RNA synthesis Flashcards
how many nucleotides does a human have?
3.2x10^9
What is a genome?
the is the entire set of DNA instructions found in a cell. only 1-2% of the human genome is DNA encoding for a gene and the remaining 98% is intergenic region.
what is a gene
unit of heredity: it contains instructions needed for an organism’s phenotype
it is a DNA segment that contains instructions for making a particular product including regulatory elements
a gene is NOT just the bits that code for a protein
a gene contains a 5’ UTR(untranslated region), 3’ UTR, an intron, and an Extron.
what is a central dogma
the central dogma was first proposed by Francis crick in 1958. it suffices to describe a generalized flow of information in the cell involving the processes of transcription of a genetic code into RNA and the processing of this by the cell via translation of the RNA into a product
describe the process of RNA synthesis
DNA is transcribed into RNA by RNA polymerase
RNA polymerase attaches to the beginning of the gene to be copied . it starts to unwind the DNA of the gene and DNA helicase breaks the hydrogen bonds between the two strands, this causes two strands to be exposed and only one strand is copied. mRNA is made by free nucleotides found in the cytoplasm. as the RNA polymerase moves along the gene, hydrogen bond is formed between the free bases and the template strand following the base pairing rule A-U; C-G(uridine is used instead of thymine since an RNA strand is being formed) . as each nucleotide arrives, it joins it to the growing mRNA strand using phosphodiester bonds.
eventually a stop message is reached and RNA polymerase then releases the completed mRNA. the mRNA then leaves the nucleus via the nuclear pores. the mRNA is a single polynucleotide chain. the RNA polymerase moves in a 5’ to 3’ direction.
what are 3 types of RNA polymerase and their genes transcribed?
RNA polymerase I- most ribosomal RNA
RNA polymerase II- protein-coding, micro RNA (mi RNA), non-coding RNA
RNA polymerase III- transfer RNA(tRNA), 5S rRNA, other small RNAs
what are some features of RNA synthesis
there are dozens of polymerases working on the same gene
many transcripts from the a gene simultaneously
it is relatively quick: 1.25-1.75kb per min
what are transcription factors
these are proteins required to initiate or regulate transcription in eukaryotes. they usually assemble on the promoter region of the gene to position RNA Pol II. example of a promoter region is the TATA box
how do transcription factors work?
TATA box is recognized by the TATA-binding protein(TBP) sub-unit of TFIID.
TFIIA and TFIIB then bind; TFIIA stabilizes the complex.
other transcription factors(E,F,&H) also bind and RNA polymerase II assembles at the promoter region and forms the transcription pre-initiation complex(PIC).
TFIIH pulls apart the DNA helix and phosphorylates the RNA polymerase II.
the phosphorylated RNA polymerase II is released from the complex and begins transcription.
What are UTRs
Untranslated Regions are regions on a gene that are transcribed but not translated. there is the:
5’ UTR which is involved in the regulation of translation.
3’ UTR which is involved in mRNA stability and mRNA building.
what are the steps involved in RNA processing?
capping
polyadenylation
splicing
what is capping
capping results in the addition of a 7 methyl guanosine to the 5’ end of the RNA via a triphosphate bridge at the same time as the pre-mRNA is being synthesized. this 5’ cap prevents the nascent mRNA from being degraded and it helps factors involved in protein synthesis recognise
what is capping
capping results in the addition of a 7 methyl guanosine to the 5’ end of the RNA via a triphosphate bridge at the same time as the pre-mRNA is being synthesized(co-transcriptional modification). this 5’ cap prevents the nascent mRNA from being degraded. factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.
what is polyadenylation?
once the elongation is completed, the pre-mRNA is cleaved by an endonuclease between an AAUAAA consensus sequence and a GU-rich sequence leaving the AAUAAA sequence on the pre-mRNA. an enzyme called poly-A polymerase adds a string of approximately 200 Adenine nucleotides called a poly-A tail. this modification further protects the pre-mRNA from degradation and signals the export of the cellular factors that the transcript needs to the cytoplasm.
what is pre-mRNA splicing
Pre-mRNA splicing involves the precise removal of introns from the primary RNA transcript and the joining of exons. The splicing process is catalyzed by protein complexes called spliceosomes which are composed of proteins and RNA molecules called snRNAs. Spliceosomes recognize sequences at the 5′ and 3′ ends of the intron.
what is an intron
an intron is a segment of a DNA or RNAwhich does not code for a product and interrupts the sequence of genes. it is usually removed by splicing and does not remain in a mature mRNA.
Size: <100 - >1,000,000 nucleotides
Median: 1747 nucleotides (exon: 121 nucleotides)
what is an exon
this is a sequence of DNA present in a mature mRNA, some of which codes for a biological product. it is not usually removed by splicing
What is alternative splicing
Alternative splicing of pre‑mRNA means that a single gene may encode more than one protein product (e.g., sex determination in Drosophila melanogaster).
in alternative splicing, some of the intron or Extron may be included or excluded during splicing.
why is capping and polyadenylation important?
- Stability
- Transport to the cytoplasm
- Integrity prior to translation
what is genotype?
The genotype is determined by the genetic composition or individual gene content. We usually refer to an individual or a cell as having a particular genotype meaning that they have a particular allele or combination of alleles.
what is referred to as phenotype/
The phenotype differs from the genotype in that it is the characteristic that is determined by the expression of a gene or the coordinated expression of several genes. The phenotype is thus a consequence of the production of specific gene products or proteins within a cell type, tissue or organism. For example mutations in a particular gene in Mycobacterium tuberculosis known a gyrA can lead to a drug resistant phenotype ie the organism may have a particular genotype conferred by the presence of a specific sequence in gyrA that leads to this phenotype. Red blood cells express the members of the globin gene family leading to the production of haemoglobin and provide their phenotypic oxygen carrier capability.
