DNA Function: Information Transmission

Transcription

1 DNA Function: Information Transmission

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

3 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.)

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

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

7 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 proteinmaking factories the RIBOSOMES! the messenger is messenger RNA (or mrna)

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9 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)

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11 Enzymes unzip DNA and RNA nucleotides link to the bases, forming mrna (see fig. 17.9)

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

13 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

14 RNA nucleotides are added only to the 3 end (mrna is synthesized in the 5 3 direction)

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

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17 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

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

19 How does mrna produce a protein?

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

22 Transfer RNA (trna) is the interpreter between the 2 forms of information: the base sequence in mrna and the amino acid sequence in polypeptides

23 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

24 Amino acid 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)

25 trna s decode the genetic message codon by codon

26 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

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

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33 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).

34 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

35 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

36 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

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38 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.

39 INTRONS AND EXONS: POST TRANSCRIPTION MODIFICATION

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