Lecture #6 Flashcards
The Central Dogma
DNA is transcribed to make RNA
-Re-write the genetic information
RNA is translated to make protein
- Going from one language to another.
The Central Dogma: Gene Expression
Gene expression:
Information in a DNA sequence is translated into a product that has an effect on the cell/organism
Regulated by the cell
- Time (cell cycle)
- Amount (cell type)
Each gene is individually regulated
Transcription: An Overview
The number of RNA copies made from each gene varies
- few copies vs. hundreds of copies
The number of protein molecules made from each RNA strand varies
- A few copies vs. hundreds of copies
The longer an RNA strand remains functional, the more protein molecules are generated
- Half life
Similarities and Differences between DNA and RNA
Similar to DNA
- Chemical make-up
Polymer of nucleotides held together by phosphodiester bonds.
- 5`→3` polarity
- Bases added in a 5`→3` direction
- Similar base-pairing
- Uracil used instead of thymine
Different than DNA
- RNA uses a ribose sugar
- RNA uses the uracil instead of thymine
- The methyl group makes the base more stable.
- RNA is single stranded
The Structure and Function of RNA
Characteristics
- More flexible than DNA double helix
- Acts similarly to a polypeptide.
- Uses convention and non-conventional base-pairing methods
- Can form many different shapes
2d and 3d structures
Functions
- Transfers information
- Provides structural support
- Catalyzes reactions
Transcription vs. Replication Similarities
Similarities to DNA replication
- DNA opened unwound
- One strand acts as a template for synthesis of an RNA strand
- Ribonucleotides added in a 5→3 direction
Base pair complimentary rule
RNA chain is produced
Transcription vs. Replication Differences
Differences between replication and transcription
- RNA strand dissociates from the DNA strand after base pair addition
Another strand may be started before the first one is complete.
- RNA strand much shorter than DNA
- Different polymerases
RNA Polymerase vs. DNA Polymerase
Similarities - Catalyzes formation of the phosphodiester bond - 5' --> 3' direction Differences - RNAP carries out transcription - RNAP adds ribonucleotides - RNAP acts as its own helicase - RNAP does not require a primer - RNAP is less accurate
Products of Transcription
Messenger RNA (mRNA) – proteins
Non-messenger RNAs rRNA: ribosomal RNA (translation) tRNA: transfer RNA (translation) snRNA: small nuclear RNA (splicing) miRNA: microRNA (gene expression) siRNA: small interfering RNA (gene expression) Others…
Basic Structural Elements of Transcription
Promoter: DNA sequence - Required
- Signals the starting point for RNA synthesis
- Asymmetrical sequence – RNAP binds in one orientation
Top or Bottom strand can act as template.
- contains transcription start site (TSS) template.
TATA Box – can be within the promoter
5’-TATAAA-3‘
~ 25 bp upstream of the Transcription start site
- Binding site for TBP (TATA binding protein)
Transcription Terminator site
Bacterial Transcription
RNAP associates with DNA
- Slides along DNA
Recognizes promoter & binds more tightly
- Sigma factor recognizes promoter.
First 10 nucleotides are joined
RNAP exits promoter
- Lose the sigma factor.
RNA strand is elongated
RNAP reaches terminator signal and dissociates
Bacterial Transcription & Translation are Coupled
Bacterial DNA is not contained in a nucleus
Ribosomes bind newly transcribed mRNA and begin translation
Eukaryotic Transcription
Very Complex
- Genes are closer together in bacteria
- Bacteria DNA is not packaged into nucleosomes.
Three types of RNAP
- I and III responsible for tRNAs, rRNAs, and other small RNAs
Eukaryotic Transcription Continued
Requires general transcription factors during initiation
- Transcription Initiation Complex forms on promoters
Positions RNAP II
TFIID interacts with the promoter
- Binds TATA box through its TBP domain
- Distorts the DNA
- Recruits other complex subunits
Transcription in Eukaryotes
Other Transcription Factors interact with DNA and RNAP II
- TFIIF, TFIIE, TFIIH
TFIIH phosphorylates the RNAP II tail
- TFIIH has a kinase domain
- RNAP II dissociates from Transcription Initiation complex
RNAP II uses ribonucleotides to start and elongate the mRNA strand
RNAP II reaches terminator signal and dissociates
