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Chapter 14 From DNA to Proteins Honors Biology Program Mountain Pointe High School
What are the byssus of a mussels? Why are these byssus so important?
Archibald Garrod STEPS OF A METABOLIC PATHWAY: First to notice that many heritable diseases were related to metabolic pathway malfunctions. ABC X D Action of enzyme 1 Action of enzyme 2 Something has interfered with the action of enzyme 3. Completion of the pathway is blocked, and C accumulates. Garrod hypothesized that each of his affected patients had inherited a single metabolic defect that interfered with an enzyme in a particular metabolic pathway.
Beadle & Tatum 33 years after Garrod’s hypothesis, these scientists were experimenting with a common bread mold that’s capable of synthesizing everything it needs to survive except for a few basic substances. Neurospora crassa and other fungal species They discovered that some of the fungal strains would only grow when supplied with vitamin B 6, others would only grow in the presence of B 12, etc.
Beadle & Tatum After careful examination, they discovered that there was a different defective enzyme in each mutant strain of the fungus. In other words, each strain of fungus possessed an inherited mutation that corresponded to a defective enzyme. This evidence supported Garrod’s “one gene, one enzyme” hypothesis!
Sickle-cell Anemia The most common lethal genetic disease in African Americans, it causes normal red blood cells to become sickle shaped, which causes an incredible variety of health problems for its victims. Normal red blood cell Sickled red blood cell
Sickle-cell Anemia hemoglobin It was discovered that this disease was caused by a defect in a protein known as hemoglobin that is found in red blood cells. Normal hemoglobin is designated HbA. Abnormal hemoglobin is designated HbS.
Pauling & Itano gel electrophoresis In 1949, these scientists subjected molecules of HbA and HbS to gel electrophoresis. In this procedure, an electric field is used to move molecules through a gel. Molecules are separated by their size, shape & surface charge. upper buffer solution electrode glass tube or plates containing gel gel lower buffer solution power supply electrode movement of proteins
Pauling & Itano HbA HbA molecules carried the greatest surface charge and therefore moved through the gel the fastest. HbS HbS molecules moved much slower. As molecules move through the gel, they’re separated into distinct bands.
Vernon Ingram HbAHbS Pinpointed the biochemical difference between HbA and HbS Hemoglobin (left) is a molecule made of 4 polypeptide chains, 2 alpha & 2 beta. Ingram discovered that the defect was caused by an incorrect amino acid substitution in one of the beta chains!
``` VALINE HISTIDINE LEUCINE PROLINE THREONINE GLUTAMATE A beta chain Hemoglobin molecule Beta chain of an HbA molecule Beta chain of an HbS molecule
The discovery of the difference between the alpha & beta chains of hemoglobin meant that… Two genes must code for hemoglobin, one for each type of polypeptide chain. Genes code for all proteins, not just enzymes. The amino acid sequences of polypeptide chains are encoded in genes.
The Three Types of RNA mRNA protein-building instructions mRNA is a single-stranded molecule that takes DNA’s protein-building instructions out of the nucleus. rRNA ribosomes rRNA is the primary component of ribosomes, the organelles that actually make proteins. tRNA delivering amino acids tRNA is the molecule responsible for delivering amino acids one by one to a ribosome in the correct order specified by the mRNA molecule. mRNA A messenger RNA molecule (mRNA) rRNA A ribosomal RNA molecule (rRNA) tRNA A transfer RNA molecule (tRNA)
Comparing DNA and RNA Double-stranded Deoxyribose sugars 4 nitrogenous bases Adenine Cytosine Guanine Thymine Single-stranded Ribose sugars 4 nitrogenous bases Adenine Cytosine Guanine Uracil Uracil Like thymine, uracil (at right, in blue) is a pyrimidine and is capable of pairing with adenine.
