4.2 DNA, mRNA, tRNA

genome

complete set of genes present in a cell

proteome

full range of proteins a cell is able to produce - usually larger than the genome

why is the proteome usually larger than the genome

large amount of post translational modification of proteins in the Golgi, each gene also capable of producing multiple different proteins via alternative splicing

mRNA

• made in the nucleus, small enough to pass thru nuclear pores

• single stranded

• carries copy of genetic code for a protein to the ribosomes

• ribose sugar

• uracil base

tRNA

• small - 80 nucleotides

• single stranded polynucleotides that twist/loop

• cloverleaf shape with one chain extending further by 3 bases - where amino acid joins

• trio of bases, anticodons on loop end

hydrogen bonds present in DNA, mRNA, tRNA

present in DNA and tRNA between complementary bases, not present in mRNA as its single stranded

transcription location

nucleus of cell

transcription steps

DNA molecule unzips as H bonds between bases break, exposes bases to be transcribed, free activated RNA nucleotides bind to complementary bases via H bonds on template strand of unzipped DNA molecule, sugar phosphate groups of RNA nucleotides bonded together by RNA polymerase to form sugar phosphate backbone of mRNA, stop codon reached, H bonds between mRNA and DNA break and DNA molecule reforms, mRNA leaves nucleus through pore

splicing

before pre-mRNA exits nucleus, non coding sections are removed and coding sections are joined together, resulting mRNA carries only exons of the gene

alternative splicing

exons of genes can be spliced in many different ways to produce different mRNA molecules - single eukaryotic gene can code for more than one polypeptide chain

translation steps

mRNA attaches to ribosome, tRNA with attached amino acid brings complementary anticodon to codon on mRNA, first tRNA forms H bonds with the start codon on mRNA, second tRNA binds with second codon on mRNA, peptide bond forms between amino acids, ribosome moves along mRNA in 5’ to 3’ direction, tRNA without amino acid detaches from mRNA, ribosome continues to read mRNA molecule until stop codon reached