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Chapter 14 From DNA to Proteins

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1 Chapter 14 From DNA to Proteins
Honors Biology Program Mountain Pointe High School

2 What are the byssus of a mussels? Why are these byssus so important?

3 Archibald Garrod First to notice that many heritable diseases were related to metabolic pathway malfunctions. STEPS OF A METABOLIC PATHWAY: Action of enzyme 1 Action of enzyme 2 Garrod hypothesized that each of his affected patients had inherited a single metabolic defect that interfered with an enzyme in a particular metabolic pathway. Something has interfered with the action of enzyme 3. X A B C D Completion of the pathway is blocked, and C accumulates.

4 Neurospora crassa and other fungal species
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. They discovered that some of the fungal strains would only grow when supplied with vitamin B6, others would only grow in the presence of B12, etc. Neurospora crassa and other fungal species

5 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!

6 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

7 Sickle-cell Anemia 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.

8 glass tube or plates containing gel
Pauling & Itano upper buffer solution movement of proteins 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. electrode glass tube or plates containing gel gel lower buffer solution electrode power supply

9 HbS molecules moved much slower.
Pauling & Itano HbA molecules carried the greatest surface charge and therefore moved through the gel the fastest. HbS molecules moved much slower. As molecules move through the gel, they’re separated into distinct bands.

10 Vernon Ingram 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!

11 Beta chain of an HbA molecule Beta chain of an HbS molecule

12 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.

13 3 Different Types of RNA

14 The Three Types of RNA mRNA is a single-stranded molecule that takes DNA’s protein-building instructions out of the nucleus. rRNA is the primary component of ribosomes, the organelles that actually make proteins. tRNA is the molecule responsible for delivering amino acids one by one to a ribosome in the correct order specified by the mRNA molecule. A messenger RNA molecule (mRNA) A ribosomal RNA molecule (rRNA) A transfer RNA molecule (tRNA)

15 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 Like thymine, uracil (at right, in blue) is a pyrimidine and is capable of pairing with adenine.

16 transcribed DNA winds up again
Transcription 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 Transcription is the process of using a portion of the DNA molecule as a template to assemble a molecule of mRNA. Only a selected stretch of one DNA strand is used as a template. Transcription is initiated at a promoter, a DNA base sequence that signals the start of a gene. transcribed DNA winds up again DNA to be transcribed unwinds Newly forming RNA transcript The DNA template at the assembly site

17 Transcription growing mRNA transcript 3’ 5’ 3’ 5’ direction of transcription 3’ 5’ A “pre mRNA” strand Once the enzyme DNA helicase has unzipped the DNA molecule at the appropriate location… RNA polymerase adds the required complementary bases to the exposed bases on one of the DNA strands.

18 Transcription Next, the pre- mRNA molecule must be modified.
unit of transcription in a DNA strand Next, the pre- mRNA molecule must be modified. A nucleotide known as a “cap” is attached to the 5` end. A nucleotide known as a “poly-A tail” is attached to the 3` end. 3’ 5’ exon intron exon intron exon transcription into pre-mRNA poly-A tail cap (snipped out) 5’ (snipped out) 3’ 5’ 3’ mature mRNA transcript Before the mRNA transcript is finished, useless sections known as introns must be snipped out, leaving only exons remaining.

19 Now that the mRNA has been transcribed, what does its message mean?
The Genetic Code DNA template codon codon codon codon codon mRNA transcript Amino acid? Amino acid? Amino acid? Amino acid? Amino acid?

20 amino acid attachment site
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. anticodon codon in mRNA anticodon tRNA MOLECULE amino acid attachment site amino acid amino acid attachment site

21 platform for chain assembly
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 tunnel Small ribosomal subunit Large ribosomal subunit Complete ribosome

22 Translation 3 Stages of Translation: Initiation
tRNA molecule is attached to small ribosomal subunit. mRNA molecule’s START codon (AUG) matches up with tRNA anticodon, attaches to small ribosomal subunit. Large ribosomal subunit attaches to small subunit.

23 P (first binding site for tRNA)
Binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA)

24 Translation Next is the elongation stage:
Ribosome complex moves along mRNA molecule. One by one, tRNA molecules deliver the amino acids coded for by the mRNA to A site. Amino acids are linked together by peptide bonds, tRNA molecules exit P site of ribosome.

25 Translation The final stage is termination.
Ribosomal complex reads STOP codon on mRNA molecule. No tRNA has an anticodon that corresponds to a STOP codon. Proteins called release factors bind to ribosome, cause enzymes to detach mRNA & polypeptide chain from ribosome. Ribosomal subunits separate.


27 Unwinding of gene regions of a DNA molecule
TRANSCRIPTION Pre mRNA Transcript Processing mRNA rRNA tRNA Mature mRNA transcripts Ribosomal subunits Mature tRNA TRANSLATION Cytoplasmic pools of amino acids, tRNAs, and ribosomal subunits Synthesis of a polypeptide chain at binding sites for mRNA and tRNA on the surface of an intact ribosome FINAL PROTEIN Destined for use in cell or for transport

28 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) Frameshifts Insertions 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: OR One DNA molecule carries the original, unmutated sequence The other DNA molecule carries a gene mutation Remember sickle-cell anemia? It’s caused by a base-pair substitution that replaces the amino acid glutamine with valine. VALINE

29 Mutations mRNA TRANSCRIPT DNA TEMPLATE ARGININE GLYCINE TYROSINE TRYPTOPHAN ASPARAGINE RESULTING AMINO ACID SEQUENCE ALTERED mRNA MESSAGE A BASE INSERTION (RED) IN DNA ARGININE GLYCINE LEUCINE LEUCINE GLUTAMATE ALTERED AMINO ACID SEQUENCE The example above is a frameshift mutation known as an insertion. This mutation causes DNA’s message to shift one base to the right. A deletion would cause a one-base shift to the left.

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