CpG dinucleotides. This process is catalyzed by enzymes called DNA methyltransferases (DNMTs).
Analyzing DNA Methylation Statements
Let's examine each statement provided in the options regarding DNA methylation:
- Statement 1: Generally, methylation occurs at the 3rd carbon position of cytosine and converts it to 3- methylcytosine.
In the context of typical eukaryotic DNA methylation, specifically in mammals, methylation occurs at the 5th carbon (C5) of cytosine within CpG sites, forming 5-methylcytosine. Methylation at the 3rd carbon (C3) or other positions like N4 is not the general mode of DNA methylation in these organisms. Therefore, this statement is factually incorrect for the common understanding of DNA methylation in higher eukaryotes.
- Statement 2: Maintenance methyltransferase acts constitutively on hemimethylated sites and converts them to fully methylated sites.
This statement accurately describes the function of maintenance DNA methyltransferases, such as DNMT1. After DNA replication, the parental DNA strand retains its methylation pattern, while the newly synthesized strand is unmethylated. This creates hemimethylated DNA. DNMT1 specifically recognizes these hemimethylated sites and methylates the cytosine on the new strand, thus maintaining the methylation pattern across cell divisions.
- Statement 3: During mammalian gametogenesis, the genomic methylation patterns are erased in primordial germ cells.
Epigenetic reprogramming, including widespread demethylation, occurs during mammalian development in primordial germ cells (PGCs) and also after fertilization. This global erasure is essential for establishing new, sex-specific methylation patterns required for proper embryonic development and germline identity. This statement is correct.
- Statement 4: Replication converts a fully methylated site to hemimethylated site.
When double-stranded DNA containing symmetrically methylated CpG sites (i.e., both strands are methylated at the cytosine within the CpG) undergoes replication, each new double helix receives one parental methylated strand and one newly synthesized, initially unmethylated, strand. This results in a hemimethylated state where only one strand of the DNA duplex is methylated at the CpG site. This is a correct description of what happens immediately after replication.
Identifying the False Statement
Based on the analysis, the statement that methylation generally occurs at the 3rd carbon position of cytosine is incorrect in the typical context of mammalian DNA methylation. The correct position is the 5th carbon.
Therefore, the false statement is the one claiming methylation occurs at the 3rd carbon position of cytosine.
Important Questions from Genes and chromosomes
Readers of histone modifications include:
The amino acid side chains of the four histones in the nucleosome are subjected to remarkable variety of post-translation modifications such as phosphorylation, acetylation and methylation. Which one of the following post-translational marks on histone tails is usually associated with transcriptional repression?
The nucleosome is the fundamental subunit of chromatin in eukaryotes. Following statements are made about nucleosome:
A. Generally, a typical nucleosome contains ~ 200bp of DNA and two copies of each histone (H2A, H2B, H3 and H4)
B. 146 bp length of DNA per nucleosome core particle is strictly maintained across the organisms
C. The histone octamers are not conserved during/after replication, however, H32-H42 tetramers are.
D. Variants have been identified for all core histones except histone H3
E. While wrapping around the core histones, the structure of DNA is altered at the middle of the nucleosome core particle and exhibits increased number of base pairs per turn
Which one of the following combination of statements is most appropriate?
Which one of the following statements about Short Interspersed Nuclear Elements (SINEs) is TRUE?
Following statements were made about transposons:
A. Transposons have inverted terminal repeats and their integration generates inverted repeats at the flanks of the target site in the host genome.
B. A composite transposon can transpose as a unit.
C. The transposition event may cause deletions or inversions or move a host sequence to a new location.
D. The transposition event may cause deletions or inversions but cannot move a host sequence to a new location.
E. Replicative transposition proceeds through cointegration.
Which one of the following represents the combination of the correct statements?
