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BIOL-L 112: Exam 3
Gene Expression |
th process by which DNA directs the synthesis of proteins (or in some cases, just RNA)
-includes transcription and translation |
Transcription |
the synthesis of RNA using information in DNA
-occurs in the nucleus |
mRNA |
Messenger RNA that carries genetic message from the DNA to the protein-synthesizing machinery of the cell |
Translation |
the synthesis of a polypeptide using the information in the mRNA
-the nucleotide sequence of mRNA is translated into the amino acid sequence of a polypeptide
-occurs in the cytoplasm |
Ribosomes |
The site of translation, molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains |
Triplet Code |
-organizes the flow of information from gene to protein
-the genetic instructions for a polypeptide chain are written in the DNA sequence of nonoverlapping, three-nucelotide words |
Template strand |
provides the pattern (template) for the sequence of nucleotides in an RNA transcript |
Codons |
the mRNA nucleotide triplets
-written in the 5'-->3' direction
-AUG: start codon, codes for methionine
-UAA, UAG, UGA: stop codons |
RNA polymerase |
An enzyme that pries the two strands of DNA apart and joins together RNA nucleotides complementary to the DNA template strand
-this elongates the RNA polynucleotide
-unlike DNA polymerases, these do not require a primer
-RNA polymerase I: most rRNA genes
-RNA polymerase II: all protein coding genes, miRNA genes
RNA polymerase III: tRNA genes, genes for small RNA |
Promoter |
the DNA sequence where RNA polymerase attaches and initiates transcription |
Terminator |
In bacteria, this is the sequence that signals the end of transcription
-the direction of transcription is referred to as 'downstream'
-the promoter sequence in DNA is upstream from the terminator |
initiation |
After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand |
Elongation
|
the polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5'-->3'. in the wake of transcription, the DNA strands re-form a double helix |
Termination |
Eventually, the RNA transcript is released, and the polymerase detaches from the DNA |
Transcription start point |
Included in the promoter of a gene, the nucleotide where RNA synthesis actualy begins |
Transcription factors |
a collection of proteins that mediate the binding of RNA polymerase and the intiation of transcription |
Transcription Initiation complex |
After transcription factors attach to the promoter, RNA polymerase II then binds to it. The whole complex of transcription factors and RNA II polymerase bound to the promoter is called the transcription initiation complex |
TATA box |
A promoter DNA sequence in eukaryotes that is crucial in forming the transcription initiation complex
-without it, RNA polymerase doesn't know where to bind to initiate transcription |
RNA processing |
-pre-mRNA is modified by enzymes in the eukaryotic nucleus before the genetic message is dispatched to the cytoplasm
-during this processing, both ends of the primary transcript are altered.
-also, certain interior sections of the RNA molecule are cut out and the remaining parts are spliced together |
5' cap |
The 5' end of the pre-mRNA molecule receives a 5' cap, a modified form of a guanine nucleotide added on to the 5' end after transcription of the first 20-40 nucleotides |
Poly-A tail |
-the 3' end of the pre-mRNA molecule receives 50-250 more adenine nucleotides
-these two modifications facilitate the export of mature mRNA from the nucleus, protect mRNA from degradation by hydrolytic enzymes, and help ribosomes attach to the 5' end of the mRNA once the mRNA reaches the cytoplasm |
RNA splicing |
after synthesis of a eukaryotic primary RNA transcript, the removal of portions of the transcript (introns) that will not be included in the mRNA and the joining together of the remaining portions (exons) |
introns |
the noncoding segments of nucleic acids that lie between coding regions (also called intervening sequences) |
Exons |
the coding regions that are eventually expressed and translated into amino acid sequences |
Spliceosome |
the removal of introns is accomplished by this large complex made of proteins and small RNAs
-it then joins together the two exons that flanked the intron |
ribozymes |
Ribosome molecules that function as enzymes
-the molecule can have 3-D structure when a region of an RNA molecule base pairs with a complementary region elsewhere in the same molecule
-some of the bases in RNA contain functional groups that can participate in catalysis
-the ability of RNA to hydrogen-bond with other nucleic acid molecules adds specificity to its catalytic activity |
Alternative RNA splicing |
-a single gene can encode more than one polypeptide
-many genes give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing |
Domains |
a discrete structural and functional region of a protein
-different exons code for the different domains of a protein |
tRNA |
transfer RNA, the translator of a genetic message consisting of a series of codons along an mRNA molecule
-its function is to transfer amino acids from the cytoplasmic pool of amino acids to a growing polypeptide in a ribosome |
anticodon |
the particular nucleotide triplet that base pairs to a specific mRNA codon
ex. the anticodon of 5'-GGC-3' is 3'CCG-5' |
aminoacyl-tRNA synthetases |
a family of related enzymes that correctly matches up tRNA and amino acids
-a tRNA that binds to an mRNA codon specifying and amino acid must carry that amino acid, and no other, to the ribosome
-there are 20 different synthetases, one for each amino acid |
Wobble |
-some tRNA's are able to bind to more than one codon
-this is possible because the rules for base pairing between the third nucleotide base of a codon and the corresponding base of a tRNA anticodon are relaxed
-for ex. the nucleotide base U at the 5' end of a tRNA anticodon can pain with either A or G in the third position (3' end) of an mRNA molecule
-this explains why the snynonymous codons for a given amino acid often differ in their third nucleotide base, but not in other bases |
rRNAs |
ribosomal RNA's are found in ribosomes, along with a large and small subunit
-this is the most abundant type of cellular RNA |
Binding sites |
P site: (peptidyl) holds the tRNA carrying the growing polypeptide chain
A site: (aminoacyl) holds the tRNA carrying the next amino acid to be added to the chain
E site: (exit) where discharged tRNA's leave the ribosome |
Translation initiation complex |
formed by the union of mRNA, initiator tRNA, and a small ribosomal subunit followed by the attachment of a large ribosomal subunit
-proteins called initiation factors are required to brings all these components together |
Targeting polypeptides to a specific location |
-Ribosomes can be free in the cytoplasm or bound to the ER
-signal peptide: targets proteins to the ER if the polypeptides for that protein are destined for the endomembrane system or secretion
- consists of a sequence of 20 amino acids
-signal-recognition particle (SRP): functions as an escort that brings the ribosome to a receptor protein built into the ER membrane |