3.01: Molecular Basis of Heredity

1. 3.01: Molecular Basis of Heredity

2. 3.01: DNA Structure

3. DNA • DNA is the called the “genetic blueprint” because it contains the instructions needed for your cells to carry out all of the functions to sustain life. DNA stands for deoxyribonucleic acid. • Information encoded in your cells’ DNA is organized into units called genes. A gene is a segment of DNA that codes for a protein or RNA molecule. By the 1950s, scientists knew that genes were made of DNA, but they didn’t know what this molecule looked like.

4. DNA • James Watson and Francis Crick, two researchers at Cambridge University, discovered the structure of DNA, which clarified how DNA served as the genetic material. • The DNA molecule is a doublehelix – two strands twisted around each other like a winding staircase or a twisted ladder.

5. DNA • Each strand is made of linked nucleotides. Remember—nucleotides are the subunits that make up nucleic acids like DNA and RNA. • Each nucleotide is made of three parts: a phosphate group, a five-carbon sugar called deoxyribose, and a nitrogen-containing base. The sugar and the phosphate group are the same for each nucleotide in a DNA molecule, but the nitrogen base may be one of four kinds.

6. DNA Guanine Cytosine Thymine Adenine • The sugar-phosphate backbones are like the side rails of a ladder, while the paired nitrogen bases are like the rungs (steps) of the ladder.

7. Discovering DNA’s Structure • In 1949, Erwin Chargraff observed that for each organism’s DNA that he studied, the amount of adenine always equaled the amount of thymine (A = T) and the amount of guanine always equaled the amount of cytosine (G = C).

8. Discovering DNA’s Structure • In 1952, Maurice Wilkins and Rosalind Franklin developed high-quality X-ray photographs of DNA and showed that it must be a tightly coiled helix composed of chains of nucleotides. • In 1953, Watson and Crick put together Chargraff’s findings and stolen X-ray data from Franklin and Wilson to come up with the three dimensional double helix model.

9. Base Pairing in DNA • Adenine always pairs with Thymine and Guanine always pairs with Cytosine (A – T) (G – C) • These base-pairing rules are supported by Chargraff’s observations.

10. Base Pairing in DNA • The nucleotides are paired together with Hydrogen bonds, resulting in two strands that contain complementary base pairs. This means, the sequence of nitrogen bases on one strand determines the sequence of nitrogen bases on the other strand.

11. Base Pairing in DNA • In other words, one strand acts as a template for figuring out the complementary strand. Ex: T C G A A C T A G C T T G A

12. DNA Replication • The process of making a copy of the DNA is called DNA replication. Remember, this occurs during the S phase of the cell cycle, before the cell divides. • Step 1: The DNA double helix unwinds using an enzyme called DNA helicase. Proteins hold each strand apart from each other, forming a Y shape called a replication fork.

13. DNA Replication • Step 2: At the replication fork, enzymes called DNA polymerase move along each of the DNA strands, adding complementary bases to the exposed nitrogen bases according to base-pairing rules.

14. DNA Replication • Step 3: DNA polymerases add nucleotides to a growing double helix until all the DNA has been copied and the polymerases are signaled to detach. Result: two new DNA molecules, each composed of one original strand and one copied strand. The nucleotide sequences of the two DNA molecules are identical. • This process is called semi-conservative because each new DNA helix is made of half original DNA molecule and half new DNA molecule.

15. DNA Replication

16. DNA Replication • During DNA replication, errors can occur in which the wrong nucleotide is added to a new strand. Changes to the DNA are called mutations. • DNA polymerases proofread the new strand as they build it to avoid these errors. This results in an error rate of only one error per 1 billion nucleotides.

17. 3.01: Transcription and Translation

18. Transcription and Translation • Traits, like eye color and hair color, are determined by proteins that are built according to instructions coded in DNA. Proteins are not built directly from DNA, though. Ribonucleic acid, RNA, is also involved.

19. Transcription and Translation • RNA differs from DNA in 3 important ways: • RNA is a single strand of nucleotides, instead of two strands as in DNA. • RNA contains the sugar ribose, instead of DNA’s sugar deoxyribose. • RNA has the nitrogen bases A, G, and C as in DNA, but instead of T it has U for Uracil. U is complementary to A, just like Thymine is in DNA.

20. Transcription • The instructions for making a protein are transferred from a gene, which is a segment of DNA, to an RNA molecule in a process called transcription. Transcription = DNA RNA • Like DNA replication, RNA transcription uses DNA nucleotides as a template for making a new molecule. The new molecule that gets made, however, is RNA in transcription instead of DNA as in replication.

21. Transcription DNA  A C G T C A G T C A A T T C G RNA  U G C A G U C A G U U A A G C • In prokaryotes, transcription occurs in the cytoplasm because prokaryotes have no nucleus. • In eukaryotes transcription occurs in the nucleus because that is where the DNA is kept

22. Translation • After transcription is complete, cells use the RNA that was made to put together the amino acids that will make up a protein. This process is called translation. Translation = RNA  protein

23. Translation • Amino acids that make up the protein are held together by peptide bonds. The protein is also called a polypeptide because of these bonds. • The entire process by which proteins are made based on information coded in the DNA is called gene expression or protein synthesis.

24. Types of RNA • Different types of RNA are made during transcription depending on what gene is being expressed: • mRNA carries a message from the DNA to make proteins • tRNA carries amino acids to build proteins • rRNA makes up the ribosome

25. Types of RNA • When a cell needs a protein, it is messengerRNA (mRNA) that is made. mRNA carries the instructions for making proteins from a gene in the nucleus and delivers it to the site of translation in the cytoplasm. • This information then gets translated from the language of RNA—nucleotides—to the language of proteins—amino acids

26. Transcription and Translation • The RNA instructions are written as a series of three-nucleotide sequences or “words” called codons. Each codon in the mRNA strand stands for an amino acid or signifies a start or stop signal for translation.

