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?

a. 5′ UTR

b. 3′ UTR

c. Protein coding region

d. All three regions

Concept Check 1

Which region of mRNA contains the Shine–Dalgarno sequence?

a. 5′ UTR

b. 3′ UTR

c. Protein coding region

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

editing

•RNA editing

Concept Check 3

Alternative 3′ cleavage sites result in:

a. multiple genes of different length.

b. multiple genes of pre-mRNA of different length.

c. multiple mRNAs of different length.

d. all of the above.

Concept Check 3

Alternative 3′ cleavage sites result in:

a. multiple genes of different length.

b. multiple genes of pre-mRNA of different length.

c. multiple mRNAs of different length.

d. 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?

a. Encoded by guide RNAs

b. By chemical changes in one of the standard bases

c. Encoded by rare bases in DNA

d. Encoded by sequences in introns

Concept Check 3

How are rare bases incorporated into tRNAs?

a. Encoded by guide RNAs

b. By chemical changes in one of the standard bases

c. Encoded by rare bases in DNA

d. 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?

a. Methylation of bases

b. Cleavage of bases

c. Nucleotides are trimmed from the ends of rRNAs.

d. All of the above

Concept Check 5

What types of changes take place in rRNA processing?

a. Methylation of bases

b. Cleavage of bases

c. Nucleotides are trimmed from the ends of rRNAs.

d. 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