Download DNA Function: Information Transmission

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)
**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 proteinmaking 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 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 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
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)
● 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 (30-200 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