Gene Expression Decoded: DNA, RNA, and Protein Synthesis

Gene Expression Fundamentals

Defining a Gene

A gene is a segment of DNA that contains instructions for making a specific protein or RNA molecule.

What is a Codon?

A codon is a sequence of three nucleotide bases on mRNA that specifies a particular amino acid or a stop signal in protein synthesis.

Why Codons are Three Nucleotides Long

A three-nucleotide codon is essential to provide enough unique combinations (64) to encode all 20 amino acids and the necessary stop signals.

Genetic Code Degeneracy

Genetic code degeneracy is the phenomenon where multiple codons code for the same amino acid, providing a crucial buffer against potential mutations.

Transcription: DNA to RNA

Promoter and Terminator Regions

• Promoter: Signals RNA polymerase where to initiate transcription.
• Terminator: Signals RNA polymerase to stop transcription and release the newly synthesized RNA transcript.

RNA Polymerase Functions

RNA polymerase unwinds DNA and synthesizes RNA by adding complementary nucleotides. It also catalyzes the formation of phosphodiester bonds between RNA nucleotides.

Base Pairing in Transcription

In transcription, RNA polymerase pairs DNA bases with complementary RNA nucleotides (e.g., Adenine with Uracil, Guanine with Cytosine).

What is Pre-mRNA?

Pre-mRNA (or primary transcript) is the initial RNA molecule transcribed directly from DNA, containing both non-coding introns and coding exons before processing.

mRNA Processing Steps

After transcription, pre-mRNA undergoes crucial processing steps. This includes the removal of non-coding sequences (introns) and the addition of modifications like the 5' cap and poly-A tail, which are vital for stability and proper translation.

Introns and Splicing

Introns are non-coding sequences within pre-mRNA that are precisely removed by a process called splicing, performed by a complex molecular machine known as the spliceosome.

Functions of 5' Cap and Poly-A Tail

• The 5' cap protects mRNA from degradation and aids in ribosome binding for translation initiation.
• The 3' poly-A tail stabilizes mRNA, protects it from enzymatic degradation, and regulates its lifespan within the cell.

Translation: RNA to Protein

Purpose of Translation

The primary purpose of translation is to synthesize a polypeptide (protein) by decoding the mRNA sequence using ribosomes, tRNA molecules, and amino acids.

mRNA Codon Sequence and Amino Acid Order

The specific sequence of codons in mRNA precisely dictates the order in which amino acids are assembled into a protein. tRNA molecules deliver the correct amino acids based on complementary base-pairing with these mRNA codons.

tRNA's Role in Amino Acid Placement

tRNA molecules ensure correct amino acid placement by complementary base-pairing with codons on mRNA. They are crucial for delivering the appropriate amino acid during the protein synthesis process (translation).

Base Pairing in Translation

In translation, tRNA anticodons pair specifically with mRNA codons to ensure that the correct amino acids are added to the growing polypeptide chain.

Release Factor Function

A release factor binds to the stop codon on mRNA, prompting the ribosome to release the newly synthesized polypeptide and subsequently disassemble, thereby terminating translation.

Essential vs. Non-Essential Amino Acids

• Essential amino acids must be obtained from the diet because the human body cannot synthesize them.
• Non-essential amino acids can be synthesized by the body from other molecules.

Key RNA Molecules

Types of RNA: mRNA and tRNA

• mRNA (messenger RNA): Carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs.
• tRNA (transfer RNA): Transfers specific amino acids to the ribosomes and matches them to the corresponding mRNA codon using its anticodon.