Gene Expression: From DNA to Protein (Chapter 17 Study Notes)

Gene Expression: From DNA to Protein

Introduction to Gene Expression

Gene expression is the process by which information encoded in DNA directs the synthesis of proteins, which are essential for cellular structure and function. This process involves two main stages: transcription and translation.

• Gene: A segment of DNA that contains instructions for making a specific polypeptide or protein.

• Polypeptide: A chain of amino acids that folds into a functional protein.

• Traditionally, it was believed that one gene codes for one polypeptide, but research shows that one gene can code for more than one polypeptide due to mechanisms such as alternative splicing.

Transcription: DNA to mRNA

Transcription is the process of synthesizing an mRNA copy from a DNA template. This mRNA carries the genetic information from the nucleus (in eukaryotes) to the ribosome, where proteins are synthesized.

• Transcription: The synthesis of RNA using DNA as a template.

• mRNA (messenger RNA): The RNA molecule that carries genetic information from DNA to the ribosome.

• In prokaryotes, transcription and translation occur simultaneously in the cytoplasm.

• In eukaryotes, transcription occurs in the nucleus, and the mRNA undergoes processing before being exported to the cytoplasm for translation.

Differences in Gene Expression: Prokaryotes vs. Eukaryotes

The processes of transcription and translation differ between prokaryotic and eukaryotic cells, primarily due to cellular compartmentalization.

• Prokaryotes: mRNA is produced in the cytoplasm and can be immediately translated by ribosomes.

• Eukaryotes: mRNA is synthesized in the nucleus and must undergo RNA processing (such as splicing, 5' capping, and poly-A tail addition) before it is transported to the cytoplasm for translation.

The Flow of Genetic Information

The central dogma of molecular biology describes the flow of genetic information:

• DNA → RNA → Protein

• Transcription: DNA is transcribed to produce mRNA.

• Translation: mRNA is translated by ribosomes to synthesize proteins.

The Genetic Code and Codons

The genetic code is a set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. Each group of three nucleotides (a codon) specifies a particular amino acid.

• Codon: A sequence of three mRNA nucleotides that codes for a specific amino acid or a stop signal during translation.

• The code is redundant (more than one codon can specify the same amino acid) but not ambiguous (each codon specifies only one amino acid).

• There are 64 possible codons (43), but only 20 amino acids, so some amino acids are coded by multiple codons.

• Start codon: AUG (codes for methionine, signals the start of translation).

• Stop codons: UAA, UAG, UGA (signal the end of translation).

Table: The Genetic Code

The following table summarizes the genetic code, showing which codons correspond to which amino acids. This table is essential for decoding mRNA sequences during translation.

First Base	Second Base: U	Second Base: C	Second Base: A	Second Base: G
U	UUU Phe UUC Phe UUA Leu UUG Leu	UCU Ser UCC Ser UCA Ser UCG Ser	UAU Tyr UAC Tyr UAA Stop UAG Stop	UGU Cys UGC Cys UGA Stop UGG Trp
C	CUU Leu CUC Leu CUA Leu CUG Leu	CCU Pro CCC Pro CCA Pro CCG Pro	CAU His CAC His CAA Gln CAG Gln	CGU Arg CGC Arg CGA Arg CGG Arg
A	AUU Ile AUC Ile AUA Ile AUG Met (start)	ACU Thr ACC Thr ACA Thr ACG Thr	AAU Asn AAC Asn AAA Lys AAG Lys	AGU Ser AGC Ser AGA Arg AGG Arg
G	GUU Val GUC Val GUA Val GUG Val	GCU Ala GCC Ala GCA Ala GCG Ala	GAU Asp GAC Asp GAA Glu GAG Glu	GGU Gly GGC Gly GGA Gly GGG Gly

Impact of Codon Redundancy

• Because multiple codons can code for the same amino acid, some mutations (changes in the DNA sequence) may not affect the resulting protein. These are called silent mutations.

• However, mutations that change a codon to one that codes for a different amino acid (missense mutations) or to a stop codon (nonsense mutations) can have significant effects on protein function.

Example: Decoding a DNA Sequence

• Given a DNA template strand: 3'-TAC GGA TGC-5'

• Transcribed mRNA: 5'-AUG CCU ACG-3'

• Translated amino acids: Met-Pro-Thr

Additional info: The ribosome is the molecular machine responsible for translating mRNA into protein. It is composed of rRNA and proteins and has distinct sites for tRNA binding and peptide bond formation.