Lecture 1: Transcription Synthesis of RNA in Eukaryotes

RNA

Polymer of nucleotides

• Nitrogenous base (A, G, C, U)

• Pentose sugar (ribose)

• Phosphate group

RNA is single stranded

Genes specify proteins via transcription/translation

Genes provide instructions for making specific proteins

RNA is bridge between genes and protein sythesis

Proteins link between genotype and phenotype

Gene expression includes 2 stages

• Transcription and translation

Transcription

Synthesis of RNA using info in RNA

Produces messenger RNA (mRNA)

Occurs in nucleus

Primary transcript (pre-RNA) is the initial RNA transcript from any gene and is modified (RNA processing) to yield the mature mRNA

Basic principles of transcription

Template strand (antisense strand)

• The DNA strand that produces a template for ordering the sequence of an RNA transcript

• The mRNA molecule produced is complementary to template strand (base-pairing rules)

RNA molecules synthesized in antiparrell direction 5’ tp 3’

Coding strand (sense strand)

• Nontemplate DNA

• Complementary to template strand

• Identical to mRNA

• T’s instead of U’s

Template strand

Determined by specific DNA sequences associated with gene

Always same strand for a given gene

Along same chromosome, the opposite strand may be template strand for a different gene

RNA Polymerase

Catalyzes RNA synthesis

• Separates 2 DNA strands; joins RNA nucleotides

• Not need any primer

• Assemble in 5’ to 3’

• RNA poly 2 synthesized pre-mRNA

Initiation and RNA polymerase binding (promotor)

Promotor

• DNA sequence where RNA poly attaches and initiates transcription (upstream)

• Several dozens nucleotide pairs upstream from starting

Initiation and RNA polymerase binding- Transcription factors

Transcription factors

• Protein help guide binding of RNA poly and initiation of transcription

• Only after transcription factors are attached to promotor that RNA poly 2 bind to it

Initiation and RNA polymerase binding- Transcription initiation complex

TIC

• Whole complex of transcription factors and RNA poly 2 bound to a promotor

Elongation of RNA strand

RNA poly untwists double helix 10-20 nucleotides at a time

Nucleotides added to 3’ end

Behind poly, RNA molecule peels away, RNA double helix reforms

A gene can be transcribed simultaneously by several RNA poly

Increase amount of mRNA

Cell can make encoded protein in large amounts

Termination of transcription

RNA poly 2 transcribes a sequence on DNA with termination signal in pre-mRNA

10-35 nucleotides downstream from the AAuAAA proteins cut the RNA transcript from the poly

pre-mRNA then undergoes processing

RNA processing

pre-mRNA is modified by enzymes

Both ends of primary transcript alters

Interior sections are cut out

Produce mRNA molecules ready for translation

Alteration of mRNA ends and their functions

Facilitate the export or mRNA to the cytoplasm

Protect mRNA from degradation by enzymes

Help ribosomes attach to 3’ end of mRNA once it reaches the cytoplasm

Rice splicing- removal of introns

Most genes and their RNA transcripts have noncoding segments of nucleotides between coding regions

Noncoding segments: Intervening sequences of Introns

Coding segments: Exons are expressed (translated into amino acids sequences)

RNA splicing- removal of introns

Splicosomes

• Variety of proteins and several small RNA recognize the splice sites

• Binds to short nucleotide sequences along an intro (key sequences)

Functional and evolutionary importance of introns

Some introns can regulate gene expression

Genes can encode more than 1 kind of poly, depending on which segments are treated as exons during splicing

• Alternative RNA splicing

Number of different proteins an organism can produce is much greater than its # of genes