How are Genes Expressed?
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Genes are instructions for assembling proteins.
How do proteins result in phenotype?
Many proteins are enzymes that catalyze and
regulate chemical reactions.
 Some genes may do things like code for enzyme that
produces pigment in a flower.
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Proteins are key to almost everything a cell
does!
RNA
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“blueprints”/Recipe
Card
Disposable
Used on the job
DNA
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“master
plan”/Cookbook
Contain all of the
information/recipes
Kept safely in an office/
kitchen (nucleus)
RNA
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Made of nucleotides
Sugar = ribose
Single strand
Uracil (instead of
Thymine)
DNA
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Made of nucleotides
Sugar = deoxyribose
Double strand
Thymine
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Most are involved in protein synthesis.
3 Types:
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1. messenger RNA (mRNA)
 Carry copies of the protein-assembling instructions
from DNA to ribosomes
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2. ribosomal RNA (rRNA)
 One component of ribosomes, which direct the
assembly of amino acids into proteins
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3. transfer RNA (tRNA)
 Bring amino acids to the ribosome according to the
mRNA’s coded message
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Definition: the copying of a nucleotide
sequence of DNA into a complementary
sequence in RNA
RNA polymerase binds to DNA at “promoters”
(specific base sequences that tell the enzyme
where to start making RNA).
It then separates the DNA strands and uses one
strand as a template from which nucleotides
are assembled into a strand of RNA.
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Before it is functional, a newly made RNA
strand is modified.
Pieces called “introns” (intervening sequences)
are cut out, while…
Pieces called “exons” (expressed sequences) are
spliced back together.
A cap and a tail are added to complete the
process.
Why? Perhaps to enable formation of multiple
RNA molecules from the same gene.
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Proteins are made of linked amino acids…
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“Polypeptides”
20 different amino acids
Sequence of amino acids determines properties of
the protein (hydrophobic, polar, charged)
The language of the mRNA instructions is the
“genetic code.”
Each word is 3 nucleotides long – a “codon” –
and specifies a certain amino acid.
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Ribosomes are the cellular factories that read
and follow the instructions for creating a
polypeptide.
Translation: decoding information from
messenger RNA to produce proteins.
The mRNA molecule made during
transcription attaches to a ribosome.
Transfer RNAs (tRNA) bring amino acids to
build the polypeptide (protein).
Anticodons of tRNA complement each codon.
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The ribosome connects the amino acids to each
other with peptide bonds.
Once amino acids are delivered, the tRNAs are
released.
The protein continues to grow until a stop
codon is reached.
The newly formed polypeptide is released.