DNA Transcription and Translation: Unveiling Protein Synthesis
DNA Transcription: Initiating Protein Synthesis
When a portion of the information contained within a DNA molecule needs to be utilized in the cell’s cytoplasm for protein construction, it undergoes a process called transcription. This results in the formation of a smaller ribonucleic acid chain known as messenger RNA (mRNA). mRNA utilizes the same fundamental base pairing rules as DNA, with the key difference being that thymine is replaced by uracil. Ribonucleotides are sequentially added in the 5′ to 3′ direction, using only one strand of the DNA as a template and RNA polymerase as a catalyst.
Transcription can only occur if two specific sequences are present in the DNA:
- Promoter: Located at the beginning of the sequence, it differs between eukaryotes and prokaryotes.
- Poly(A) Tail: A sequence of up to 200 adenine nucleotides, found only in prokaryotes at the end of the sequence. Eukaryotes have a signal in this region that induces the coupling of a larger precursor.
DNA Translation: Decoding Genetic Information
The genetic information carried by mRNA must be translated in the cytoplasm by a protein-producing structure called the ribosome. The ribosome, composed of various proteins and ribosomal RNA (rRNA), initiates the reading of mRNA at a specific sequence, which differs between eukaryotes and prokaryotes.
Once mRNA is bound to the ribosome, a third type of RNA, transfer RNA (tRNA), comes into play. Multiple types of tRNA exist, each recognizing specific groups of three mRNA bases called codons. Each codon corresponds to one of the twenty amino acids that make up polypeptide chains, the building blocks of proteins.
In bacterial DNA, genetic information is continuous. However, in higher organisms, it was discovered that genetic information is fragmented. The coding sequences, called exons, are interrupted by non-coding sequences, called introns. In eukaryotic cells, the initial mRNA transcript includes both introns and exons. The introns are subsequently removed, and the exons are joined together in a process called splicing. This process can be regulated, leading to alternative splicing, where different combinations of exons can be joined, resulting in the formation of multiple functional polypeptides from a single initial transcript. Only after splicing is the mature mRNA molecule transported out of the nucleus through nuclear pores, aided by specific ribonuclear proteins.
Key Differences Between DNA and RNA
DNA
- Double-stranded helix structure.
- Strands are complementary: a purine base pairs with a pyrimidine base (G-C and A-T).
- Guanine and cytosine are joined by three hydrogen bonds.
- Adenine and thymine are joined by two hydrogen bonds.
- Strands are antiparallel.
- Contains deoxyribose sugar.
RNA
- Single-stranded linear chain of nucleotides.
- Thymine is replaced by uracil.
- Contains ribose sugar.
DNA Denaturation and Renaturation
- Denaturation: The separation of the two DNA strands, occurring at approximately 100°C.
- Renaturation: The rejoining of the two separated DNA strands, occurring at approximately 65°C.