The Central Dogma: DNA to RNA to Protein

Central Dogma of molecular biology

The Central Dogma explains how genetic information flows from DNA → RNA → Protein.

How does DNA encode proteins?

DNA contains genes that are transcribed into RNA, which is then translated into proteins.

Role of RNA in protein production

RNA is involved in transcription (copying DNA into mRNA) and translation (synthesizing proteins based on mRNA).

Types of RNA

mRNA (messenger RNA): Carries the genetic code from DNA to the ribosome. tRNA (transfer RNA): Brings amino acids to the ribosome based on mRNA codons. rRNA (ribosomal RNA): Makes up the ribosome and facilitates protein synthesis.

Gene

A gene is a segment of DNA that contains the instructions for making a specific protein.

Transcription

During transcription, an RNA molecule is synthesized using one strand of DNA as a template. The RNA is complementary to the DNA template.

Stages of transcription

Initiation: RNA polymerase binds to the promoter region of the gene and unwinds the DNA. Elongation: RNA polymerase synthesizes the RNA molecule by adding complementary RNA nucleotides. Termination: RNA polymerase reaches a termination signal, and the RNA molecule is released.

Promoter in transcription

The promoter is a DNA sequence where RNA polymerase binds to start transcription.

RNA synthesis direction

RNA is synthesized in the 5' to 3' direction, complementary to the DNA template strand.

Transcription in prokaryotes vs eukaryotes

Prokaryotes: Transcription and translation can occur simultaneously in the cytoplasm because they lack a nucleus. Eukaryotes: Transcription occurs in the nucleus, and RNA must be processed before it leaves the nucleus for translation.

RNA processing in eukaryotes

A 5' cap is added for stability and protection. A poly-A tail is added to the 3' end to protect from degradation. Introns (non-coding regions) are removed, and exons (coding regions) are spliced together.

Purpose of RNA splicing

RNA splicing removes introns and joins exons to create a mature mRNA that can be translated into protein.

Translation location in eukaryotic cells

Translation occurs in the cytoplasm at the ribosome.

Role of mRNA in translation

mRNA provides the code for the amino acid sequence of a protein, using sets of three nucleotides called codons.

Codon

A codon is a sequence of three nucleotides in mRNA that specifies one amino acid.

Function of tRNA in translation

tRNA molecules carry amino acids to the ribosome, where their anticodons match with mRNA codons to add the correct amino acids to the growing protein chain.

Translation initiation

The small ribosomal subunit binds to the mRNA, and the initiator tRNA binds to the start codon (AUG). The large ribosomal subunit then joins to complete the ribosome.

Translation elongation

tRNA molecules bring amino acids to the ribosome, matching their anticodons with the mRNA codons. Amino acids are linked together by peptide bonds, forming a polypeptide chain.

Translation termination

The ribosome reaches a stop codon, and a release factor binds to the ribosome, causing the polypeptide to be released and translation to end.

Release factor in translation

A release factor is a protein that binds to the stop codon, causing the ribosome to release the completed polypeptide and dissociate.

Genetic code

The genetic code is the set of rules that defines which mRNA codons correspond to which amino acids.

Gene expression regulation in prokaryotes

Gene expression is often regulated through operons, which are groups of genes that are transcribed together. The lac operon is an example of an operon that regulates lactose-digesting enzymes.

Repressor in prokaryotes

A repressor is a protein that binds to DNA and blocks transcription. For example, in the lac operon, the repressor binds to the operator region to prevent transcription when lactose is absent.

Lactose effect on gene expression

When lactose is present, it binds to the repressor, causing the repressor to release from the DNA, allowing transcription of the lactose-digesting enzymes.

Gene expression regulation in eukaryotes

Gene expression in eukaryotes is regulated through several mechanisms, including DNA packaging, transcription factors, alternative splicing, RNA export, RNA degradation, and protein folding and activity.

Alternative splicing

Alternative splicing allows a single gene to produce multiple protein variants by including different combinations of exons.

Transcription factors

Transcription factors are proteins that bind to the promoter region of DNA and help recruit RNA polymerase, affecting the rate of transcription.

RNA degradation

Some mRNA molecules are rapidly degraded before they can be translated, while others are more stable and translated into more protein.

Role of ribosomes in translation

Ribosomes are the cellular machines that read mRNA, facilitate the binding of tRNA, and catalyze the formation of peptide bonds between amino acids during protein synthesis.

Function of the poly-A tail in mRNA processing

The poly-A tail protects the mRNA from degradation and aids in its export from the nucleus to the cytoplasm for translation.