Chromosomes & DNA Replication TSW explain how the genetic code is contained within DNA

Discovering DNA: A Team Effort Frederick Griffith (1928) Question: How do bacteria make people sick? Answer: Transformation

Transformation Process in which one strain of bacteria is changed by a gene(s) from another strain of bacteria

Oswald Avery (1944) Question: Which molecule is important for transformation? Answer: DNA stores & transmits genetic info from 1 generation to the next

Hershey & Chase (1952) Question: Are genes made of DNA or protein? Experiment: Which part of a bacteriophage enters the bacterium? Answer: DNA

Hershey/Chase Experiment

Erwin Chargaff DNA base-pairing rules A-T G-C

“Race for the Double-Helix” Rosalind Franklin (1952): Used X-ray to determine that DNA is spiral-shaped Watson & Crick (1953): Created 1st accurate model of DNA (double-helix), won the Nobel Prize

DNA’s Structure Adenine Thymine Guanine Cytosine DNA is a double helix 4 Nucleotide bases Adenine Thymine Guanine Cytosine

Chargaff’s Rule Bases form the rungs on the “DNA ladder” by complementary pairing

More DNA Structure Sugar-Phosphate backbone Nucleotides held together by H bonds

DNA & Chromosomes Prokaryotes: single, circular chromosome Eukaryotes: DNA in nucleus of the cell in the form of multiple chromosomes

Chromosome # Varies widely between species Example: Humans: 46 Giant Sequoia Tree: 22 Drosophila:8

DNA Length Extremely long (more than 1m in the nucleus of a human cell!) Must be tightly folded to fit inside of the nucleus Q: How does this work? A: Chromosomes

Chromosome Structure CHROMATIN: DNA that is tightly coiled around proteins called HISTONES NUCLEOSOME: DNA+Histone Pack together to form a thick fiber with loops and coils Allows a lot of DNA to fit in such a small space

Chromatin Usually dispersed until MITOSIS where the chromatin condenses into individual chromosomes Do you remember which phase?

DNA Replication Each strand of the DNA double-helix has all the info needed to construct the other half via “base-pairing” 2 strands are complementary

Duplicating DNA Copying DNA called REPLICATION Results in 2 DNA molecules that are identical to each other & the original strand

DNA Replication Replication = Duplication Happens in the S phase of Interphase Must replicate before mitosis or meiosis I Part of the double helix is unwound Replication in small pieces (Okazaki fragments) Enzyme stitches pieces together later

How does replication occur? Enzymes “unzip” the DNA molecule “unzipping”: H bonds break between nucleotides & the 2 strands of DNA unwind

DNA Polymerase Enzyme (-ase) Joins individual nucleotides “proofreads” new strand to maximize odds that each molecule is a perfect copy

What does replication look like?

What is a Gene? A length of DNA on a chromosome May be in several parts Exon – the expressed parts of the DNA sequence Intron – the intervening, “junk DNA”, not expressed Parts are cut, put together and then used to make RNA and proteins

Telomere Poly A tail Telomerase Cancer Fountain of Youth