DNA and RNA structures

Genetic Information Translation

• Genes provide instructions for protein synthesis, but RNA acts as an intermediary.
• DNA is transcribed into RNA, then RNA is translated into protein.
• This reflects the flow of information from DNA to RNA to protein.
• Transcription and translation are linguistic terms describing genetic information transfer from genotype to phenotype.
• Nucleic acids and proteins have distinct "languages."

Nucleic Acid Language

• The language of nucleic acids is the nucleotide sequence in DNA and RNA.
• DNA has four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
• RNA has uracil (U) instead of thymine.
• Genes are specific base sequences on DNA with defined beginnings and ends, typically hundreds or thousands of nucleotides long.
• Transcription rewrites DNA's nucleic acid language as a complementary base sequence on RNA.
• RNA is synthesized using DNA as a template.

Polypeptide Language

• Translation converts the nucleic acid language to the polypeptide language.
• Polypeptides are polymers of 20 different amino acids.
• The nucleotide sequence in RNA determines the amino acid sequence in the polypeptide.
• RNA acts as a messenger carrying genetic information from DNA to guide protein synthesis.

Codons

• During translation, the RNA nucleotide sequence is converted into an amino acid sequence.
• Nucleotide sequences are read as triplets of bases called codons.
• A triplet code allows for 64 possible codons (4^3), sufficient for all 20 amino acids.
• Genetic instructions for a polypeptide's amino acid sequence are written in DNA and RNA as nonoverlapping three-base "words" called codons.
• DNA codons are transcribed into complementary RNA codons, which are then translated into amino acids forming a polypeptide.
• A minimum of 300 nucleotides are necessary to code for 100 amino acids.

Key Terms

• Transcription involves the synthesis of RNA using a DNA template.
• Transfer RNA (tRNA) functions as an interpreter in translation, with a specific anticodon that picks up a specific amino acid and conveys it to the appropriate mRNA codon.
• Codons are three-nucleotide sequences in mRNA that specify a particular amino acid or polypeptide termination signal.
• Translation is the synthesis of a polypeptide using the genetic information encoded in an mRNA molecule, changing from the language of nucleotides to amino acids.
• An anticodon is a sequence of three bases on tRNA complementary to a specific codon on mRNA, ensuring the correct amino acid is added to the protein chain.
• Start codons are specific mRNA nucleotide sequences that signal the beginning of protein synthesis, with AUG being the most common.
• A triplet code is a set of three-nucleotide long "words" specifying amino acids.
• RNA polymerase is the large molecular complex that links together the growing chain of RNA nucleotides during transcription, using a DNA strand as a template.
• Messenger RNA (mRNA) encodes genetic information from DNA and conveys it to ribosomes, where the information is translated into amino acid sequences.

The Genetic Code

• Molecular biologists deciphered the genetic code in the 1960s.
• An artificial RNA molecule composed solely of uracil (UUU) produced a polypeptide with phenylalanine, demonstrating that UUU specifies phenylalanine.
• Of the 64 codons, 61 code for amino acids.
• AUG codes for methionine and signals the start of a polypeptide chain.
• Three codons (UAA, UGA, and UAG) function as stop codons, marking the end of translation.
• RNA codons have a straightforward, complementary relationship to DNA codons.
• Codons are arranged linearly in both DNA and RNA, with no gaps.
• There is redundancy in the genetic code, but no ambiguity, as each codon represents only one specific amino acid.

Universality of the Genetic Code

• The genetic code is nearly universal across all organisms.
• Its universality enables modern DNA technologies to combine genes from different species.
• The shared genetic language highlights the evolutionary kinship connecting all life on Earth.

Transcription Process

• Transcription is the transfer of genetic information from DNA to RNA.
• One DNA strand serves as a template for the new RNA molecule, while the other strand is unused.
• RNA polymerase moves along the gene, forming an RNA strand by following base-pairing rules, with U replacing T.
• The process begins at a specific promoter sequence and continues until the enzyme reaches a terminator sequence, signaling the end of the gene.

Stages of Transcription

• Initiation: RNA polymerase attaches to the promoter region of the DNA, opens the double helix, and starts synthesizing RNA.
• The promoter acts as a binding site for RNA polymerase and determines where transcription starts.
• Elongation: The RNA strand grows as RNA polymerase moves along the gene.
• The newly formed RNA molecule peels away from its DNA template, allowing the two separated DNA strands to come back together.
• Free nucleotides form hydrogen bonds with the nucleotide bases of the template DNA, following base-pairing rules (with U replacing T in RNA).
• Termination: RNA polymerase reaches the terminator DNA sequence, detaches from the RNA strand and the DNA.
• Special DNA sequences mark the start (promoter) and end (terminator) of a gene.

Eukaryotic RNA Processing

• In eukaryotic cells, mRNA is transcribed in the nucleus and must travel to the cytoplasm for polypeptide synthesis.
• Before leaving the nucleus, eukaryotic transcripts undergo modifications, including the addition of a cap (a modified G nucleotide) at the 5' end and a tail (50 to 250 A nucleotides) at the 3' end.
• These additions facilitate mRNA export from the nucleus, protect it from degradation, and assist ribosomes in binding.
• Eukaryotic genes are longer than the mRNA that leaves the nucleus because they contain introns.
• Another modification is RNA splicing, necessitated by the presence of noncoding stretches of nucleotides within the coding sequences.
• Introns (noncoding regions) interrupt nucleotide sequences that code for amino acids (exons).
• RNA splicing allows the production of multiple polypeptides from a single gene by varying the combination of exons included in the final mRNA.

RNA Splicing

• Both exons and introns are transcribed from DNA into RNA.
• Before the RNA leaves the nucleus, the introns are removed, and the exons are joined together to form a continuous coding sequence.
• RNA splicing is catalyzed by a complex of proteins and small RNA molecules.

Translation Requirements

• Translation requires processed mRNA, enzymes, chemical energy sources like ATP, ribosomes, and transfer RNA (tRNA).

Transfer RNA (tRNA)

• Cells use tRNA as a molecular interpreter to convert the genetic message from mRNA into proteins.
• tRNA transfers amino acids from the cytoplasm to the growing polypeptide in a ribosome.
• tRNA picks up the correct amino acids and recognizes the corresponding codons in mRNA.
• tRNA is made from a single strand of RNA consisting of about 80 nucleotides in a cloverleaf structure with four arms.
• The anticodon region varies between different tRNAs.
• tRNAs contain chemically modified bases.
• The anticodon on tRNA recognizes and pairs with a specific codon on mRNA using base-pairing rules.
• At the other end of the tRNA molecule, there is a site where one specific kind of amino acid attaches.
• Each amino acid is joined to the correct tRNA by a specific enzyme, with 20 different enzymes for 20 amino acids.
• These enzymes use ATP to bind the appropriate amino acid to its corresponding tRNA.
• It is the base triplet of a tRNA molecule that couples the tRNA to a complementary codon in the mRNA and then the message is translated into polypeptides.