Earth Science: Biology Human – Genetics and Heredity- A Feature Characteristics: Transcription
Following the one-gene-one-polypeptide hypothesis, one particular gene is always responsible for forming one polypeptide. Ribonucleic acid (RNA) plays an essential role in converting genetic information into proteins.
RNA is found in the cell nucleus as well as in cytoplasm, the cell interior; in mitochondria, essentially the cell’s power plant; ribosomes, sites of protein production; and chloroplasts, the plant organelles.
It has a similar structure to DNA, but contains the sugar ribose (instead of deoxyribose), and the base uracil (instead of thymine), which can, however, also bond with adenine. RNA is normally single-stranded, but loops can develop within a strand due to base pairing. One can distinguish three different RNA types, which are based on their occurrence and function.
Ribosomal RNA (rRNA) is-apart from proteins—the principal component of ribosomes. Transfer-RNA (tRNA) bonds amino acids and organic compounds with at least one amino and one carboxyl acid group.
Then tRNA transports them to the ribosomes, where they are linked to a polypeptide chain with the aid of messenger RNA(mRNA.)
Transcription
The mRNA develops as a bit-by-bit copy of DNA in a process called transcription and subsequently brings the genetic information to the ribosomes where the transformation of information into proteins takes place (protein biosynthesis).
During transcription, the hydrogen bridges between the complementary DNA strands are split at those locations where genes are read and the single DNA strands diverge (initiation) so that the coded, genetic information can be transferred to mRNA.
At a starting point of transcription (promoter), complementary base pairing begins between one of the DNA strands and free RNA nucleotides, linked by the enzyme RNA-polymerase, which moves along the DNA strand section-by-section, attaching one complementary nucleotide after another to the ribonucleotid chain (elongation).
DNA thus serves as a matrix for the synthesis of single mRNA strands. After reaching a certain frequency range (a terminator), the enzyme interrupts the synthesis. The completed RNA separates from the DNA, which now closes again, and carries the information to the ribosomes for biosynthesis.
PROTEINS
Proteins fulfill diverse tasks in all living beings. In protein biosynthesis, a sequence of DNA bases is transformed into a specific amino acid sequence in the protein molecule. Proteins are macromolecules composed of amino acids, which are arranged in chains and joined together by pep tide bonds.
These polypeptide chains (primary structure) can fold into spatial structures (secondary and tertiary structure).
BASICS
SPLICING Since eukaryote DNA has coded and non-coded segments (introns and exons), the pre-mRNA formed by transcription initially also contains exons, which are spliced out before specific enzymes reunite the parts to form active mRNA again.