27. Transcription and Translation • Ex: See the genetic code on the screen. Whenever we use this table, we plug mRNA into the chart to find the amino acids coded by the DNA.

28. Transcription and Translation • Start with DNA: T A C A T G T G T • Translate to RNA: A U G U A C A C A, then split into 3-letter codons • Transcribe into proteins: (use the table) • Methionine, Tyrosine, and Threonine • Translation takes place in the cytoplasm. Here, transfer RNA molecules and ribosomes work together to make proteins.

29. Transcription and Translation • Transfer RNA (tRNA) carries a specific amino acid on one end. Each tRNA molecule has a three-nucleotide anticodon that is complementary to an mRNA codon. Write the anticodons for the mRNA codons you translated above: • tRNAanticodons: U A C—A U G—U G U

30. Transcription and Translation • Ribosomes are made of ribosomal RNA (rRNA) and proteins. Remember—ribosomes are the site of protein synthesis

31. Steps of Translation • Transcription, in the nucleus, creates an mRNA molecule. • mRNA leaves the nucleus and enters the cytoplasm. • A ribosome attaches to mRNA at a “start” codon, a tRNA molecule containing the anticodon to mRNA’s codon attaches to the ribosome as well. This signals the beginning of protein synthesis. • The ribosome slides down the mRNA molecule while tRNA molecules, carrying amino acids, join up to the mRNA in the ribosome just long enough to drop off their amino acids, which join together to form a protein chain (a polypeptide).

32. Steps of Translation • This continues until a “stop” codon is reached. Then the newly made protein is released into the cell.

34. DNA/RNA Review

35. DNA/RNA Review • DNA and RNA are both nucleic acids. This means they are composed of subunits called nucleotides, which are made of a sugar, a phosphate, and a nitrogen base. • The structure of DNA is a double helix, which was discovered by Watson and Crick • One strand of DNA acts as a template for the other strand, following base pairing rules: A bonds with T, G bonds with C

36. DNA/RNA Review • DNA replication means making an exact copy of the DNA. • The structure of RNA is a single strand of nucleotides, not a double helix. • RNA has the sugar Ribose, while DNA has the sugar deoxyribose. • RNA has A, G, and C nitrogen bases, but has U instead of T.

37. DNA/RNA Review • Transcription is making mRNA from DNA, which occurs in the nucleus. • Translation is making proteins from mRNA, which occurs at the ribosome. • Transcription and translation together make up the process called protein synthesis (making proteins).

38. DNA/RNA Review • Practice the following processes: • DNA replication—using base pairing rules, make the complementary strand of DNA from the original: • T A C C G G C T A A T A • A T G G C C G A T T A T

39. DNA/RNA Review • Transcription—using the new DNA strand you just created as a template, tell what the mRNA would be : • U A C - C G G - C U A - A U A • Translation— split your mRNA into codons and use p. 303 in your book to write out the amino acids that would result from this strand of mRNA: • Tyrosine, Arginine, Leucine, Isoleucine

40. Gene Regulation and Mutations

41. Gene Regulation • For the most part, all of the cells in an organism’s body have the same DNA. However, cells are specialized for specific tasks and parts of the body. How can they be specialized but have the exact same DNA, the exact same instructions?

42. Gene Regulation • Cell differentiation is the way that cells become different from each other as they go through mitosis. At first, all cells are the same and are not specialized. These are called stem cells. As they grow and divide they become differentiated and specialized into heart cells, brain cells, liver cells, etc.

43. Gene Regulation • The differentiation of cells, despite the fact that all cells have the same DNA, occurs due to gene regulation—organisms’ cells can regulate which genes are expressed and which are not, depending on the cell’s needs. • As different cells respond to the environment they produce different types and amounts of proteins by “turning on” some genes and “turning off” other genes. This protein production is what specializes different cells for different jobs.

44. Mutations • A mutation is a change in the nucleotide-base sequence of a gene or DNA molecule. • When the cell replicates its DNA, the enzyme DNA Polymerase is in charge of proofreading the new DNA strand for errors. If it makes an error and doesn’t correct it, a mutation occurs.

45. Mutations • Mutations can disrupt the functions of proteins. Look at the example to see how they can change proteins: • DNA  TAC – GGC – GAG – TAG – CCT • mRNA AUG – CCG – CUC – AUC – GGA • Aminoacids: Methionine, Proline, Leucine, Isoleucine, Glycine • These amino acids in this order make up a specific protein that is needed by the body. If one DNA nucleotide is changed, look what happens to the protein:

46. Mutations • DNA  TAC – GTC – GAG – TAG – CCT • mRNA AUG – CAG – CUC – AUC - GGA • Amino acids: Methionine,Glutamine, Leucine, Isoleucine, Glycine • Now a different protein has been made, or perhaps an amino acid sequence that is unstable and will do nothing.

47. Mutations • A different type of mutation can occur called a frameshift mutation. What would happen if the cell accidentally removed one of the nucleotides in the original DNA sequence? • DNA  TAC – GGC – GAG – TAG – CCT TAC – GGC – AGT – AGC – CT… • mRNA AUG – CCG – UCA – UCG – GA • Amino acids: Methionine, Proline, Serine, Serine, __________

48. Mutations • In these ways, protein synthesis can be disrupted by even a small mutation in the DNA sequence.