Molecular Biology: DNA Transcription, Translation, and Genetics
DNA Transcription and RNA Synthesis
During transcription, an RNA molecule, called messenger RNA (mRNA), is synthesized. This mRNA is complementary to the DNA fragment that carries the information for a specific protein. This molecule provides the information necessary for ribosomal protein synthesis. Cells produce proteins, such as enzymes and structural proteins, that may not always be needed. This can lead to a progressive deterioration of cellular functions and activities.
Peptide Mutations
a) AAT CCA ACA GG
b) The peptide mutations will be different because there will be at least one less amino acid. The change in the last triplet ACC ACT will result in the corresponding mRNA being UGA, which does not encode any amino acid but signals a stop codon. Therefore, the mutant would lack tryptophan.
Trisomy Explained
The statement that mosaic trisomy is always caused by a defect in meiosis is false. While classical or free trisomy can result from meiotic errors, mosaic trisomy is not present in all cells of the affected individual. If the source is an error in meiosis during gametogenesis, where there are two copies of chromosome 21 in either the egg or sperm, the resulting zygote after fertilization will have three copies of chromosome 21. Consequently, all cells derived from this zygote through mitosis will also have three copies of chromosome 21.
DNA Transcription Process
AUA AUG GUU CUU GUC GUC ACC. DNA transcription into ribonucleotides is facilitated by enzymes, including RNA polymerase, in the cell nucleus. RNA polymerase recognizes a DNA sequence that is transcribed (the “meaningful” strand) and uses ribonucleotides complementary to that strand in the 5′-3′ direction. The DNA fragment corresponding to a transcribed gene (in prokaryotes, this can be more than one gene).
mRNA Translation and Peptide Synthesis
DNA – RNA or mRNA mass – transfer RNA or tRNA – amino acid – peptide or protein // DNA transcription mRNA / peptide mRNA Translation // Search for mRNA sequence. Given the DNA sequence 3′-TACAAATTCGTCAAC-5′, the complementary mRNA sequence is 5′-AUGUUUAAGCAGUUG-3′. Using the genetic code table, the corresponding amino acid peptide sequence is: Phe-Met-Leu-Gln-Asn.
NCBI contains one of the two DNA strands, and we know it contains Thymine (T), which is absent in RNA. Note that a “codon” is formed by a triplet (3) bases and encodes an amino acid. For 810 bases, there will be 810/3 = 270 amino acids. The DNA of eukaryotic genes contains introns (non-coding sequences), meaning the number of amino acids encoded is lower (only exons are encoded). The gene sequence in the mouse will resemble that of the cow more than that of the chicken, due to greater evolutionary proximity and therefore more similarity in gene sequences between two mammal species (mouse and cow) than between a mammal and a bird. The evolutionary distance between two species usually corresponds to differences between their genomes.