14.1 Many Genes Have Complex Structures

14.1 Many Genes Have Complex Structures

Gene Organization • The concept of colinearity and noncolinearity

Gene Organization • Introns • Regions of RNA that are not reflected in protein sequence • INTervening sequences • Exons • Regions of RNA that are reflected in protein sequence • EXpressed sequences

The Concept of the Gene • The gene includes DNA sequence that codes for all exons, introns, and those sequences at the beginning and end of the RNA that are not translated into a protein, including the entire transcription unit – the promoter, the RNA coding sequence, and the terminator.

14.2 Messenger RNAs, Which Encode the Amino Acid Sequences of Proteins, Are Modified after Transcription in Eukaryotes • The structure of messenger RNA: • A mature mRNA contains 5′ untranslated region (5′ UTR, or leader sequence) • Shine–Dalgarno sequence • Protein coding region • 3′ untranslated region

Concept Check 1 Which region of mRNA contains the Shine–Dalgarno sequence? • 5′ UTR • 3′ UTR • Protein coding region • All three regions

Pre-mRNA processing • The addition of the 5′ cap: • A nucleotide with 7-methylguanine; 5′-5′ bond is attached to the 5′-end of the RNA. • The addition of the poly(A) tail: • 50 ~ 250 adenine nucleotides are added to the 3′-end of the mRNA.

Pre-mRNA processing • RNA splicing: • Consensus sequences: • 5′ consensus sequence: GU A/G AGU: 5′ splice site • 3′ consensus sequence: CAGG • Branch point: the adenine “A”: 18 ~ 40 nucleotides upstream of 3′-splicing site • Spliceosome: five RNA molecules + 300 proteins • The process of splicing

Intron removal, mRNA processing, and transcription take place at the same site in the nucleus. • Self-splicing introns happen in some rRNA genes in protists and in mitochondria genes in fungi. • There are alternative processing pathways for processing pre-mRNA.

RNA editing • RNA editing: The coding sequence of an mRNA molecule is altered after transcription. • Guide RNA is used to serve as template for RNA editing

Concept Check 3 Alternative 3′ cleavage sites result in: • multiple genes of different length. • multiple genes of pre-mRNA of different length. • multiple mRNAs of different length. • all of the above.

14.3 Transfer RNAs, Which Attach to Amino Acids, Are Modified after Transcription in Bacteria and Eukaryotic Cells

The Structure of Transfer RNA • Rare modified RNA nucleotide bases • Ribothymine • Pseudouridine • Common secondary structure – the cloverleaf structure

The Structure of Transfer RNA • Transfer RNA gene structure and processing

Concept Check 3 How are rare bases incorporated into tRNAs? • Encoded by guide RNAs • By chemical changes in one of the standard bases • Encoded by rare bases in DNA • Encoded by sequences in introns

14.4 Ribosome RNA, a Component of the Ribosome, Also Is Processed after Transcription

The Structure of the Ribosome • Large ribosome subunit • Small ribosome subunit

Ribosomal RNA Gene Structure and Processing

Concept Check 5 What types of changes take place in rRNA processing? • Methylation of bases • Cleavage of bases • Nucleotides are trimmed from the ends of rRNAs. • All of the above

14.5 Small RNA Molecules Are Present Extensively in Eukaryotes and Participate in a Variety of Functions • RNA interference: limits the invasion of foreign genes and censors the expression of their own genes

14.5 Small RNA Molecules Are Present Extensively in Eukaryotes and Participate in a Variety of Functions • Model Genetic Organism – the Nematode Worm

14.1 Many Genes Have Complex Structures