Unit Three (chap 13,14, 15, 16, 17 Flashcards
Differentiate between the structures of DNA and RNA
- RNA contains uracil in place of thymine.
- RNA has a hydroxyl group on the 2’-carbon atom of its sugar, making RNA more reactive (less stable) than DNA
- RNA molecules fold to form secondary structures
- DNA is deoxyribose sugar and RNA is ribose
- RNA is usually single stranded, DNA is usually double strande
- DNA secondary structure is a double helix. RNA secondary structure has many types
The same between RNA and DNA: - composed of nucleotides
- A,G,C
- nucleotides joined together by phosphodiester bonds
What is the location and function of rRNA
Cell type: prokaryotic and eukaryotic
Location of function in eukaryotic cells: cytoplasm
Function: structural and functional components of the ribosome
what is the location and function of mRNA
Cell type: prokaryotic and eukaryotic
Location of function in eukaryotic cells: nucleus and cytoplasm
Function: carries genetic code for protiens
What is the location and function of tRNA
Cell type: prokaryotic and eukaryoitc
Location and Function in Eukaryotic cells: cytoplasm
Function: helps incorporate amino acids into polypeptide chain
define transcription
Transcription: synthesis of an RNA molecule from a DNA template
RNA synthesis is complementary and antiparallel to template strand
Only certain regions of the DNA are transcribed (mostly gene regions) rather than the entire chromosome
Only 1 of the 2 DNA strands is transcribed for each gene
Nucleotides are added to the 3’ end of the RNA molecule so the direction of synthesis is 5’-3’
* refer to Fig 13.8
draw and label a transcription unit
draw the substrates for transcription
describe the bacterial transcription apparatus
Bacterial cells only possess one type of RNA polymerase, which catalyzes the synthesis of all classes of bacterial RNA.
5 sub units make up the core enzyme: 2 copies of alpha, 1 beta, 1 beta prime, 1 omega
The core enzyme catalyzes the elongation of RNA molecules by the addition of RNA nucleotides.
The sigma factor controls the binding of RNA polymerase to the promoter forming a holoenzyme.
It is said to move downstream during transcription: it binds to the promoter and moves toward the terminator.
What is the difference between bacterial RNA polymerase and eukaryotic RNA polymerases
Bacteria only have one RNA polymerase that transcribes all RNA (mRNA, tRNA, rRNA), but eukaryotes have many RNA polymerases (names with roman numerals) that transcribe different RNAs.
RNA polymerase II transcribes pre-mRNA
what is the function of the promoter
the promoter is a DNA sequence that the transcription factor recognizes and binds. It indicates which of the 2 DNA strands is to be read as the template and the direction of transcription. In most cases, the promoter is located next to the transcription start site but is not itself transcribed
What is a consensus sequence and why is its presence important within a promoter
A consensus sequence is the set of most commonly encountered nucleotides among sequences that possess considerable similarity, or consensus. The presence of a consensus within a set of nucleotides usually implies that the sequence is associated with an important function.
The holoenzyme initially binds weakly to the promoter but then undergoes a change in structure that allows it to bing more tightly and unwind the double stranded DNA
What are the steps of bacterial transcription
Initiation:
- sigma and core RNA polymerase bind to promoter
- unwinds DNA
- nucleotides incorporated – no primer required
- 2 phosphates cleaved for each new nucleotide added
- sigma released
Elongation:
- RNA polymerase continues adding nucleotides
- unwinding at front of bubble and rewinding behind bubble
- proofreading
Termination- different genes can use different termination mechanisms. This is the only one you need to know:
1. RNA polymerase transcribes a terminator sequence, which consists of a sequence that forms a hairpin structure in the RNA followed by several uracils in the RNA
2. the hairpin structure causes the RNA polymerase to pause
3. at this point, only A-U bonds (weak bonds) are holding DNA and RNA together – DNA and RNA separate
What is a transcription factor
transcription factors are accessory protiens that bind to DNA sequences and affect levels of transcription. General transcription factors combine with RNA polymerase to form the basal transcription apparatus. Other transcription factors bind to other DNA sequences and bring about higher levels of transcription by stimulating the assembly of the basal transcription apparatus at the start site.
what is the difference between general transcription factors and other transcription factors
General Transcription Factors:
- necessary for any transcription to occue
- combine with RNA polymerase and other proteins to form the basal transcription apparatus
Other Transcription Factors:
- can increase or decrease transcription levels but not required for transcription.
what is a basal transcription apparatus
a group of proteins that assembles near the transcription start site and is sufficient to initiate minimal levels of transcription
what is the difference between a core promoter and a regulatory promoter within eukaryotes
the core promoter is located immediately upstream of the gene and is the site to which the basal transcription apparatus binds (required for transcription). It usually includes one or more consensus sequences. Must know the TATA box is at -25
The regulatory promoter is located immediately upstream of the core promoter. It affects the rate of transcription, but it is not required for transcription.
what are the steps of eukaryotic transcription (for RNA polymerase II)
Initiation:
- TFIID contains a TATA- binding protein (TBP) which binds to the TATA box within the core promoter
- General TFs + RNA polymerase + mediator bind to core promoter via TFIID
- TBP of TFIID positions active site of RNA polymerase over start site
- other transcription factors:
- may bind to regulatory promoter
- may bind to enhancers
- affect transcription rate by interacting with the basal transcription apparatus via the mediator
Elongation:
- similar to bacterial elongation
Termination:
- there are proteins that assist to remove the RNA polymerase and the RNA transcript from the DNA
what is the difference between an exon and an intron
exons are RNA coding regions, and the noncoding regions are called introns. All the introns and exons are initially transcribed into RNA but during or after transcription the introns are removed and the exons are joined to yield mature RNA.
what organisms are introns common within and what organisms are introns rare within
common in eukaryotic genes but rare in bacterial genes
introns have been observed in archaea, bacteriophages, and some bacteria
present in mitochondrial and chloroplast genes as well as in the nuclear genes of eukaryotes.
Draw and label the structure of a mature mRNA
what are the 3 main steps in pre-mRNA processing
- Addition of the 5-cap
- Addition of the poly(A) tail
- RNA splicing
what type of cells does pre-mRNA processing occur and where inside the cell
it occurs in eukaryotic cells within the nucleus
How is the 5’ cap added (in pre-mRNA processing) and what is its function
It is a guanine nucleotide added backwards (5’ to 5’ bond) to the 5’ end of the mRNA.
then methyl groups are added to the mRNA
Function:
1. assist with binding of ribosome to mRNA for translation
2. stabilizes mRNA
How is the poly(A) tail added and what is its function
- cleavage 11-30 nucleotides downstream of AAUAA consensus near 3’ end
- Polyadenylation: many adenines are added
Function: - increases stability of mRNA
- required for ribosome binding to 5’ cap