Cell Protein Synthesis

Cell Protein Synthesis

• An mRNA copy of the gene is created in the nucleus
• Amino acids are linked together at the ribosome to form the protein encoded by the mRNA
• Protein synthesis involves both transcription and translation

Transcription

• Transcription is RNA synthesis
• Specific sequences called promoters bind RNA polymerase and allow transcription initiation
• Prokaryotic RNA polymerase directly recognizes the promoter sequence

Translation

• Translation is protein synthesis
• Specific sequences called terminators determine the end of transcription
• In eukaryotes, transcription factors are needed to recognize the promoter

Gene Expression

• Gene expression relates to transcription and translation for a protein coding gene

Chemical Differences Between DNA and RNA

• There are key structural chemical differences
• Ribose is used in RNA
• Deoxyribose is used in DNA
• Uracil is used in RNA
• Thymine is used in DNA

Transcription and DNA Replication

• Transcription is likened to DNA replication

RNA Molecules

• Cells produce numerous types of RNA molecules
• Prokaryotic cells have a single type of RNA polymerase
• Eukaryotic cells possess three distinct RNA polymerases

Promoters

• Promoters are DNA regions defining the start of transcription
• They are key in identifying the template strand
• Promoters contain consensus sequences
• Consensus sequences are short sequences identified by transcription factors and RNA polymerase
• TATA box in eukaryotes and -10 and -35 regions in prokaryotes exemplify promoter consensus sequences

Transcription Termination

• In prokaryotes, terminators signal where transcription ends
• Eukaryotes use a polyadenylation signal to mark the end of the RNA transcript
• After the polyadenylation signal, pre-mRNA is cleaved, and adenine nucleotides are added to the 3' end

Eukaryotic pre-mRNA processing

• Eukaryotic genes have noncoding sequences called introns
• The process of intron removal of pre-mRNA is called splicing
• They are spliced through alternative splicing
• The coding regions are called exons
• In the process of splicing, various mRNAs and, consequently, proteins are generated from a pre-mRNA through alternative splicing

mRNA Processing in Eukaryotes

• Guanine nucleotide addition at the 5’ end in a typical 5’ to 5' linkage
• This nucleotide addition is called mRNA capping
• It protects the mRNA, enabling mRNA export to the cytoplasm and then translation
• Key steps involve capping, polyadenylation, and splicing of introns, leading to a mature mRNA transcript

Transcription and Translation location

• In prokaryotes transcription and translation occur at the same time and location
• In eukaryotes transcription happens first in the nucleus, then translation in the cytoplasm

Genetic Code

• Every three ribonucleotides is called a "triplet code" or "codon"
• Each amino acid is specified by a codon
• In principle, an mRNA molecule is translated in three ways (reading frames)
• All codons form the genetic code
• Genetic code comprises 64 codons, containing 61 sense codons and 3 nonsense codons
• Different amino acids are specified by two, three, or four codons
• Three amino acids are encoded by six different codons
• Tryptophan and methionine are encoded by a single codon each
• Start codon (AUG) and three stop codons (UAA, UAG, and UGA) are included

Features of The Genetic Code

• The code is made of triplets, nonoverlapping, commaless etc.
• It is degenerate and ordered
• It is nearly universal for all organisms