From DNA to Proteins Flashcards
Gene expression-
steps in which DNA in the genes code for proteins
Archibald Garrod-
studied rare genetic disease called alkaptonuria
alkaptonuria
recessive disease in which affected individuals were unable to break down tyrosine, an amino acid. Instead, homogentisic acid accumulates in urine and
turns it black. Lacked the enzyme which breaks down homogentisic acid–Mutation in one gene is associated with the absence of this specific enzyme
Beadle and Tatum-
studied the fungus Neurospora.
Neurospora Wild Type
individual with normal phenotype, which can grow
normally on simple growth medium/basic nutrients
Mutant strain (produced by radiation)- Neospora
cannot make a substance such as an amino acid, can only grow if that substance is added to growth medium
o Each mutant strain had mutation at only one locus- each gene locus affected only one enzyme
Called this the one-gene, one-enzyme hypothesis
Linus Pauling-
Mutation at single locus alters the structure of one polypeptide chain of hemoglobin, causing sickle cell anemia
o Therefore- we now conform to the one-gene, one-polypeptide chain hypothesis
Ribonucleic acid, or RNA,
link between DNA to protein
Differences between RNA and DNA:
o RNA is single stranded
o Contains the sugar ribose, not deoxyribose
o Base uracil substitutes for thymine
Transcription-
the “copying” of the template strand of DNA and formation of RNA. Forms three types of RNA
three types of RNA
mRNA, tRNA, rRNA
Messenger RNA (mRNA)-
single, uncoiled strand of RNA that carries information for making a protein
Transfer RNA (tRNA)-
single strand of RNA that folds back
on itself to form a specific shape. Each tRNA bonds to
one type of amino acid and carries it to the ribosome
Ribosomal RNA (rRNA)-
globular RNA that is part of the ribosomes of RNA and helps catalyze protein synthesis
Differences between transcription and DNA replication:
Only part of the DNA molecule is
used as a template
o Enzyme
RNA polymerase is used as opposed to DNA polymerase
o Results in one free RNA strand, not a double helix
Transcription happens constantly–DNA replication happens during S of cell cycle
Initiation: (transcription)
first stage of transcription
a. RNA polymerase attaches to promoter- region in
DNA that is not transcribed, contains the sequence
TATA (TATA box)
b. RNA polymerase then unwinds the DNA and initiates transcription
Elongation: (transcription)
second stage of transcription
a. RNA polymerase assembles RNA nucleotides using
1 strand of the DNA as a template
b. Occurs in
5’ to 3’ direction (new strand is made in 5’ to 3’ direction
like in replication)
Termination: (transcription)
third stage of transcription
RNA polymerase reaches a special sequence of
nucleotides that serve as a termination point
In eukaryotes- often contains the DNA sequence AAAAAAA
RNA polymerase releases the DNA template and the new RNA strand
In eukaryotic cells, each new molecule of mRNA is only what? what does this necessitate?
“pre-
mRNA”- must be modified before it can code for proteins
Modification of mRNA–5’ end
A modified GTP (guanosine triphosphate- or a guanine nucleotide with two extra phosphate groups) is added to the 5’ end to form a 5’ cap. Provides stability to new mRNA and a point of attachment for the ribosome.
Modification of mRNA–3’ end
3’ end of mRNA- 150 – 200 adenine nucleotides are added- poly-A
tail (-A-A-A….A-A-3’)
o Provides stability
o Controls movement of mRNA across the nuclear envelope
heterogenous nuclear RNA,
Pre-mRNA
Alternative splicing-
Different exons can be kept in or snipped
out- will change the types of proteins that are produced. Explains how human cells can make hundreds of
thousands of proteins from only about 20k genes
Exons-
sequences that express a code for a polypeptide. exported from nucleus.
Introns-
intervening base sequences that must be removed before translation–stay in the nucleus
Small nuclear ribonucleoproteins, or snRNPs,
delete out the introns and splice the exons
Translation-
information found in mRNA is used to specify the amino acid sequence of
a polypeptide
codons
The mRNA is read as codons- sequences of three consecutive bases of mRNA which specifies one amino acid
codon code
Code is called triplet code because each codon consists of three nucleotides. All of the codons collectively form the genetic code
cracking of genetic code
By 1967, the genetic code was “cracked”- scientists had
identified the amino acids for all 64 possible codons. The genetic code is universal- the same codons code for the same amino acids in all organisms. Since there are 64 possible codons and only 20 amino acids,
more than one codon specify certain amino acids
anticodon
A particular tRNA can recognize a specific codon because it has a sequence of three bases, called the anticodon, that is complementary to the mRNA codon, forming hydrogen bonds with one another.
