Chromosomes Chromosome Supercoils Coils Nucleosome Histones DNA double helix.
The structure of the DNA double helix was described by Watson and Crick in 1953. Explain the...
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Transcript of The structure of the DNA double helix was described by Watson and Crick in 1953. Explain the...
The structure of the DNA double helix was described by Watson and Crick in 1953. Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.(Total 8 marks)
Please have that paperwork out & your journal… HW check!
• subunits are nucleotides;• one base, one deoxyribose and one phosphate in each
nucleotide;• description / diagram showing base linked to deoxyribose C1
and phosphate to C5;• four different bases – adenine, cytosine, guanine and thymine;• nucleotides linked up with sugar-phosphate bonds;• covalent / phosphodiester bonds;• two strands (of nucleotides) linked together;• base to base;• A to T and G to C;• hydrogen bonds between bases;• antiparallel strands;• double helix drawn or described;
Accept any of the points above if clearly explained in a diagram.• [8]
11 DNA and Its Role in Heredity
• 11.1 What Is the Evidence that the Gene Is DNA?
• 11.2 What Is the Structure of DNA?
• 11.3 How Is DNA Replicated?
• 11.4 How Are Errors in DNA Repaired?
• 11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?
11-1:• Not in IB, but ...Not in IB, but ...
• Experiments on bacteria and Experiments on bacteria and viruses demonstrated that DNA IS viruses demonstrated that DNA IS the GENETIC MATERIAL!the GENETIC MATERIAL!
11.1 What Is the Evidence that the Gene Is DNA?
By the 1920s: chromosomes consisted of DNA & proteins.
A new dye stained DNA, provided circumstantial evidence—DNA’s the genetic material:
It was in the right place
It varied among species
It was present in the right amount
11.1 What Is the Evidence that the Gene Is DNA?
Frederick Griffith, working w/2 strains of
Streptococcus pneumoniae
“transforming principle” from dead cells of 1 strain produced a heritable change in the other strain.
11.1 What Is the Evidence that the Gene Is DNA?
To identify the transforming principle, Oswald Avery:
Treated samples to destroy different molecules (RNA, DNA, Protein)
If DNA was destroyed, the transforming principle was lost.
11.1 What Is the Evidence that the Gene Is DNA?
Hershey-Chase experiment:
• Is DNA or protein the genetic material?using bacteriophage T2 virus
• Bacteriophage proteins labeled with 35SDNA labeled with 32P
Figure 11.3 Bacteriophage T2: Reproduction Cycle
Bacteriophage T2
Protein coat
DNA
DNA
Bacteriophage T2 attaches to the surface of a bacterium and injects its DNA.
Viral genes take over the host’s machinery and synthesizes new viruses.
The bacterium bursts, releasing about 200 viruses.
DNA Origami!• http://www.dnai.org/teacherguide/pdf/
origami_inst.pdf
11-2 SUMMARY:• DNA: double helix of 2 DNA: double helix of 2
ANTIPARALLEL polynucleotide ANTIPARALLEL polynucleotide chainschains
• 2 chains joined by H bonds 2 chains joined by H bonds between nucleotide bases—pair A-between nucleotide bases—pair A-T , G-CT , G-C
11-2 Recap ?s:
• What’s the evidence that Watson & What’s the evidence that Watson & Crick used to come up with the Crick used to come up with the double helix model for DNA?double helix model for DNA?
• How does the double helical How does the double helical STRUCTURE of DNA relate to its STRUCTURE of DNA relate to its FUNCTION?FUNCTION?
Figure 11.9 Base Pairing in DNA Is Complementary
5′ end
3′ end
TA pairs have twohydrogen bonds.
CG pairs have three hydrogen bonds.
The strands both run in a 5′-to-3′ direction—they are antiparallel.3′ end
Each phosphate group links the 3′ carbon of one sugar to the 5′ carbon of the next sugar along the backbone.
Pairs of complementary bases form hydrogen bonds that hold the two strands of the DNA double helix together.
11-3 Recap:• Meselson and Stahl showed that DNA Meselson and Stahl showed that DNA
replication is replication is semiconservativesemiconservative: each : each parent strand serves as a template for a parent strand serves as a template for a new strandnew strand
• A complex of proteins, most notably A complex of proteins, most notably DNA polymerasesDNA polymerases, is involved, is involved
• New DNA is polymerized in New DNA is polymerized in one one direction onlydirection only
• Since the 2 strands are antiparallel, 1 Since the 2 strands are antiparallel, 1 strand is made continuously and the strand is made continuously and the other is made in other is made in Okazaki fragmentsOkazaki fragments that that are eventually joinedare eventually joined
11-3 Recap ?s:
• How did the Meselson-Stahl expt How did the Meselson-Stahl expt work?work?
• What are 5 enzymes needed for What are 5 enzymes needed for DNA replication? Role of each?DNA replication? Role of each?
• How does the leading strand of How does the leading strand of DNA differ from the lagging DNA differ from the lagging strand?strand?
11.3 How Is DNA Replicated?
Human chromosome telomeres (TTAGGG) are repeated about 2500 times.
Chromosomes can lose 50–200 base pairs with each replication. After 20–30 divisions, the cell dies.
11.3 How Is DNA Replicated?
Some cells—bone marrow stem cells, gamete-producing cells—have telomerase that catalyzes the addition of telomeres.
90% of human cancer cells have telomerase; normal cells do not. Some anticancer drugs target telomerase.
11-4 Recap:• DNA replication ain’t perfect!DNA replication ain’t perfect!
• DNA can also be damaged or DNA can also be damaged or naturally alterednaturally altered
• Repair mechanisms exist that Repair mechanisms exist that detect and repair mismatched or detect and repair mismatched or damaged DNAdamaged DNA
11.4 How Are Errors in DNA Repaired?
The newly replicated DNA is scanned for mistakes by other proteins.
Mismatch repair mechanism detects mismatched bases—the new strand has not yet been modified (e.g., methylated in prokaryotes) so it can be recognized.
If mismatch repair fails, the DNA is altered.
11.4 How Are Errors in DNA Repaired?
DNA can be damaged by radiation, toxic chemicals, and random spontaneous chemical reactions.
Excision repair: enzymes constantly scan DNA for mispaired bases, chemically modified bases, and extra bases—unpaired loops.
11-5 Recap:• Knowledge of the mechanisms of Knowledge of the mechanisms of
DNA replication led to DNA replication led to development of techniques for development of techniques for making multiple copies of DNA making multiple copies of DNA sequences of DNA molecules!sequences of DNA molecules!
11-5 Recap ?s:
• What do primers do in PCR?What do primers do in PCR?
• Why are dideoxyribonucleosides Why are dideoxyribonucleosides used in DNA sequencing?used in DNA sequencing?
11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?
Copies of DNA sequences can be made by the polymerase chain reaction (PCR) technique.
PCR is a cyclical process:• DNA fragments are denatured by
heating.• A primer, plus nucleosides and DNA
polymerase are added.• New DNA strands are synthesized.
11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?
PCR results in many copies of the DNA fragment—referred to as amplifying the sequence.
Primers are 15–20 bases, made in the laboratory. The base sequence at the 3′ end of the DNA fragment must be known.
11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?
DNA polymerase that does not denature at high temperatures (90°C) was taken from a hot springs bacterium, Thermus aquaticus.
11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?
DNA sequencing determines the base sequence of DNA molecules.
Relies on altered nucleosides with fluorescent tags that emit different colors of light.
DNA fragments are then denatured and separated by electrophoresis.