Protein Synthesis

1. Protein Synthesis James Watson and Francis Crick demonstrating a DNA double helix model.

2. Importance of Proteins • Much of the “work” done by the cells and tissues of the body is actually the result of a protein. • Examples include: • Carrier proteins, which facilitate many forms of cell membrane transport, including endocytosis. • Recognition proteins, which identify the cell as self to the immune system. • Antibodies, which bind to and disable infectious viruses and bacteria. • Structural proteins, which make up connective tissues such as ligaments and tendons. • Hormones, which deliver messages throughout the body. • Enzymes, which speed up the rate of chemical reactions within the body.

3. Enzymes usually end with –ase, and are a necessary component of every form of chemical change in the body, such as the steps of the citric acid cycle.

4. Proteins are made of one or more polypeptides, or chains of amino acids. • Insulin, a protein hormone that helps regulate blood sugar, is made of two polypeptides.

5. Proteins form very complex three-dimensional structures that are directly the result of their primary structure, or sequence of amino acids in their polypeptides. • A change of even a single amino acid can alter the entire shape of the protein.

6. DNA to Protein • The nucleus is able to control the cell by directing the amount and types of proteins produced by the ribosomes in the cytoplasm and rough endoplasmic reticulum. • This entire process is called protein synthesis and begins with the DNA that makes up the chromatin/chromosomes in the nucleus.

7. DNA and RNA • DNA and RNA are both nucleic acids; polymers of nucleotides. • Nucleotides are made of three units: • A sugar (ribose or deoxyribose) • A phosphate group (PO4) • A nitrogenous base • The sugar and phosphateare part of the structure ofthe DNA molecule, butdon’t play a direct rolein protein synthesis.

8. There are four types of nitrogenous bases found in nucleotides. • The sequence of these bases determines the type of protein that will be made from the DNA template. • Each base is given an abbreviation. • A = Adenine • T = Thymine • C = Cytosine • G = Guanine

9. The structure of the DNA molecule was first observed by Rosalind Franklin, who used X-ray crystallography. • Two other scientists, James Watson and Francis Crick, deduced that the nitrogenous bases were at the center of the molecule, with the phosphate and sugars making up the sides.

10. The final piece of the puzzle was that adenine (A) bonded best with thymine (T), and cytosine (C) bonded best with guanine (G). • DNA forms a double-helix shape, with two spiraled polynucleotide strands bonded down the middle.

11. DNA Replication • During replication, the double-helix will be untwisted and the base pairs separated. • This process is catalyzed by an enzyme, DNA helicase. • Each strand becomes a template for the assembly of a new complementary strand. • This process is called the semiconservative model of replication, because each daughter DNA molecule will actually have one strand from the original parent DNA molecule.

12. The enzyme responsible for producing the new DNA strand is DNA polymerase. • DNA polymerase can only work on one direction – from the end labeled 5’ (five-prime) to the end labeled 3’.

13. DNA replication will begin at multiple sites, called origins of replication and form “bubbles” that gradually enlarge until the entire molecule is synthesized.

14. Following base-pair rules, what complementary strand would be formed from this parent molecule?

15. Protein Synthesis • Remember, each of the heritable traits in our bodies are the result of genes, or specific segments of DNA.

16. Eye color, for example, is the result of two or more genes that collectively determine the amount of a pigment protein called melanin produced in the irises of the eyes.

17. One Gene, One Polypeptide • Each gene transcribes for one polypeptide. • Proteins may be made of one or more polypeptides. • Polypeptides are produced in ribosomes, which are in the cytoplasm. • Chromatin is too large to easily exit nuclear pores, so a “copy” of the gene is made from a single-stranded molecule of RNA. • This process is called transcription. • The overall process of protein synthesis can be summarized as: • DNA → RNA → Polypeptide

18. Transcription begins when an enzyme, RNA polymerase, binds to a specific gene, or segment of DNA on a chromosome. • The new molecule, called mRNA, is formed based on the DNA template.

19. mRNA has a few important structural differences, compared to DNA: • The sugar in the molecule’s backbone is ribose instead of deoxyribose. • It is only single-stranded. • The nitrogenous base uracil (U) is used in place of thymine. • RNA synthesis within transcription follows this pattern: • A U • T  A • C  G • G  C

20. Nitrogenous bases are read by RNA polymerase three at a time, a unit called a codon. • Each codon will eventually translate into a single amino acid. • The first codon in the mRNA sequence is called a promoter, or “start” codon. • AUG

21. Transcription continues until one of the three terminators, or “stop” codons is reached. • UAA • UGA • UAG

22. The mRNA must next be used to generate the required polypeptide in a ribosome. • This stage is called translation. • Each codon on the mRNA molecule is matched up to a complementary anticodon, or group of three bases on a tRNA molecule. • For example, if the codon on the mRNA is CCG, the anticodon on the tRNA will be GGC. • tRNA carriesan amino acid, which is added to the chain of the new protein.

23. The genetic code is non-ambiguous, meaning that each codon specifies for one amino acid. • For example, AUU always codes for isoleucine.

24. The genetic code is also redundant, meaning that each amino acid may be coded by multiple codons. • For example, isoleucine is coded by AUU, AUC, and AUA. • Redundancy helps to avoid damage if an error occurs in transcription.

25. Example: Insulin • Insulin is a protein hormone that regulates blood sugar. • The protein is made of two polypeptide chains joined by a connecting peptide and capped by a signal peptide.

26. How Insulin is Built • The double helix of the relevant section of chromosome 11 is unzipped by DNA helicase. • The mRNA molecule is transcribed by RNA polymerase using one of the DNA strands as a template. • The mRNA molecule is sent through a nuclear pore to the rough endoplasmic reticulum.

27. How Insulin is Built • The mRNA is received by a ribosome in the rough E.R., which starts translating the mRNA codons into amino acids. • The protein is built, one amino acid at a time, using tRNA molecules. • The insulin is sent to the Golgi, where it is packaged before released through the cell membrane.

28. Mutations • Any change in the nucleotide sequence of DNA is called a mutation. • Substitution mutations occur when the wrong nucleotide is inserted during DNA replication. • Example: A thymine (T) is inserted instead of an adenine (A) in a complementary DNA strand. • Insertion mutations occur when an extra nucleotide is inserted into the complementary DNA strand. • Deletion mutations occur when a nucleotide is skipped and not added to the complementary DNA strand.

29. Substitution Mutations • If a substitution mutation does not affect the primary structure of the polypeptide, it is called a silent mutation. • A missensemutation results in the insertion of a single incorrect amino acid in the poly peptide.

30. Sickle-cell anemia is the result a missense mutation in the gene that codes for one of the four polypeptides of the hemoglobin protein in red blood cells.

31. Substitution Mutations • A nonsense mutation changes an amino acid codon to a stop codon, prematurely terminating the protein. • This is the most damaging of the substitution mutations.

32. Frameshift Mutations • Insertions and deletions are also called frameshift mutations because they completely disrupt the reading frame, or triplet grouping of the gene. • Nearly every amino acid following the mutation will be incorrect, completely altering the structure of the protein.

33. Mutagens • Mutations can occur spontaneously during DNA replication or can be the result of physical or chemical agents called mutagens. • Examples include ultraviolet radiation, X-rays, asbestos, and tobacco smoke.