DNA Function: Information Transmission



NOTES: CH 17, part 1 - 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…

-

- (skin, muscles, etc.)

- (hemoglobin, etc.)

- (antibodies)

- (digest food, copy DNA, etc.)

How does the information in DNA get converted to PROTEINS?

**Remember…proteins are (there are 20 different amino acids)

**the order / sequence of amino acids is CRUCIAL….DNA determines the order of amino acids

Protein Synthesis / Gene Expression consists of 2 steps:

1)

2)

TRANSCRIPTION:

● DNA has the information (“blueprints”) to make proteins, BUT…it can’t leave the nucleus (too big!)

● So, it needs a to carry the blueprints from the nucleus to the protein-making factories…the !

● the “messenger” is “ ” (or mRNA)

● RNA (ribonucleic acid) is different from DNA in 3 ways:

1)  

2)  (instead of deoxyribose)

3) Instead of thymine (T),

(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 ) is transcribed

( transcription of mRNA from template DNA is catalyzed by which and link RNA nucleotides as they base-pair along the DNA template

( RNA nucleotides are added only to the 3’ end ( )

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

( During transcription, mRNA grows about 30-60 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 on the DNA

How does mRNA produce a protein?

TRANSLATION:

( During translation, proteins are synthesized according to the genetic message of sequential along the mRNA

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

( 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

( Molecules of tRNA are specific for only 1 amino acid

-one end of tRNA attaches to -the other end attaches to mRNA codon by base pairing

(anticodon = )

( tRNA’s decode the genetic message

( as tRNA’s deposit amino acids in the correct order, ribosomal enzymes

(peptidyl transferases) catalyze formation of peptide bond between polypeptide

and the 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 (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 ( )

- from hydrolytic enzymes;

-serves as an “ ” for small ribosomal subunits

*at the 3’ end, an enzyme (30-200 adenine nucleotides)

-

-may facilitate the export of mRNA from the nucleus to the cytoplasm

2) RNA Splicing

*INTRONS = ; intervening

sequences (between the coding segments)

*EXONS = ;

exons are eventually expressed

( Both introns and exons are transcribed to form an oversized

mRNA molecule;

( Enzymes excise the introns and join the exons to form a .

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