Wobble effect-
The codon- anticodon interactions are not as strong for the third base in a codon
Stop codons
UAA, UAG, UGA
start codons
AUG
effect of binding of anticodon
The amino acids carried by the tRNA can then be linked
together by peptide bonds in the order specified by the sequence of codons in the mRNA.
Ribosomes
the site of translation
Consist of two subunits, small and large
Each is made of proteins and rRNA
During translation, the mRNA molecule fits between the two subunits
Contain 2 binding sites for tRNA: P, A, (and E)
Translation
mRNA moves out of nucleus to the cytoplasm. Requires the coordinated functioning of protein and RNA components of the ribosomes, mRNA, and amino acids linked to tRNAs.
tRNA action translation
Folds on itself to form three or more loops of unpaired nucleotides
Bottom loop contains an anticodon- 3 bases at bottom of molecule that will complementary base pair with the
codon found on the mRNA
Each kind of tRNA binds to a specific type of amino acid at the three prime top of the molecule
SEE DIAGRAM
P site-
peptidyl site-tRNA holding growing polypeptide chain binds here
A site-
aminoacyl site- tRNA delivering the next amino acid in the sequence binds here
E site-
Exit site–tRNA exits here after dropping off its amino acid
Initiation-
SEE DIAGRAM
first stage of translation:
The small ribosomal unit binds to the five prime end of the mRNA
An initiator tRNA binds to an mRNA at the start
sequence AUG. The initiator tRNA carries with it the
amino acid met
The large ribosomal unit binds to the small unit. The
initiator tRNA occupies the P site
initiation complex
ribosome + mRNA + tRNA
Elongation-
SEE DIAGRAM
second stage of translation:
a. A new tRNA bearing an amino acid binds to the A site of the ribosome.
b. Methionine is removed from the first tRNA and
attaches to the amino acid on the newly arrived tRNA
That first tRNA is then released into the cytoplasm
where it can once again bind to its amino acid
Meanwhile, the second tRNA can now move from the
A site into the vacated P site. The A site is now open, exposing the next codon. A new tRNA carrying a new amino acid enters the A site. The two amino acids in the P site bind to this new amino acid, forming a chain of 3 amino acids. This continues over and over, and the chain elongates by one amino acid at a time- dictated by the codons in the mRNA.
Termination-SEE DIAGRAM
Ribosome encounters a “stop” codon. No tRNA molecule binds
Instead, a release factor (special protein) binds to A site
Therefore, when the tRNA in the P site separates from
the amino acid, the newly synthesized polypeptide chain is free
What happens after translation has completed?
amino acids of the polypeptide chain can associate to form secondary and tertiary structures
polysomes-
clusters of many ribosomes translating an mRNA transcript at the same time. found in bacterial cells that rapidly use or secrete proteins.
Mutation-
change in the nucleotide sequence of DNA. not every mutation causes an observable change (some codons code for same amino acids, change in eye color in an skin cell). Mutations provide diversity
Base substitution-
change in only one pair of nucleotides
Missense mutations-
Base substitution. replacement of one amino acid by another
Can alter active site of enzymes
silent mutation-
Base substitution. codes for the same amino acid/no observable change
Nonsense mutations-
Base substitution. convert a code for an amino acid into a stop codon correct protein can’t be produced
Frame shift mutation-
one or two nucleotide pairs are inserted or
deleted from the molecule, altering the reading frame (sometimes more –not multiples of 3) Codons downstream from this mutation specify a new sequence of amino acids resulting in a new polypeptide chain
Transposons, or transposable genetic elements-
DNA sequences that jump from one area into the middle of a gene
Can alter the timing of the gene or block its activity
Discovered by Barbara McClintock- noticed that certain genes in corn would spontaneously turn on or off
Transposons require an enzyme transposase for incorporation into a new location
Most are retrotransposons
retrotransposons
form copies of itself by producing an RNA intermediate, then an enzyme called reverse transcriptase forms DNA from that RNA and the DNA jumps into a gene
Causes of Mutations
Spontaneous or mutagens
Spontaneous mutations
Mistakes in DNA replication • Defects in mitotic or meiotic separation • Hot spots
Hot spots-
areas of DNA that are more likely than others to mutate–generally areas that have
stretches of repeated nucleotides
Mutagens-
anything that causes a mutation
* Radiation
• Chemical mutagens can modify bases, leading to
mistakes in base pairing
- Although mutations in the somatic cells cannot be passed on to offspring,
some mutations can lead to cancer. In addition, many mutagens are
carcinogens-agents that can cause cancer
