• Understand nucleic acids as molecules that carry instructions for protein synthesis, including the structure of DNA and the roles of mRNA, rRNA, and tRNA.
• Explain the genetic code as a universal triplet code, and describe the steps in gene expression: transcription, RNA processing in eukaryotes, and translation by ribosomes.

Nucleic acids, including DNA and RNA, are crucial molecules that store and transmit the genetic information required to build proteins, which are vital for cellular functions.

DNA: The Master Blueprint

• Structure: DNA is a double-stranded helix made up of nucleotides, each consisting of a sugar (deoxyribose), a phosphate group, and one of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair specifically (A with T, C with G) to form the rungs of the DNA ladder.

• Function: DNA holds the genetic instructions for protein synthesis, with each gene being a specific sequence of bases that dictates the sequence of amino acids in a protein.

In DNA replication, the leading strand is synthesized continuously in the 5' to 3' direction. The lagging strand is made in short segments called Okazaki fragments, as DNA polymerase only works in the 5' to 3' direction. These fragments are later joined to complete the process. Image by Clker-Free-Vector-Images, Pixabay.

RNA is responsible for translating the instructions from DNA into the proteins that perform various functions in the cell. Unlike DNA, RNA is typically single-stranded and contains the sugar ribose.

mRNA (Messenger RNA)

• Role: mRNA carries the genetic code from DNA in the nucleus to the ribosomes, the sites of protein synthesis.
• Structure: mRNA is a single strand of nucleotides, organized into codons, each corresponding to a specific amino acid.

rRNA (Ribosomal RNA)

• Role: rRNA forms the core structure of ribosomes, where it helps position the mRNA and tRNA and catalyzes the assembly of amino acids into proteins.
• Structure: rRNA combines with proteins to form the large and small subunits of ribosomes.

tRNA (Transfer RNA)

• Role: tRNA transports amino acids to the ribosome, matching its anticodon with the corresponding mRNA codon to ensure the correct amino acid sequence in the protein.
• Structure: tRNA is a cloverleaf shaped molecule with an anticodon at one end and an attachment site for a specific amino acid at the other.

Diagram of the three types of RNA below. Image licensed under CC BY-SA 4.0.

The central dogma of molecular biology describes the flow of genetic information within a biological system: DNA is transcribed into RNA, which is then translated into proteins, essential molecules that perform various functions in the cell.

In this process, all three types of RNA—mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA)—converge at the ribosome, where they work together to facilitate translation, the synthesis of proteins. mRNA carries the genetic code, tRNA brings the corresponding amino acids, and rRNA forms the core of the ribosome’s structure, ensuring efficient protein assembly.

Transcription: DNA to RNA

• Overview: A specific DNA sequence within a gene is transcribed into a complementary RNA molecule, which will serve as a template for protein synthesis.
• Location:
• Eukaryotes: Transcription occurs in the nucleus.
• Prokaryotes: Transcription occurs in the cytoplasm, as they lack a nucleus.
• Process: The DNA double helix unwinds and unzips around the gene region. RNA polymerase binds to the promoter region on the DNA and synthesizes pre-mRNA (in eukaryotes) or mRNA (in prokaryotes) by joining RNA nucleotides that are complementary to the DNA template strand.
• Complementary Base Pairing: RNA nucleotides pair with complementary DNA bases, where adenine (A) pairs with uracil (U) instead of thymine (T). This produces an RNA sequence that mirrors the coding strand of DNA.

RNA Processing (in Eukaryotes)

• Overview: Pre-mRNA undergoes modifications to become mature mRNA, ensuring it is properly prepared for translation into a protein.
• Splicing: The initial mRNA transcript, known as pre-mRNA, contains exons (coding regions) and introns (non-coding regions). Introns are removed, and exons are spliced together to create a continuous coding sequence in the mature mRNA.
• Alternative Splicing: Different combinations of exons can be spliced to produce multiple mature mRNA variants from a single gene, allowing for the creation of different protein products.
• 5' Methyl Cap: A modified guanine nucleotide is added to the 5' end of the mRNA. This methyl cap protects the mRNA from degradation and aids in ribosome recognition during translation.
• Poly-A Tail: A poly-A tail is added to the 3' end, which stabilizes the mRNA and assists in its export from the nucleus to the cytoplasm.

Translation: RNA to Protein

• Overview: The mRNA sequence is translated into a sequence of amino acids to build a functional protein.
• Location:
• Eukaryotes: Translation occurs at ribosomes either in the cytoplasm or on the rough endoplasmic reticulum (Rough ER).
• Prokaryotes: Translation occurs in the cytoplasm at ribosomes.
• Process: Ribosomes read the mRNA sequence in groups of three bases (codons). Each codon specifies a particular amino acid or a stop signal for the process.
• tRNA and Codon-Anticodon Interaction: Transfer RNA (tRNA) molecules, each carrying a specific amino acid, have anticodons that pair with complementary mRNA codons, ensuring the correct order of amino acids in the growing chain.
• Protein Assembly: The ribosome links amino acids with peptide bonds, forming a polypeptide chain. As the chain elongates, it folds into a specific three-dimensional structure, ultimately forming a fully functional protein.

The Genetic Code

• The genetic code is a universal set of instructions used by nearly all organisms to translate DNA information into proteins. In DNA, the sequence of three nucleotide bases is called a triplet. During transcription, each DNA triplet is transcribed into a corresponding three-base sequence in mRNA, called a codon. For example, the codon AUG codes for the amino acid methionine and also serves as the start codon for translation.

Redundancy

• The genetic code is redundant, meaning multiple codons can encode the same amino acid. For instance, the amino acid leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This redundancy helps protect against mutations, as a change in one nucleotide might still result in the same amino acid being incorporated into a protein.

Explore the Genetic Code: Use this codon wheel to decode mRNA sequences and discover the amino acids they translate to. Simply follow the nucleotide bases from the center outward to reveal the corresponding amino acid, represented by its single-letter abbreviation.