12.1 DNA

DNA Function Controls production of proteins Heredity Proteins form structures of cells and controls all chemical process in cells Heredity Every new cell made in the body needs an exact copy of DNA (Mitosis) Parents pass DNA to their offspring

Dna Structure Nucleotides have 3 parts: Composed of repeating units called nucleotides Nucleotides have 3 parts: 5 – carbon sugar called deoxyribose A phosphate group A nitrogen base

There are 4 kinds of bases: Adenine Purines Guanine = 2 rings ------------------------------------- Thymine Pyrimidines Cytosine = 1 ring

Purines

Pyrimidines

Dna Structure Nucleotides bond together to form a long strand 2 strands bond together The two strands twist around a central axis to form a spiral structure that is called a double helix

DNA structure 2 strands bond together Held together by hydrogen bonds Bonds form between the purines & pyrimidines Adenine – Thymine (2 H-bonds) Guanine --- Cytosine (3 H-bonds) This forms two complementary strands

Nitrogenous base pairing

Dna’s double helix

copyright cmassengale Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars) copyright cmassengale

DNA History Griffith Experimented on mice and observed some harmless strains of bacteria could change into disease-causing strains. He called this transformation. Avery Discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation to the next.

More DNA History Hershey-Chase Concluded that the genetic material in bacteria was DNA not proteins

Discovery of dna They all, except for Chargaff & Franklin, won the Nobel Prize in 1962.

Discovery of dna Structure of DNA was discovered by: Chargaff: A=T & G=C : His rule is that the # of adenines must equal the # of thymines; cytosines=guanines Maurice Wilkins & Rosalind Franklin: She took the famous X-ray diffraction picture of DNA James Watson & Francis Crick: Discovered the helix shape

copyright cmassengale Question: If there is 30% Adenine, how much Cytosine is present? copyright cmassengale

copyright cmassengale Answer: There would be 20% Cytosine Adenine (30%) = Thymine (30%) Guanine (20%) = Cytosine (20%) Therefore, 60% A-T and 40% C-G copyright cmassengale

Question: If there is 15% Thymine, how much Cytosine is present?

copyright cmassengale Answer: Adenine (15%) = Thymine (15%) Therefore, 30% A-T and 70% C-G Guanine (35%) = Cytosine (35%) There would be 35% Cytosine copyright cmassengale

12.2 DNA Replication

How Is DNA copied? Replication Fork` DNA replication is the process of making a copy of DNA Enzyme called DNA helicase opens up DNA helix by breaking the hydrogen bonds (unzips it) The ends where the DNA separates are called replication forks An enzyme, DNA polymerase, attaches at the replication forks and will begin the replication process Replication Fork`

3. DNA polymerase and the replication process DNA polymerase moves along each of the DNA strands As it moves, unpaired bases in each strand connect with complementary bases of nucleotides that are floating freely in the nucleus The nitrogen bases attach via hydrogen bonds

Practice If you are given the following sequence, what would be the complementary sequence? Old strand : Atcgagt New strand : tagctca

How is dna copied? DNA polymerase continues until all of the DNA has been copied a. The end results is two identical DNA molecules b. Each of these DNA molecules is half “old” and half “new” = semiconservative replication c. The 2 DNA strands should be identical to each other and to the original

Synthesis of the New DNA Strands The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork RNA Primer DNA Polymerase Nucleotides 3’ 5’

Synthesis of the New DNA Strands The Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin RNA Primer Leading Strand DNA Polymerase 5’ 3’ Lagging Strand 5’ 3’

Lagging Strand Segments Okazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzyme Lagging Strand RNA Primer DNA Polymerase 3’ 5’ Okazaki Fragment

Joining of Okazaki Fragments The enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase Okazaki Fragment 1 5’ 3’

Replication of Strands Replication Fork Point of Origin

How are Errors Fixed? Incorrect nucleotides will be put into DNA sometimes DNA polymerase also has a proofreading role It can add nucleotides only if the previous nucleotide is paired up correctly This prevents most errors Only 1 error per billion occurs

How is speed of replication increased? In circular bacterial DNA, two forks begin at a single point and work away from each other In eukaryotes, about 100 replication works open in each DNA molecule at the same time a. Replication occurs at the same time along each fork until all of the forks meet