U1: Central Dogma

Transcription & Translation

Central Dogma

DNA is transcribed into mRNA and gets translated into proteins

RNA polymerase

separates strands of DNA to build new RNA molecules in 5’—> 3’ end. Complementary to DNA strands 

What are the benefits of RNA poly?

Can start it own strand without primers, can open DNA strands and only transcribed one strand at a time so no leading/lagging strand.

Coding strand

Non transcribed strand of DNA, template strand is the transcribed strand.

Starting point in transcription?

Promotor region is where is starts and its needs to identify a location and dire action of transcription

TBP

TATA box binding protein.

In Eukaryotic cells how does RNA polymerase begin coding?

Promoters attract general transcription factors bind onto promoter and recruit RNA polymerase like TBP

Why do Eucaryotic Cells Modify RNA after Transcription?

Transcriptions happens in the nucleus and in order to move it out the cell into the cytoplasm for translation to occur 5’ cap and 3’ tail are essential to export from nucleus.

End modifications

5’ Methyl-G cap and 3’ poly-A tail. Protect from degradation, facilitate translation (ribosome recruitment) 

Exons

protein coding (expressed; exit nucleus)

Introns

non-protein coding

RNA splicing

removes introns & connect exons to make mature mRNA for translation

Spliceosome

Protein-RNA complex that carries out RNA splicing

snRNPs “snurps”

Small nuclear ribonucleoproteins; snRNA (small nuclear RNA) targets complex based don intron sequences

What modification occur when entering the Cytoplasm?

Once the protein sequence leaves the nucleus the 5’ cap and 3’ tail get exchanged into proteins that are initiation factors used for beginning of translation.

Translation direction

5’ to 3’ sequences of mRNA is translated from N-terminus (5’) to C-terminus (3’) direction as proteins

Codon

3 RNA nucleotides corresponds to one amino acid and never overlap.

Redundant

multiple codons per amino acid

Unambiguous

only one amino acid per codon and evolutionarily conserved

tRNA

transfer RNA a non coding RNA that carries a amino acid and has 3 anticodon base pair that attaches to the mRNA codon

Amino-acyl tRNA synthetase

specifically recognizes amino acid tRNA, attaches appropriate amino acid to tRNA

Site of translation

Ribosome

Ribosome

3 tRNA binding sites; A site (amino acid) , P site (polypeptide) and E site (exit)

Initiation

translation begins at the AUG codon (Met)

Shine-Delgarno Sequence 

mRNA ribosome binding site base pairs with rRNA from small subunit lining up correct AUG start 

Which sequence helps bacteria find AUG codon?

Shine-Delgarno Sequence 

Which sequence helps eukaryotes find AUG codon?

kozak sequence

Kozak Sequence

small ribosomal subunit binds to 5’cap of mRNA, this sequence helps ribosome find AUG 

Reading Frame 

Where in the sequence does the frame start translating (beginning, middle, etc..) start codon determines reading frame

Elongation

tRNA with correct anticodon will bind in A site —> connecting amino acid chain to A site —> p site tRNA moves to e site —> A site moves to P site 

Termination

stop codon recruits release factor (protein), translation complex disassembled 

Polyribosomes

multiple ribosomes simultaneously translate the same mRNA (polysomes)

Prokaryotic Translation

Dont have introns and the cells don’t have nucleus, so transcription & translation happens at the same time

Open reading frame

Contain a start codon and no premature stop codon; this help predict gene locations especially in prokaryotes 

Substitution mutation

one nucleotide is replaced by another 

Missense

Wrong amino acid placement

Silent

Same amino acid after nucleotide replacement

Nonsense

Stop codon created (shortens the most)

Deletion

one or more nucleotides are removed from the gene

Insertion

one or more nucleotides are added to a gene

What mutations causes a frameshift?

Deletion and insertion