DNA Function: Information Transmission

● DNA is called the “code of life.” What does it code for? *the information (“code”) to make proteins!

Why are proteins so important? ● Nearly every function of a living thing is carried out by proteins… -DNA replication -structural proteins (skin, muscles, etc.) -transport proteins (hemoglobin, etc.) -fight infection (antibodies) -enzymes (digest food, copy DNA, etc.)

How does the information in DNA get converted to PROTEINS? **Remember…proteins are long chains of amino acids (there are 20 different amino acids) CRUCIAL **the order / sequence of amino acids is CRUCIAL …DNA determines the order of amino acids

Protein Synthesis / Gene Expression consists of 2 steps: 1) TRANSCRIPTION 2) TRANSLATION

TRANSCRIPTION: ● DNA has the information (“blueprints”) to make proteins, BUT…it can’t leave the nucleus (too big!)… ● So, it needs a MESSENGER to carry the blueprints from the nucleus to the protein- making factories…the RIBOSOMES! ● the “messenger” is “messenger RNA” (or mRNA)

● RNA (ribonucleic acid) is different from DNA in 3 ways: 1) RNA is single stranded 2) the sugar in RNA is RIBOSE (instead of deoxyribose) 3) Instead of thymine (T), RNA contains URACIL (U) (base pairing rules still apply: C with G; A with U)

● Enzymes “unzip” DNA and RNA nucleotides link to the bases, forming mRNA (see fig. 17.9)

● for each gene, only 1 of the 2 strands (the template strand) is transcribed Template strand

● transcription of mRNA from template DNA is catalyzed by RNA polymerases which separate the 2 DNA strands and link RNA nucleotides as they base-pair along the DNA template

● RNA nucleotides are added only to the 3’ end (mRNA is synthesized in the 5’  3’ direction)

● RNA polymerases bind to DNA at regions called PROMOTERS. This binding site is where transcription begins (initiation site)

Transcription (continued) ● During transcription, mRNA grows about nucleotides per second *as the mRNA strand elongates, it peels away from the DNA template; *2 strands of DNA double helix are reunited

● Transcription proceeds until RNA polymerase reaches a termination site on the DNA

How does mRNA produce a protein?

TRANSLATION: ● During translation, proteins are synthesized according to the genetic message of sequential codons along the mRNA

● Transfer RNA (tRNA) is the interpreter between the 2 forms of information: the base sequence in mRNA and the amino acid sequence in polypeptides

● tRNA aligns the appropriate amino acid to form a new polypeptide by transferring the amino acid from the cytoplasm to a ribosome and recognizing the correct codons in mRNA Amino acid location 3 exposed bases

● Molecules of tRNA are specific for only 1 amino acid -one end of tRNA attaches to a specific amino acid -the other end attaches to mRNA codon by base pairing (anticodon = a sequence of 3 bases on tRNA) Amino acid

● tRNA’s decode the genetic message codon by codon

● as tRNA’s deposit amino acids in the correct order, ribosomal enzymes (peptidyl transferases) catalyze formation of peptide bonds between the polypeptide and each new amino acid

**figure 17.5 shows the “dictionary” for the codons and their corresponding amino acids

Protein Synthesis in Prokaryotes vs. Eukaryotes ● In bacteria, transcription and translation happen in the same location and often happen simultaneously! ● In eukaryotic cells, the nuclear envelope separates transcription from translation…this provides time for RNA processing (this is an extra step between transcription and translation that does not occur in prokaryotes).

Post-transcription modification of mRNA in eukaryotes: 1) Alteration of mRNA ends: *the 5’ end is “capped” with a modified form of guanine (G) -protects the mRNA from hydrolytic enzymes; -serves as an “attach here” signal for small ribosomal subunits

Post-transcription modification of mRNA in eukaryotes: *at the 3’ end, an enzyme adds a poly-A tail ( adenine nucleotides) -inhibits degradation of the mRNA -may facilitate the export of mRNA from the nucleus to the cytoplasm

Post-transcription modification of mRNA in eukaryotes: 2) RNA Splicing *INTRONS = noncoding segments of DNA; intervening sequences (between the coding segments) *EXONS = coding regions of DNA; exons are eventually expressed

● Both introns and exons are transcribed to form an oversized mRNA molecule; ● Enzymes excise (cut out) the introns and join the exons to form a continuous coding sequence.

INTRONS AND EXONS: POST TRANSCRIPTION MODIFICATION