Chapter 11 Flashcards
What does the central dogma explain?
What about viruses?
Are they reversible?
Genetic information usually flows from
DNA to DNA during its transmission from generation to generation and from DNA to protein during its phenotypic expression in an organism
During the replication of RNA viruses, information is also transmitted from RNA to RNA and RNA to DNA during the conversion of the genomes of RNA tumor viruses to their DNA proviral forms.
DNA to RNA is sometimes reversible
RNA to protein is always irreversible
What are the two steps involved in DNA being processed into proteins?
- Transcription (DNA to RNA)
- Translation (RNA to Proteins)
Give a quick overview of what happens in transcription and translation.
Transcription:
-One strand of DNA in a gene is used as
a template to synthesize a complementary strand of RNA, called the gene transcript
Translation:
-The sequence of nucleotides in the RNA
transcript is converted into the sequence of amino acids in the polypeptide gene product.
What is the difference in the product of transcription in prokaryotes and eukaryotes?
Prokaryotes:
-The product of transcription, the primary
transcript, usually is equivalent to the mRNA molecule.
Eukaryotes:
-Primary transcripts often must be processed by the excision of specific sequences and the modification of both termini before they can be translated. Thus, the primary transcripts usually are precursors to mRNAs and, as such, are called pre-mRNAs.
What are the 5 different types of RNA molecules and what are their jobs?
- mRNA:
-The intermediaries that carry genetic info from DNA to the ribosomes where proteins are synthesized - tRNA:
-Small RNA molecules that function as adaptors between amino acids and the codons in mRNA during translation - rRNA:
-Structural and catalytic components of the ribosomes, the machines that translate nucleotide sequences of mRNAs into amino acid sequences of polypeptides - snRNA:
-Structural components of spliceosomes, the nuclear organelles that excise introns from gene transcripts - miRNA:
-Short single-stranded RNAs that block the expression of complementary or partially complementary mRNAs by either causing their degradation or repressing their translation.
What RNA molecules are not translated?
tRNA
rRNA
snRNA
miRNA
How is the produced RNA molecule oriented to the DNA template strand?
It will be complementary and antiparallel to the DNA template strand and identical, except that
uridine replace thymidines, to the DNA nontemplate strand
mRNA molecules are coding strands of RNA and are also called sense strands of RNA because their nucleotide sequences “make sense” in that they specify sequences of amino acids in the protein gene products.
An RNA molecule that is complementary to an mRNA is referred to as antisense RNA
RNA synthesis occurs by a mechanism that is similar to that of DNA synthesis except for a few differences.
What are these differences?
- The precursors are ribonucleoside triphosphates rather than deoxyribonucleoside triphosphates,
- Only one strand of DNA is used as a template for the synthesis of a complementary RNA chain in any given region.
- RNA chains can be initiated de novo, without any requirement for a preexisting
primer strand
How does the initiation of RNA molecules begin?
RNA polymerases bind to specific nucleotide
sequences called promoters, and with the help of proteins called transcription factors, initiate the synthesis of RNA molecules at transcription start sites near the promoters
What is a transcription unit?
1.A segment of DNA that is transcribed to produce one RNA molecule
What are the 3 stages of transcription?
- Initiation of a new RNA chain
- Elongation of the chain
- Termination of transcription and release of the nascent RNA molecule
What are the three steps involved in the initiation of RNA chains in prokaryotes?
- Binding of the RNA polymerase holoenzyme to a promoter region in DNA
- The localized unwinding of the two strands of DNA by RNA polymerase, providing a
template strand free to base-pair with incoming ribonucleotides - The formation of phosphodiester bonds between the first few ribonucleotides in the nascent (new) RNA chain
What makes up the complete RNA polymerase molecule in prokaryotes?
What makes up the core enzyme?
What does each do?
What is the difference in function of the complete holoenzyme and the core enzyme?
- The complete RNA polymerase molecule, the holoenzyme, has the composition α2ββ′σ.
- α2ββ′ (no sigma)
- α:
-Involved in the assembly of the tetrameric core (α2ββ′) of RNA polymerase.
β:
-Contains the ribonucleoside triphosphate binding site
β′:
-Harbors the DNA template-binding region.
σ:
-Involved only in the initiation of transcription; it plays no role in chain elongation
-To recognize and bind RNA polymerase to
the transcription initiation or promoter sites in DNA
4.The core enzyme (with no σ) will catalyze RNA synthesis from DNA templates in vitro (in dish), but, in so doing, it will initiate RNA chains at random sites on both strands of DNA.
In contrast, the holoenzyme (σ present)
initiates RNA chains in vitro (in dish) only at sites used in vivo (living subject).
By convention, the nucleotide pairs are numbered relative to the transcript initiation site (designated +1).
What do each indicate?
Base pairs preceding (before) the initiation site are given minus (−) prefixes (upstream)
Those following (after) the initiation site are given plus (+) prefixes. (downstream)
Within prokaryotes, what are the midpoints of the two conserved sequences and where do they occur?
The midpoints of the two conserved sequences occur at about 10 and 35 nucleotide pairs before the transcription-initiation site.
Thus they are called the −10 sequence and the −35 sequence, respectively.
They are consensus sequences
