ch 17- transcription Flashcards
gene expression
the process by which DNA directs protein synthesis
includes two stages : transcription and translation
basic principles of transcription
is the synthesis of RNA using DNA as a template
produces RNA of all kinds (mRNA)
mrna is the type that eventually leads to building polypeptides
RNA structures
differs from DNA
RNA uses ribose sugar
bases are A,G,C, and U
single stranded
types of RNA
mRNA (messenger)
tRNA (transfer
rRNA (ribosomal)
primary/pre RNA
mRNA
carries information specifying amino acid sequences of proteins from DNA to ribosomes
transfer RNA
serves as adapter molevcule in protein synthesis
translates mRNA codons into amino acids
primary/pre RNA
serves as precursor to mRNA, rRNA, or tRNA before being processed by splicing or cleavage
properties that enable RNA to perform many different functions
single stranded
can hydrogen bond to other nucleic acids
can assume a three-dimensional shape
has functional groups that allow it to act as a catalyst
messenger rna how it is built
strand of 1000 to 10000 nucleotides
blueprint for a protein
built from DNA template
goes to ribosomes in cytoplasm
ribosomal RNA
made from DNA in nucleolus
combined with proteins
2 ribosomal subunits passed out through nuclear pores
anticodon
3 base code on lower loop of transfer RNA
bonds to complementary CODON on mRNA and brings correct amino acids to ribosome/mRNA complex
how is translation and transciption different in prokaryotes and eukaryotes
in prokaryotes transcription and translation occur together
in eukaryotes rna transcripts are modified before becoing true mrna
what is the flow of information from gene to protein based on
a triplet code
codon
a series of nonoverlapping, three-nucleotide words
these triplets are the smallest units of uniform length that can code for all the amino acds
what provides a template for ordering the sequence of nucleotides in an RNA transcript
a DNA strand called the template strand
what does the GENE determine in transcription
the sequence of bases along the length of an mRNA molecule
how is a language shared by all living things
same codon codes for same AA in all organisms
RNA synthesis
catalyzed by RNA polymerase enzyme
follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine
RNA polymerase
separates the DNA strands at the appropriate point and bonds the RNA nucleotides as they base-pair along the DNA template
difference and similarities between RNA and DNA polymerase
like DNA, RNA polymerase can only assemble a polynucleotide in its 5’ to 3’ direction
unlike DNA RNA are able to start a chain from scratch
stages of transcript
initiation
elongation
termination
before RNA synthesis
specific sequence of nucleotides on DNA strand mark beginning and ending of gene
initiation - rna transcript
rna pol binds to promoter
segment of DNA unwinds
RNA polym initiates RNA synthesis by binding to promoter at start point on DNA template strand
promoter
includes TATA box and proteins called transcription factirs
it is crucial in forming the initiation complex in eukaryotes
what does the promoter tell DNA
where to start transcription
which of the two DNA strands to transcribe
elongation
polymerasae complex moves and unwinds DNA
adds RNA nucleotides to 3’ end
New RNA peels away from DNA template
DNA helix reforms
termination
rna transcript is released and polymerase detaches from DNA
how is termination diferent from prokaryotes and eukaryotes
in prokaryotes the polymerase stops transcription at the end of the terminator, both RNA and DNA are then released by RNA pol
in eukaryotes the pre-mRNA is cleaved from the growing RNA chain while RNA pol II continues to transcribe the DNA
termination in eukaryotes
the polymerase transcribes a DNA sequence called the polyadenylation signal sequence that codes for a polyadenylation sequence in the pre-mRNA
at a point past the sequence the pre-mrna is cut from the enzy=me
the polymerase keeps transcribing for nucleotides
transcriptionis terminated when polymerase eventually falls off
key concept of termination
in most cases termination stops when RNA polymerase reachs a DNA sequence that functions as a transcription termination signal
how does transcription differ from DNA replicatin
only ne region on DNA strand is used as a template
rna polym is used not dna pol
results in single stranded dna
no need for primer
how is rna modified after transcription
enzymes modify pre-mrna before the genetic messages are dispatched to cytoplasm
during rna processing, both ends of the primary transcript are usually altered
usually some interior parts of the molecule are cut out and other parts are spliced together
how is the 5’ end of premrna modified
receives a modified nucleotide cap
helps tell ribosome where to attacah when translation begins
how is the 3’ end of pre-mrna modified
the 3’ end gets a poly-tail
facilitates transport of mRNA out og nucleus and inhibits degradation by enzymes
RNA splicing
removes introns and joins exons
introns
in between sequences = junk dna
exons
expressed seuqneces
ribozymes
catalytic rna molecules that function as enzymes and can splice rna
spliceosomes
sometimes carries out RNA splicing
consist of a variety of proteins and small nuclear ribonucleoproteins that recognize the splice sites
alternative RNA splicing
some genes being able to encode more than one kind of polypeptide depending on which segments are treated as exons during RNA splicing
makes the number of different proteins an organism can produce, much greater than its number of genes
translation
the actual synthesis of a polypeptide, which occurs under the direction of mRNA
occurs on ribosomes in the sytoplasn