Transcription Transcription template mRNA Transcription is the process of using a portion of the DNA molecule as a template to assemble a molecule of mRNA. selected stretch of one DNA strand Only a selected stretch of one DNA strand is used as a template. promoter gene Transcription is initiated at a promoter, a DNA base sequence that signals the start of a gene. sugar-phosphate backbone of one strand of nucleotides in a DNA double helix sugar-phosphate backbone of the other strand of nucleotides part of the sequence of base pairs in DNA transcribed DNA winds up again DNA to be transcribed unwinds Newly forming RNA transcript The DNA template at the assembly site
Transcription DNA helicase Once the enzyme DNA helicase has unzipped the DNA molecule at the appropriate location… RNA polymerase RNA polymerase adds the required complementary bases to the exposed bases on one of the DNA strands. growing mRNA transcript direction of transcription 3’5’ 3’5’ 3’ pre mRNA A “pre mRNA” strand
Transcription Next, the pre- mRNA molecule must be modified. cap A nucleotide known as a “cap” is attached to the 5` end. poly-A tail A nucleotide known as a “poly-A tail” is attached to the 3` end. unit of transcription in a DNA strand exon intron cap 5’ poly-A tail 3’ transcription into pre-mRNA (snipped out) mature mRNA transcript intronsexons Before the mRNA transcript is finished, useless sections known as introns must be snipped out, leaving only exons remaining. 5’ 3’
Now that the mRNA has been transcribed, what does its message mean? DNA template mRNA transcript Amino acid? The Genetic Code codon
anticodonattachment site for an amino acid tRNA molecules have a nucleotide triplet known as an “anticodon” on one end and an attachment site for an amino acid on the other end. tRNA MOLECULE anticodon amino acid attachment site anticodon amino acid attachment site codon in mRNA amino acid
Ribosomes are composed of two subunits made of rRNA. These subunits are created in the nucleolus, travel separately out of the nucleus and only unite when mRNA messages need to be translated into proteins. platform for chain assembly Small ribosomal subunitLarge ribosomal subunitComplete ribosome tunnel
Translation 3 Stages of Translation: Initiation Initiation tRNA molecule is attached to small ribosomal subunit. START codonAUG anticodon mRNA molecule’s START codon (AUG) matches up with tRNA anticodon, attaches to small ribosomal subunit. Large ribosomal subunit attaches to small subunit.
Binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA)
Translation elongation Next is the elongation stage: Ribosome complex moves along mRNA molecule. amino acidsA site One by one, tRNA molecules deliver the amino acids coded for by the mRNA to A site. peptide bondsP site Amino acids are linked together by peptide bonds, tRNA molecules exit P site of ribosome.
Translation termination The final stage is termination. STOP codon Ribosomal complex reads STOP codon on mRNA molecule. anticodon STOP codon. No tRNA has an anticodon that corresponds to a STOP codon. release factors enzymes Proteins called release factors bind to ribosome, cause enzymes to detach mRNA & polypeptide chain from ribosome. Ribosomal subunits separate. www.johnkyrk.com/DNAtra nslation.html www.johnkyrk.com/DNAtra nslation.html
TRANSCRIPTION TRANSLATION Unwinding of gene regions of a DNA molecule Pre mRNA Transcript Processing mRNArRNAtRNA Mature mRNA transcriptsRibosomal subunitsMature tRNA Synthesis of a polypeptide chain at binding sites for mRNA and tRNA on the surface of an intact ribosome FINAL PROTEIN Cytoplasmic pools of amino acids, tRNAs, and ribosomal subunits Destined for use in cell or for transport
Mutations mutations Changes in the nucleotide sequence of genes are known as mutations. The most common types of gene mutations are: Base-pair substitutions (shown at left) Base-pair substitutions (shown at left) Frameshifts Frameshifts Insertions Insertions Deletions Deletions original base triplet in a DNA strand a base substitution within the triplet (red) As DNA is replicated, proofreading enzymes detect the mistake and make a substitution for it: POSSIBLE OUTCOMES: One DNA molecule carries the original, unmutated sequence OR The other DNA molecule carries a gene mutation VALINE sickle-cell anemia Remember sickle-cell anemia? base-pair substitution glutamine valine It’s caused by a base-pair substitution that replaces the amino acid glutamine with valine.
Mutations ARGININE GLYCINE TYROSINETRYPTOPHANASPARAGINE LEUCINE GLUTAMATE DNA TEMPLATE mRNA TRANSCRIPT RESULTING AMINO ACID SEQUENCE ALTERED mRNA MESSAGE A BASE INSERTION (RED) IN DNA ALTERED AMINO ACID SEQUENCE insertion The example above is a frameshift mutation known as an insertion. This mutation causes DNA’s message to shift one base to the right. deletion A deletion would cause a one-base shift to the left.