Genes, RNA, and Protein Synthesis

• A gene is a heredity unit passed from parent to offspring.
• A gene specifies one or more organism traits.
• Each gene corresponds to specific DNA regions.
• Inherited DNA dictates protein synthesis, leading to specific traits.
• Proteins link genotype and phenotype.
• Gene expression, encompassing transcription and translation, directs DNA based protein synthesis.

RNA and Transcription

• RNA serves as a bridge between genes and protein synthesis.
• Transcription synthesizes RNA using DNA information.
• Transcription produces messenger RNA (mRNA).
• Translation synthesizes a polypeptide using mRNA information.
• Ribosomes are translation sites.

Genetic code

• Assembling instructions for amino acids into proteins are encoded into DNA.
• There are 20 amino acids with only four nucleotide bases in DNA.
• The flow of information from gene to protein follows the triplet code, with three-nucleotide words.
• Gene words are transcribed into complementary, nonoverlapping three-nucleotide mRNA words.
• These words are then translated into an amino acid chain that form a polypeptide.
• The template strand of DNA orders the complementary nucleotide sequence in an RNA transcript.
• The mRNA molecule is complementary to the template strand.
• During translation, mRNA base triplets termed codons are read from 5' to 3'.
• Of 64 triplets, 61 code for amino acids and 3 are "stop" signals ending translation.
• The genetic code has redundancy, as a particular amino acid may be specified by more than one codon.
• Genetic code is unambiguous, as no codon specifies more than one amino acid.

Transcription

• RNA polymerase catalyzes RNA synthesis, separating DNA strands and joining RNA nucleotides.
• RNA is complementary to the DNA template strand.
• RNA polymerase does not need a primer.
• RNA synthesis base-pairing rules follow those of DNA, but uracil substitutes for thymine.
• Transcription has three stages: initiation, elongation, and termination.
• Promoters signal the transcription start point and usually extend upstream.
• In eukaryotic cells, transcription factors help guide RNA polymerase binding and transcription initiation.
• Transcription initiation complex is completed assembly of transcription factors and RNA polymerase II bound to a promoter.
• Eukaryotes require a TATA box promoter element to form the initiation complex.

RNA processing

• Pre-mRNA is modified in the eukaryotic nucleus before genetic messages dispatch into the cytoplasm (RNA processing)
• Each end of a pre-mRNA molecule is modified.
  • The 5' end receives a modified nucleotide 5' cap.
  • The 3' end gets a poly-A tail.
• These modifications facilitate mRNA export to the cytoplasm, protect mRNA from hydrolytic enzymes, and help ribosomes attach to the 5' end.

RNA Splicing

• Most eukaryotic genes and RNA transcripts have long noncoding nucleotides between coding regions.
• These are removed through RNA splicing.
• Noncoding segments in a gene are called intervening sequences, or introns.
• Other regions are called exons because they are eventually expressed, usually translated into amino acid sequences.
• Spliceosomes accomplish intron removal.
• Spliceosomes consist of proteins and small RNAs that recognize the splice sites.
• The RNA of the spliceosome catalyzes the splicing reaction.
• Some introns contain sequences that regulate gene expression and affect gene products.
• Some genes encode multiple polypeptides, depending on which segments are exons during slicing.
• Alternative RNA splicing refers to this.
• An organism can produce more proteins than genes.
• Proteins often have a modular architecture of discrete regions called domains.
• Exon shuffling may lead to evolution of new proteins by mixing and matching exons between different genes.