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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