Transcription and RNA Processing

what are proteins?

• polypeptides that are made up of amino acids

  • amino acids are linked by peptide bonds

what is gene expression?

• the process that DNA directs the synthesis (creation) of proteins

• this includes transcription and translation

  • happens in all organisms

what is transcription and translation?

• transcription

  • making RNA by copying information from DNA

  • allows for the “message” of DNA to be transcribed

  • happens in the nucleus

• translation

  • making polypeptide using information from RNA

  • happens in the ribosome

  • nucleotide sequence becomes an amino acid sequence

what are the 3 types of RNA used in transcription and translation?

• messenger RNA (mRNA)

• ribosomal RNA (rRNA)

• transfer RNA (tRNA)

messager RNA

• mRNA is made during transcription by copying DNA

• It carries the instructions from the DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are made.

transfer RNA

• imporant for translation

• each rRNA carries a specific amino acid

• that can attach to the mRNA via their anticodon

  • complementary codon

  • mRNA

• this allows for information to be translated into a peptide sequence

ribosomal RNA

• rRNA forms ribosomes

• linked amino acids together

genetic code

• DNA contains the sequence of nucleotides that code for proteins

  • the sequence is read in groups of 2 called triplet code

• during transcription, only one DNA strand is being transcribed

  • this is the template strand

  • also knowen has noncoding strand, minus strand, or antisense strand

genetic code pt.2

• mRNA is the opposite (antiparallel and complementary) of the DNA strand.

• A pairs with U, and C pairs with G

• mRNA is read in groups of three bases called codons

• Each codon tells the cell which amino acid to add

genetic code pt.3

• there are 64 different codon combinations

  • 61 code for amino acids

  • 3 are STOP codons

  • same for all living things

• redundancy: more than one codon can code for the same amino acid

genetic code pt.4

• reading frame

  • how codon are grouped

  • where the ribosome starts reading the mRNA in sets of three

  • codon is one of those three-letter groups, but the reading frame is the overall structure that keeps everything in the right order

  • the codons must be in the correct spot during translation to make the correct proteins

what are the steps of transcription?

• initiation

• elongation

• termination

initiation

• beings with RNA polymerase attaches to a promoter region of the DNA

  • does not need a primer to attach

  • promoter regions are upstream (before) the desired gene to transcribe

• eukaryotes

  • promoter region is called the TATA box

  • transcription factors help RNA polymerase bind

• prokaryotes

  • RNA polymerase can bind DIRECTLY to promoter

elongation

• RNA polymerase opens the DNA and reads the triplet code of the template strand

• moves in the 3’ to 5’ direction

  • so the mRNA transcript elongates 5’ to 3’

elongation pt.2

• RNA polymerase moves downstream

  • opens small sections of DNA at a time

  • pairs complementary RNA nucleotides

  • the growing mRNA strand peels away from the DNA template strand

  • then DNA double helix comes back

• many RNA polymerases can copy the same gene at once, making multiple mRNAs.

termination

• prokaryotes

  • transcription proceeds through a termination sequence

  • causes a termination signal

    • RNA polymerase detaches

    • mRNA transcript is released and proceeds to translation

      • mRNA does NOT need modifications

• eukaryotes

  • RNA polymerase transcribes a sequence of DNA at the end called the polyadenylation signal sequence

  • polyadenylation signal is a “wrap it up” signal for transcription in eukaryotes — it helps end transcription and prepare the pre-mRNA for processing.

    • pre-mRNA MUST undergo modifications before translation

pre-mRNA modifications

• there are 3 modifications that happen to eukaryotes pre-mRNA before translation

• 5’ cap

• Poly-A tail

• RNA splicing

what is 5’ cap (GTP) and Poly-A tail?

• 5’ cap (GTP)

  • the 5’ end of the pre-mRNA receives a modified guanine nucleotide “cap”

• poly-A tail

  • the 3’ end of the pre-mRNA receives 50-250 adenine nucleotides

• these help the mature mRNA leave the nucleus, help protect mRNA from degradation, and help ribosomes attach to the 5’ end of the mRNA when it reaches cytoplasm

what is RNA spilcing?

• sections of the pre-mRNA are called introns and are removed so that exons are joined together (first image)

  • introns - do not code for amino acids

  • exons - expressed sections that code for amino acids

• alternative slicing - means the same gene can be cut and rearranged in different ways → this makes different proteins from the same gene.

what happens when were all done?

• now the pre-mRNA is now mature mRNA and can leave the nucleus and go to cytoplasm for translation at the ribosomes