1. From DNA to Protein
Chapter 14

2. The Human Code
The human genome contains approximately 3 billion base pairs which reside in the 23 pairs of chromosomes within the nucleus of all our cells.
Each of the estimated 30,000 genes in the human genome makes an average of three proteins.
Humans share
99.9% of DNA with each other (We only differ by .1% genetically!!!!)
98.4% of DNA with Chimpanzees
50% of DNA with Bananas

3. Impacts, Issues: Ricin and your Ribosomes
Ricin (found in castor- oil plant used in plastics, paints, cosmetics) is toxic because it inactivates ribosomes, the organelles which assemble amino acids into proteins, critical to life processes

4. The Nature of Genetic Information
Each strand of DNA consists of a chain of four kinds of nucleotides: A, T, G and C
The sequence of the four bases in the strand is the genetic information

5. Differences between DNA and RNA
Sugar = Deoxyribose
Double stranded
Bases
Cytosine
Guanine
Adenine
Thymine
Sugar = Ribose
Single Stranded
Bases
Cytosine
Guanine
Adenine
URACIL (U)
These chemical differences make it easy for the enzymes in the cell to tell DNA and RNA apart

6. Ribonucleotides and Nucleotides

7. The 3 main types of RNA Messenger RNA (mRNA)
Carry a copy of the instructions from the nucleus to other parts of the cell
Ribosomal RNA (rRNA)
Makes up the structure of ribosomes
Transfer RNA (tRNA)
Transfers amino acids (proteins) to the ribosomes to be assembled

8. Gene Expression
A cell’s DNA sequence (genes) contains all the information needed to make the molecules of life
Gene expression
A multistep process including transcription and translation, by which genetic information encoded by a gene is converted into a structural or functional part of a cell or body

9. Transcription: DNA to RNA
In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotides
Uracil (U) nucleotides pair with A nucleotides
RNA polymerase adds nucleotides to the transcript

10. Base-Pairing in DNA Synthesis and Transcription

11. The Process of Transcription
RNA polymerase and regulatory proteins attach to a promoter (a specific binding site in DNA close to the start of a gene)
RNA polymerase moves over the gene in a 5' to 3' direction, unwinds the DNA helix, reads the base sequence, and joins free RNA nucleotides into a complementary strand of mRNA
 Videos:
Transcription
3 Minute Transcription/Translation

12. Transcription

13. Figure 14.5
Transcription.
Fig. 14-5b, p. 219

14. Transcription
Many RNA polymerases can transcribe a gene at the same time

15. Post-Transcriptional Modifications
In eukaryotes, RNA is modified before it leaves the nucleus as a mature mRNA
Introns = Nucleotide sequences that are removed from a new RNA
Exons = Sequences that stay in the RNA

16. Allows one gene to encode different proteins
Alternative Splicing
Alternative splicing
Allows one gene to encode different proteins
Some exons are removed from RNA and others are spliced together in various combinations
After splicing, transcripts are finished with a modified guanine “cap” at the 5' end and a poly-A tail at the 3' end

17. mRNA The Messenger
mRNA carries protein-building information to ribosomes and tRNA for translation
Codon
A sequence of three mRNA nucleotides that codes for a specific amino acid
The order of codons in mRNA determines the order of amino acids in a polypeptide chain

18. Consists of 64 mRNA codons (triplets)
Genetic Code
Genetic code
Consists of 64 mRNA codons (triplets)
Some amino acids can be coded by more than one codon
Some codons signal the start or end of a gene
AUG (methionine) is a start codon
UAA, UAG, and UGA are stop codons

19. Codons of the Genetic Code

20. rRNA and tRNA tRNAs deliver amino acids to ribosomes
tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon
Ribosomes, which link amino acids into polypeptide chains, consist of two subunits of rRNA and proteins

21. tRNA

22. Translation Translation = mRNA to protein
Occurs in the cytoplasm on the ribosomes
Translation occurs in three stages
Initiation
Elongation
Termination
Video: Translation

23. Initiation An initiation complex is formed
A small ribosomal subunit binds to mRNA
The anticodon of initiator tRNA base-pairs with the start codon (AUG) of mRNA
A large ribosomal subunit joins the small ribosomal subunit

24. The ribosome assembles a polypeptide chain as it moves along the mRNA
Elongation
The ribosome assembles a polypeptide chain as it moves along the mRNA
Initiator tRNA carries methionine, the first amino acid of the chain
The ribosome joins each amino acid to the polypeptide chain with a peptide bond

25. Figure 14.12
An example of translation as it occurs in eukaryotic cells. (a, b) In initiation, an mRNA, an intact ribosome, and an initiator tRNA form an initiation complex. (c–e) In elongation, the new polypeptide chain grows as the ribosome catalyzes the formation of peptide bonds between amino acids delivered by tRNAs. (f) In termination, the mRNA and the new polypeptide chain are released, and the ribosome disassembles.
Fig d, p. 223

26. Termination
When the ribosome encounters a stop codon, polypeptide synthesis ends
Release factors bind to the ribosome
Enzymes detach the mRNA and polypeptide chain from the ribosome

27. Polysomes
Many ribosomes may simultaneously translate the same mRNA, forming polysomes

28. Mutated Genes and Their Protein Products
If the nucleotide sequence of a gene changes, it may result in an altered gene product, with harmful effects
Mutations
Small-scale changes in the nucleotide sequence of a cell’s DNA that alter the genetic code

29. Common Mutations Base-pair-substitution
May result in a premature stop codon or a different amino acid in a protein product
Example: sickle-cell anemia
Deletion or insertion (frame shift)
Can cause the reading frame of mRNA codons to shift, changing the genetic message
Example: Huntington’s disease

30. Sickle cell Mutation

31. Sickle Cell Mutation
The sickle cell mutation is a point mutation that affects the hemoglobin in red blood cells.
A single base change from and adenine to uracil changes glutamic acid to valine

32. Sickle Cell Mutation
This mutation however seems to “help” populations where malaria is endemic. (e.g. certain parts of Africa)….. Why?
Malaria affects 400 million people annually and kills million
People with the sickle cell trait are more likely to survive acute malaria illness

33. Sickle Cell Mutation Red cells affected with malaria become deformed.
People with the sickle cell trait remove those cells more affectively which reduce the parasite burden in people

34. What Causes Mutations? Transposable elements
Segments of DNA that can insert themselves anywhere in a chromosomes
Spontaneous mutations
Uncorrected errors in DNA replication
Harmful environmental agents
Ionizing radiation, UV radiation, chemicals

35. McClintock’s Transposable Elements
100 Greatest Discoveries

36. Mutations Caused by Radiation
Ionizing radiation damages chromosomes, nonionizing (UV) radiation forms thymine dimers

37. Inherited Mutations
Mutations in somatic cells (the cells that are not sperm or eggs) of sexually reproducing species are not inherited
Mutations in a germ cell or gamete may be inherited, with evolutionary consequences

38. Summary: Protein Synthesis in Eukaryotic Cells

39. Practice 1) Transcribe and translate the following sequence
TACCCCAGCGTTACT
2) The DNA strand is mutated to the following:
TACCCCAGCGTTACC
What type of mutation is this?
How does this mutation change the protein?
3) Figure out a mutation in the DNA that will not change the protein.