DNA Deoxyribonucleic acid DNA is a nucleic acid
DNA
• Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms & some viruses. • The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, since it contains instructions needed to construct other components of cells, such as proteins & RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
DNA • Genes are made of DNA = deoxyribonucleic acid • How could DNA code for everything genes do? – Genes had to carry information from one generation to the next – Genes had to put that information to work by determining the inheritable characteristics of organisms – Genes had to be easily copied, because it is replicated every time the cell divides
DNA’s Structure • A long molecule made of units called nucleotides • Each nucleotide is made of 3 basic parts: – A 5 -carbon sugar called deoxyribose – A phosphate group – A nitrogenous base (There are 4 kinds…) A (Adenine) G (Guanine) Purines T (Thymine) C (Cytosine) Pyrimidines
A Single DNA Nucleotide Phosphate Group Deoxyribose Sugar Nitrogenous Base
te Phospha Group DNA Structure Nitrogenous Base Weak Hydrogen Bonds Deoxyribose Sugar
Figure 12– 7 Structure of DNA Section 12 -1 Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Go to Section:
The Chargaff “rule” • It was discovered that the percentages of guanines (G) and cytosines (C) are almost equal and the percentages of adenines (A) and thymines (T) are almost equal • Erwin Chargaff came up with a “rule” that guanine (G) pairs with cytosine (C) and adenine (A) pairs with thymine (T) (DNA Base Pairing)
Let’s pair up DNA! • What would pair up with ATCG? TAGC • What would pair up with ATTAGC? TAATCG • What would pair up with ATACGGTC? TATGCCAG
The Double Helix • Rosaline Franklin’s x-ray showed that two DNA strands are twisted around each other. • James Watson and Francis Crick used Franklin’s x-ray to make a model of DNA. • Watson and Crick’s model of DNA was a double helix, in which two strands were wound around each other. • Watson and Crick also discovered that hydrogen bonding held the nitrogenous bases together.
• The two strands of DNA unwind or “unzip” breaking the hydrogen bonds and separating. Then each strand becomes the guide or “template” for the making of a new strand. • A protein called an enzyme called DNA polymerase breaks the nitrogen base bonds and the two strands of DNA separate, polymerizes individual nucleotides to produce DNA and “proof reads” the new DNA.
STEP 1 Replication fork • Two original strands of DNA separates by unwinding • This occurs through the action of an enzyme that breaks the hydrogen bonds between the strands • The two areas on either end where the DNA separates is the REPLICATION FORK
STEP 2 • At the replication fork, DNA polymerase moves along the DNA Strands adding nucleotides • As DNA polymerase moves along two strands of DNA form
Step 3 • DNA Polymerase continues until all the nucleotides have been added • Two new identical molecules of DNA are formed • Another enzyme connects all the pieces of DNA together
Proofreading • At the end of Replication, DNA polymerase goes through the DNA to make sure there are no errors • This prevents mistakes in the DNA sequences • Mutations- Change in the sequence of the DNA
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