chapter 17

transcription

the synthesis of RNA under the direction of DNA

translation

the synthesis of a polypeptide which occurs under the direction of mRNA. The cell translates the base sequence of an mRNA molecule into the amino acid sequence of a polypeptide. Done in ribosomes.

the number of nucleotides making up a genetic message must be _____ the number of amino acids in the protein product

three times

on DNA 3'-ACC-5' will be transcribed as ___ on the mRNA molecule

5'-UGG-3'

codons

mRNA base triplets, read in 5' to 3' direction

Amino acid methionine and and a start signal for ribosomes to begin translating the mRNA at that point

5'-AUG-3' genetic messages begin here

stop codons

UAA, UAG, UGA

redundancy v ambiguity

redundancy yes, different codons can create the same amino acid, but one codon will never create different amino acids

the template strand of a gene contains the sequence 3'-TTCAGTCGT-5'. Draw the nontemplate sequence and the mRNA sequence.

nontemplate: 5'-AAGTCAGCA-3', mRNA sequence: 5'-AAGUCAGCA-3'

RNA polymerase

pries the two strands of DNA apart and joins the RNA nucleotides as they base-pair along the DNA template. 5' to 3'

the DNA sequence where RNA polymerase attaches and initiates transcription

promotor, also determines which strand is a template

downstream and upstream

downstream" is direction of transcription. So, the promotor sequence is upstream from the terminator.

transcription unit

the stretch of DNA that is transcribed into an RNA molecule

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 polumerase moves downstream, unwindng 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 factors

a collection of proteins that mediate the binding of RNA polymerase and the initiation of transcription. Then RNA polymerase II can bind to it.

transcription initiation complex

the whole complex of transcription factors and RNA polymerase II bound to the promoter. Moves down and pries them open and RNA synthesis begins.

TATA box

nucleotide sequence containing TATA, upstream from transciptional start point. Signals the transcription factors to bind before RNA Polymerase II

compare and contrast DNA polymerase and RNA polymerase

both assemble nucleic acid chains from onomer nucleotides whose order is determined by complementary base pairing to a template strand. Both synthesize in the 5'-3' directin, antiparallel to the template. DNA polymerase requires a primer, but RNA polymerase can start a nucleotide chain from scratch. DNA polymerase uses nucleotides with the sugear deoxyribose and the base T, whereas RNA polymerase uses nucleotides with the sugar ribose and the base U.

What is a promoter, and is it located at the upstream or downstream end f a transcription unit?

promoter is the region of DNA to which RNA polymerase binds to begin transcriptin, and it is at the upstream end of the gene/transcription unit

What makes RNA polymerase start transcribing a gene at the right place on the DNA in a bacterial cell? Eukaryotic cell?

In a bacterial cell, RNA polymerase recognizes the gene's promoter and binds to it. In a eukaryotic cell, transcription factor mediate the binding of RNA polymerase to the promoter

RNA processing

the modifying of pre-mRNA, in which both ends of the primary trancript are altered before exiting the nucleus

A modified form of a guanine nucleotidde added into the 5' end after transcription

5' hat

AAUAAA

polyadenylation signal, signals the 3' end of pre-MRNA

Poly-A tail

at 3' end, an enzyme adds 50-250 adenine nucleotides. Facilitate export, protect RNA, help ribosomes attach to 5' end

RNA splicing

removal of large portions of the RNA molecule

introns

noncoding segments

extrons

coding segments of pre-mRNA

snRNPs

small nuclear ribonucleoproteins, signals for RNA splicing, they recognize splicing sites

spliceosome

RNA and snRNP and additional proteins, releases intron and joins together extrons

ribozymes

RNA molecules that function as enzymes. the intron RNA functions as a ribozyme and catalyzes its own excision

alternative RNA splicing

genes can give rise to different polypeptides based on which parts of the pre-mRNA is treated as an extron. explains biodiversity

In nematode worms, a gene that codes for an ATPase has two alternatives for exon 4 and three alternatives for exon 7. how many different forms of the protein could be made from this gene?

six different forms could be made because alternative splicing could generate six different mRNAs (two possibilities for exon 4 * 3 possibilities for exon 7)