Lecture 4b Flashcards
Overall process of DNA–RNA…
Nucleus has 46 chromosomes
Each chromosome has hundreds of genes
Each gene has instructions on how to make a different RNA molecule, but the instructions are coded as DNA & you can’t take DNA out of the nucleus
Decode the instructions, write them as RNA (mRNA, rRNA, tRNA). RNAs leaves the nucleus
Bring all the RNAs to a ribosome which will use them to make the protein.
Gene expression
Process by which information stored in ageneis used to make a geneproduct: mRNA, tRNA, rRNA, protein
2 steps in gene expression
Transcription
Translation
Transcribing DNA
Enzymes add RNA nucleotides to make an RNA molecule that is complementary to the DNA template strand (information from ageneis used to make RNA: mRNA, rRNA, tRNA)
Template DNA strand: strand of the DNA molecule whose info is used to make the RNA
Transcription
information in DNA (genes) is used to make RNA molecules (mRNA, rRNA & tRNA).
DNA does NOT get turned into RNA
enzymes “read” the nucleotide sequence in DNA to make a complementary RNA stand.
Transcription make RNA (mRNA, rRNA, tRNA)
Translation
Information in mRNA used to make a protein. mRNA does NOT get turned into a protein.
Info in mRNA used to make a polypeptide
-Sequence of amino acids in the polypeptide chain is determined by the
mRNA sequence
mRNA & tRNA meet rRNA at the ribosomes
Amino acids bonded together to make a protein
Ingredients for translation
mRNA tRNA rRNA Amino acids Release factor Ribosome Energy
mRNA
polynucleotide that determines the aa sequence of a polypeptide
Codons
sequence of 3 mRNA nucleotides that corresponds to a specific amino acid or a stop signal
tRNA
polynucleotide containing an anti-codon region that binds to codons AND another region that binds to amino acids
Ribosomes
structures that contain binding sites for mRNA & tRNA and
contain rRNA that forms peptide bonds b/w amino acids
Genetic code, codons
Set of codons, each of which corresponds to a specific amino acid or a stop signal
64 codons
3 stop codons: end translation, no aa added
61 codons for aa, 20 are distinct aa
includes 1 start codon: AUG which codes for the aa methionine: the 1st aa (at least initially) in all polypeptides
mRNA determines ….
mRNA codon determines amino acid sequence
Order of codons → sequence of amino acids in the protein
1 codon codes for 1 aa
Codon
group of 3 mRNA nucleotides
tRNA sites
Anticodon region + aa binding site
Anticodon: pairs with complementary codon in mRNA
where do the Amino acids, mRNA, tRNA meet up at ^
Amino acids, mRNA, tRNA meet up at the RIBOSOMES (contains rRNA)
Complex made up of 2 different subunits (each has rRNA + proteins)
Binding sites for mRNA & tRNA
rRNA joins the aa (amino acid) together (peptide bonds).
tRNA brings the aa to the ribosome: binds in complementary fashion to the mRNA
Translation steps
- mRNA binds to ribosome
- Anti-codon region of 1st tRNA binds to codon of mRNA (“P site” of ribosome)
- 2nd tRNA binds at the “A site” to the 2nd codon of mRNA
- rRNA of ribosome forms peptide bond b/w aa 1 + 2
- 1st tRNA (had Met) leaves without its amino acid
Terminating translation
Occurs when the ribosome reaches a stop codon in the mRNA Release factor (protein, not a tRNA) binds to the stop codon (same protein recognizes all 3 stop codons) Binding promotes the breaking of the bond b/w tRNA & the completed polypeptide
function of polypeptides
Polypeptides: peptide bonds join AA
Where do the aa come from ?
Your cells can make some aa: these are called nonessential aa (since you don’t have to get them from your diet)
9 Essential aa: those that you can’t make at all or make enough of to meet your needs.
Have to get from diet
*** Essential aa & Nonessential aa are
EQUALLY IMPORTANT FOR THE CELLS
