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Peptide Bond Formation
Walk the Dogma
RECALL: The 4 types of organic molecules…
CARBOHYDRATES
LIPIDS
PROTEINS
(amino acid chains)
NUCLEIC ACIDS (DNA and RNA)
The CENTRAL DOGMA of CELL BIOLOGY
Remember: DNA contains the
instructions that tells the cell
how to make proteins
Protein Synthesis has 2 main phases:
• Transcription (DNA → RNA)
• Translation (RNA → Protein)
DNA vs. RNA
DNA
-Deoxyribose Sugar
-Double-stranded
- A binds to T
RNA
- Ribose
Sugar
- Single-stranded
- A binds to U (Uracil)
Ribonucleic Acid (RNA)
There are 3 types
of RNA:
Messenger RNA (mRNA)
• Structure: Linear Single Strand
• Function: Carries genetic info from the
DNA in the nucleus to the cytoplasm
Transfer RNA (tRNA)
• Structure: Hairpin Loop
• Function: Binds and carries specific amino acids
Ribosomal RNA (rRNA)
• Structure: Globular
• Function: Combines with proteins to form ribosomes
Ribonucleic Acid (RNA)
Protein Synthesis: TRANSCRIPTION
Transcription: the process by which genetic
information is copied from DNA to RNA
• DNA double-strand “unzips”
• RNA polymerase (an enzyme) binds to a
specific region on DNA called a promoter
• RNA polymerase travels along the gene,
creating a chain of mRNA that is
complementary to the strand of DNA
• RNA polymerase reaches the termination signal
at the end of the gene & falls off the DNA strand
DEFINITION: GENE
A specific segment
on a strand of DNA,
which encompasses
the space between a
promoter and a
termination signal,
that contains the
instructions to make a
single polypeptide
Post-Transcriptional Modification
• Occurs before translation can begin
• Introns are removed, exons are joined
– Intron: Non-coding sections of a gene
– Exon: Region of a gene that does code for protein
• This process a.k.a. mRNA SPLICING
• Catalyzed by a spliceosome (RNA/protein complex)
Intron = “Intervening”
Exon = “Expressed”
Translation’s Triplet Code
REMEMBER : Through transcription, the cell
has created a strand of mRNA
• The next step is to translate that sequence of (4) nucleotides
(A, U, G, C) into a correlating sequence of (20) amino acids
• The genetic code of the mRNA is “read”
by the ribosome 3 nucleotides at a time
• Each 3 consecutive nucleotides is called a CODON
• Each codon codes for a specific amino acid
Protein Synthesis: TRANSLATION
• mRNA leaves the nucleus and travels into the cytoplasm
• A ribosome finds and binds to mRNA’s START CODON
• A free-floating, amino-acid-carrying tRNA bearing the
appropriate ANTICODON binds to the start codon
• The amino acid detaches from its tRNA
• The ribosome slides 3 nucleotides down the mRNA strand
• (THE PROCESS NOW REPEATS ITSELF) A free-floating,
amino-acid-carrying tRNA bearing the appropriate anticodon
delivers the next amino acid to the forming polypeptide
• The ribosome reaches the STOP CODON and falls off the
mRNA, releasing the polypeptide (FINAL PRODUCT)