SOCR 330 (Genetics) - Exam 1, Set 3
how is genetic information coded?
*in triplets (codons)*
genes to proteins
*post-transcriptional* and post-translational modifications contribute to the *production of more than one protein from a single gene*
Elongation
-*RNA polymerase moves along the template strand* -RNA transcript is synthesized in the 5' to 3' direction -as it travels, the polymerase unwinds the DNA helix ahead of itself and the DNA rewinds behind it
two of the most conserved sequences among eukaryotic promoters
-*TATA box* (most common sequence: TATAAA, located between -25 and -30 base pairs from the transcription site) -*CCAAT/CAAT box* (consensus sequence: GGCCAATCT, located between -60 and -100 base pairs from the transcription start site)
eukaryotic transcription
-*multiple polymerases* > RNA pol I, RNA pol II, and RNA pol III -general transcription factors (GTFs) bind to the promoter first and help recruit the appropriate RNA polymerase forming the "transcription initiation complex" -pre-mRNA (primary transcript) requires *processing* into mRNA before translation -mRNA is exported to cytoplasm for translation > transcription and translation are always separate events (takes place in different parts of the cell)
prokaryotic transcription
-*one RNA polymerase* makes all RNA types -RNA polymerase binds directly to promoters -mRNA is ready to be translated right after transcription -closely-coupled transcription possible > has the advantage of *speed* -mRNA can be translated immediately (even before transcription is finished)
what is the most common start codon?
-AUG (codes for Methionine) -the first codon in the mRNA that gets translated
RNA processing
-addition of 7-methylguanosine cap *(5' cap)* -addition of 3' *poly-A tail* -intron removal *(splicing)*
RNA splicing
-introns are removed by a complex structure called a *spliceosome* > consists of 5 snRNAs associated w/ proteins, forming small ribonucleoproteins (snRNPs) -spliceosome recognizes and cuts at specific sequences found at 5' and 3' splice sites
Termination: intrinsic mechanism
-inverted repeats in DNA (downstream of stop codon) -when transcribed, RNA folds and forms a *"hairpin"* shape -RNA polymerase then encounters poly-A series -RNA polymerase pauses and dissociates -new RNA transcript released -*there are other forms of transcription termination*
RNA editing
-involves altering the mRNA sequence to generate alternative protein products -RNA editing may include the insertion, deletion, and base substitution of nucleotides within the mRNA
role of promoters
-needed to initiate transcription -contain sequences that are *recognized* by the *RNA polymerase* and also by the *transcription factors* in eukaryotic cells -located upstream of the *transcription start site*(where transcription starts, position +1)
alternative splicing
-process by which particular exons of a gene may be included within or excluded from the final mRNA produced from that gene -same pre-mRNA is spliced in different ways in different tissues / developmental stages, generating different mRNAs (and different proteins) -alternative splicing generates a tremendous amount of proteomic diversity
why do introns exist?
-regulate alternative splicing -regulation of gene expression -can contain functional non-coding RNAs -indirect effect as buffer of mutations -many more reasons that we are still learning about
Initiation
-the *RNA polymerase holoenzyme is a multi-subunit enzyme* (core [5 subunits] + sigma factor) that binds to the promoter -*core* enzyme has polymerase activity -*sigma factor* recognizes and binds to promoter region containing the -35 and -10 boxes > allows the core enzyme to position in the right place to start transcription
which codon sets up the reading frame?
-the *start codon* -all subsequent codons are read in order until a *stop codon* is encountered on the mRNA
how many possible reading frames are there when reading a single-strand nucleotide sequence into a set of consecutive, non-overlapping triplets?
-three possible reading frames -these three reading frames will specify proteins with *entirely different* amino acid sequences
how many stop codons are there and what do they do?
-three stop codons -terminate translation
promoter consensus sequences in prokaryotes
-two short DNA sequences located at the -10 and -35 positions from from the transcription start site -*the more closely the sequence at the -10 and -35 boxes resemble the consensus sequence the stronger the promoter will be*
stages of transcription in prokaryotes
1. Initiation 2. Elongation 3. Termination
Poly-A tail
100-250 adenines added to the 3' end of pre-mRNA immediately after transcription (poly-A polymerase)
the genetic code
a set of *rules* used by living cells to translate the information encoded in the genetic material into proteins
the basic building block for a polypeptide structure is...
an amino acid
weak promoter
little RNA polymerase binding > low levels of transcription
strong promoter
lots of RNA polymerase binding > high levels of transcription
5' cap (7-methylguanosine cap)
methylated guanosine (guanine + ribose) added to 5' end while transcription is in progress
what are amino acids joined by to form a polypeptide?
peptide bonds
in eukaryotes, specific sequences at a promoter region are recognized by what?
recognized by both *transcription factors* and *RNA polymerase*
what is the code when most amino acids are encoded by more than one codon?
the code is *degenerate*
what is the code when consecutive amino acids are specified by consecutive codons?
the code is *over-lapping*
what is the code when the same genetic code used by almost all organisms on Earth?
the code is *universal*
open-reading frame (ORF)
the sequence between *start* and *stop* codons
promoters in eukaryotes
these promoters are much more complex and diverse
how many steps are there in the splicing process?
two steps: 1. the 5' end of the intron is cleaved and attached to the branch point to form a lariat 2. the 3' end of the intron is cleaved and the two ends of the exon are spliced together
