Download ONE GENE – ONE POLYPEPTIDE

ONE GENE – ONE POLYPEPTIDE
Gene: sequence of nucleotides in DNA that performs a specific function, ex.
coding for a protein.
Proteins: phenotypic characteristics, antibodies, hormones, drive cellular
processes (metabolism), their absence or presence in an altered form can
result in genetic disorders.
Each gene in our DNA codes for one particular protein.
Evidence:
Archibald Garrod (British physician, early 20th century):
Studied patients with alkaptonuria, they lack the enzyme to break
down alkapton, which leads to black urine and noticed the illnesses
recurs in some families.
Concluded error in hereditary material results in error of enzyme.
Beadle and Tatum (23 years later) demonstrated the hypothesis.
Using mutant strains of red bread mould and the synthesis of amino
acids they were able to show that the lack of a particular enzyme to
produce an amino acid corresponded to a mutation in a specific
gene.
Evidence continued:
Hemoglobin is made of 2 alpha polypeptides and 2 beta polypeptides
Each chain is controlled by a different gene.
Ingram found that in sickle cell anemia red blood cells, the 6th amino
acid of the beta-chain, normally glutamic acid, is substituted with
valine
Significance: linked a hereditary abnormality to a single alteration in
the amino acid of a protein – ultimately controlled by a single gene
Hemophilia, cystic fibrosis also caused by single gene alterations.
PROTEIN SYNTHESIS
Occurs in the cytoplasm on ribosomes.
But DNA (code) is not able to exit the nucleus (could be damaged).
Messenger RNA (mRNA): copies and transports the code to the ribosome.
Transcription: the copying of the DNA info into mRNA
Transcribe (one form to another)
Translation: ribosomes use the mRNA blueprint to synthesize proteins.
Translate (one language to another)
RIBONUCLEIC ACID - RNA
Ribose sugar (one more O than deoxyribose – an OH at C2)
Uracil base instead of thymine (H on C1 instead of CH3), pairs with adenine
Single stranded
3 major classes:
mRNA: copy of a gene – template for protein building at the ribosome
Transfer RNA (tRNA): transfers amino acids to the ribosome to build a
protein
Ribosomal (rRNA): structural component of the ribosome along with
proteins
next...Transcription
Extra slides follow...
GENETIC CODE
There are 20 amino acids found in proteins, only 4 bases in mRNA (U C A
G)
Codons: sequences of three bases used to code for an a.a.
43=64 possible codons (some amino acids have more than one codon)
Ex. UUU UUC, UCU, UCC all code for phenylalanine (a.a.)
This redundancy helps to reduce errors
AUG: start codon (Methionine) used 99% of the time
UAA, UAG, UGA: stop codons
The mRNA transcript is read in sets of 3 nucleotides (one codon) to
determine which a.a. is next.
TRANSCRIPTION OVERVIEW
3 processes: initiation, elongation and termination.
Initiation: RNA polymerase binds to DNA at a promoter site near the
beginning of the gene.
Elongation: RNA polymerase uses the appropriate ribonucleotides to
build the mRNA transcript.
Termination: RNA polymerase passes the end of the gene, recognizes a
stop signal.
mRNA transcript is then completely released from the DNA.
TRANSLATION OVERVIEW
Also initiation, elongation and termination
Initiation: Ribosome binds to a specific sequence on the mRNA
Elongation: the ribosome moves along the mRNA and tRNA delivers
the appropriate amino acid and the polypeptide is elongated.
Termination: the ribosome reaches a stop signal, the ribosome falls of
the mRNA and the polypeptide chain is released.