Unit 3 notes from textbook + slides Flashcards
(53 cards)
where does transcription and translation occur in bacteria?
cytoplasm
where does transcription and translation occur in eukaryotes?
transcription occurs in the nucleus and translation in the cytoplasm
exception to the central dogma
viruses are an exception and can go from RNA->DNA or RNA->RNA
difference between uracil and thymine
thymine has a methyl group on its 5’ carbon whereas uracil has a hydrogen
transcription definition
as a region of DNA unwinds, one strand is used as a template for the synthesis of an RNA transcript that is complementary in sequence
uses RNA polymerase
acts by adding nucleotides to the 3’ end of the growing transcript
stages of transcription
1) initiation: RNA polymerase and other proteins are attracted to double-stranded DNA, the DNA strands are seperated and transcription of the template strand begins
2) elongation: successive nucleotides are added to the 3’ end of the growing RNA transcript as the RNA polymerase proceeds along the template strand
3) termination: the RNA polymerase encounters a sequence in the template strand that causes transcription to stop and the RNA transcript to be released
directionality of the RNA transcript
synthesized in the 5’ to 3’ direction, the DNA template is read in the 3’ to 5’ direction
promoter definition
regions of typically a few hundred base pairs where RNA polymerase and associated proteins bind to DNA
Promoters in eukaryotes
includes the sequence 5’-TATAAA-3’ or something similar which is known as the TATA box
terminator sequence
transcription continues until the RNA polymerase encounters a sequence known as a terminator. transcription stops at the terminator and the transcript is released
how is promoter recognition mediated in bacteria?
a protein called sigma factor which associates with RNA polymerase and facilitates its binding to specific promoters
once transcription is initiated, sigma factor dissociates and the RNA polymerase continues transcription on its own
how is promoter recognition mediated in eukaryotes?
transcription in eukaryotes requires the combined action of at least 6 proteins known as general transcription factors that assemble at the promoter of a gene
also transcriptional activator proteins help control when and in which cells transcription of a gene will occur. they are able to bind with enhancer DNA sequences as well as with proteins that allow transcription to begin.
messenger RNA definition
RNA molecules that combines with ribosome to direct protein synthesis, it carries the genetic “message” from the DNA to the ribosome
RNA processing definition
converts the primary transcript into the finished mRNA which can then be translated by the ribosome
happens in the nucleus
RNA processing (3 types of chemical modifications)
1) the 5’ end of the primary transcript is modified by the addition of a special nucleotide. called the 5’ cap. the cap is linked to the RNA transcript by a triphosphate bridge between the 5’ carbons of both ribose sugars. the ribosome recognizes an mRNA by its 5’ cap
2) polyadenation: the addition of a string of about 250 consecutive A-bearing nucleotides to the 3’ end, forming a poly(A)tail. it stabilizes the RNA transcript until it is translated in the cytoplasm
3) not every stretch of the RNA transcript ends up being translated into protein. exons: regions of protein-coding sequence that are expressed introns: non-coding regions that are interspersed.
process of removal of the non coding introns and joining of the exons known as RNA splicing
ribosomal RNA
makes up the bulk of ribosomes and is essential in translation
transfer RNA
carries individual amino acids for use in translation
large and small subunit of the ribosome
the large subunit of the ribosome includes three binding sites for molecules of tRNA, which are called aminoacyl(A)site, peptidyl(P)site and the exit(E)site
use of ribosomes in translation
structures made up of RNA and protein that bind with mRNA and are at the site of translation
anticodon (tRNA)
three bases in the anticodon loop
the 3 nucleotides that undergo base pairing with the corresponding codon
anti parallel and complementary to the codon on the mRNA
aminoacyl tRNA synthetases
enzymes that connect specific amino acids to specific tRNA molecules
these enzymes are directly responsible for actually translating the codon sequence from a nucleic acid to a specific amino acid in a polypeptide chain
binds to multiple sites on any tRNA that has an anticodon corresponding to the amino acid and it catalyses formation of the covalent bond between the amino acid and tRNA
directionality of synthesis of polypeptide
from the amino end to the carboxyl end
translation stages
1) initiation: the intiator AUG codon is recognized and Met is established as the first amino acid of the new polypeptide chain
2) elongation: successive amino acids are added one by one to the growing chain
3) termination: addition of amino acids stops and the completed polypeptide chain is released from the ribosome
what does the initiation of translation require?
a number of protein intiation factors that bind to the mRNA. In eukaryotes, one group of intiation factors binds to the 5’ cap. Initiation factors recruit the small ribosomal subunit and tRNA met and scan the mRNA for an AUG codon. When the complex reaches an AUG the large ribosomal subunit joins and intiation factors are released.
