17.1 An Overview of Transcription

• Enzymes called RNA polymerases are front and center in transcription

• The strand that is read by RNA polymerase is the template strand.

• The other strand is the non-template strand, or coding strand

• If transcription enables the use of genetic information, then the ability to begin transcription-initiation-is the key to its control

• Researchers soon discovered that bacterial RNA polymerase，the enzyme that transcribes genes一the core enzyme-wasn’t sufficient to recognize the start of genes.

• Another protein, called sigma, must first bind to the core enzyme to recognize sites where transcription should begin.

• These sites were named promoters because they are regions of DNA that promote the start of transcription.

• Together, the bacterial RNA polymerase core enzyme and sigma form a holoenzyme

• DNA that is located in the direction RNA polymerase moves during transcription is said to be downstream from a point of reference

• DNA located in the opposite direction is said to be upstream from the same point of reference

• Once RNA polymerase leaves the promoter region as it synthesizes RNA, the elongation phase of transcription is under way.

• Termination ends transcription.

• In bacteria, transcription stops when RNA polymerase transcribes a DNA sequence called a transcription-termination signal.

• Many eukaryotic promoters include a sequence called the TATA box, centered about 30 base pairs upstream of the transcription start site, and other important sequences that vary more widely.

• Instead of using a sigma protein, eukaryotic RNA polymerases recognize promoters using a group of proteins called general transcription factors.

• In eukaryotes, a DNA sequence near the end of each gene called the polyadenylation signal, or poly(A) signal, is transcribed.

17.2 RNA Processing in Eukaryotes

• Work from many teams confirmed that eukaryotic genes are initially copied into nonfunctional RNAs called primary transcripts.

• For protein-coding genes, the primary transcript is a pre-mRNA.

• The primary transcript requires multistep modification, called RNA processing, within the nucleus to generate the mature, functional RNA.

• RNA processing is critically important for gene expression in eukaryotes.

• Regions of a gene that are transcribed but not represented in the final RNA came to be called introns

• Regions that are transcribed and represented in the final mature RNA were called exons

• As transcription proceeds, the introns are removed from the growing RNA strand by a remarkable process known as splicing.

• These protein-plus-RNA macromolecular machines are known as small nuclear ribonucleoproteins

• The spliceosome assembles as more snRNPs join the complex.

• As soon as the 5’ end of a eukaryotic pre-mRNA emerges from RNA polymerase, enzymes add a 5 ’ cap The cap consists of a modified guanine nucleotide linked to 1e transcript in an unusual way.

• The cap enables ribosomes to bind to the mRNA, and it also protects the 5’ end of the mRNA from enzymes that degrade RNA

• This string of adenines is known as the poly(A) tail. Like the 5’ cap, the poly(A) tail is required for ribosomes to start translation and to protect the end of mRNA from attack by enzymes.

17.3 An Introduction to Translation

• When two or more ribosomes simultaneously translate one mRNA, the structure is called a polyribosome,

• Transcription and translation can be coupled in bacteria because there is no nuclear envelope to separate the two processes.

17.4 The Structure and Function of Transfer RNA

• The novel class of RNA eventually became known as transfer RNA (tRNA).

• When a tRNA has an amino acid attached, it is known as an aminoacyl tRNA.

• Anticodon is a triplet of ribonucleotides able to form base pairs with the codon for the amino acid in mRNA.

• The right amino acid for a particular tRNA to be attached is allowed by:

  • An n input of energy from ATP is required to attach an amino acid to a tRNA.

  • Enzymes called aminoacyl-tRNA synthetases catalyze the addition of amino acids to tRNAs-what biologists call “charging” a tRNA.

  • For each of the 20 major amino acids, there is a different aminoacyl-tRNA synthetase and one or more tRNAs.

• Wobble pairing allows one tRNA to read more than one codon.

17.5 Ribosome Structure and Function in Translation

• Biologists have known since the 1930s that ribosomes contain many proteins and ribosomal RNAs (rRNAs).

• To translate an mRNA, a ribosome must begin at the first codon in a message, translate the mRNA up to the message ’ s termination codon, and then stop.

• Translation begins when a section of rRNA in a small ribosomal subunit binds to a complementary sequence on an mRNA (ribosome binding site or Shine-Dalgarno sequence).

• The interactions between the small subunit, the message, and the tRNA are mediated by proteins called initiation factors

• Translation initiation in bacteria is a three step process:

  • The mRNA binds to a small ribosomal subunit

  • The initiator tRNA bearing f-Met binds to the start codon

  • The large ribosomal subunit binds, completing the complex.

• Translocation is the process in which the ribosome moves one codon down the mRNA once a new peptide bond is formed.

• Translocation requires a type of protein called an elongation factor.

• The three steps in elongation:

  • arrival of aminoacyl tRNA

  • peptide-bond formation

  • translocation-repeat at each codon along the mRNA

• Instead of tRNAs working to terminate translation, translation is brought to an end when the translocating ribosome reaches one of the stop codons and a protein called a release factor recognizes the stop codon and fills the A site

• Although folding can occur spontaneously, it is frequently guided and accelerated by proteins called molecular chaperones.