DNA- Structure and Replication

Discovering DNA For a long time, scientists thought cells were only composed of carbs, lipids, and proteins It was obvious that in reproduction SOMETHING inside the cells transferred from parent to offspring, but what? In the late 1800s, Meischer discovered “nuclein” – a fourth macromolecule found in the nucleus, later renamed the “Nucleic Acid” Nucleic Acids come in two forms – DNA and RNA

DNA Structure DNA (a nucleic acid) is made up of monomers called nucleotides each nucleotide is made up of three parts: 1. 5-carbon sugar (deoxyribose) 2. phosphate group 3. nitrogenous base FOUR types of nitrogenous bases adenine (A) & guanine (G) – purines cytosine (C) & thymine (T) – pyrimidines

Nitrogenous bases purines (A & G): two rings on the nitrogen base pyrimidines (C & T): one ring on the nitrogen base

But how do the Pieces fit together?

Chargaff’s Rules

Chargaff’s Rules Erwin Chargaff analyzed the DNA of many different species and noticed this pattern # of A = # of T # of G = # of C # of purines = # of pyrimidines

Franklin’s Picture Scientists were RACING to try and be the first one to figure out what DNA looked like Rosalind Franklin used X-Ray technology to take this picture of DNA which she shared with…

DNA “double helix” …Watson & Crick – the scientists who used the picture to discover the shape of DNA! DNA consists of two strands wound around each other like a twisted ladder

DNA “double helix” base pairing between nitrogen bases in the middle explains Chargaff’s Rules A always bonds with T (“points”) – double bond C always bonds with G (“curves”) – triple bond

DNA “double helix” The nitrogen bases pair with a hydrogen bond that is very WEAK! They can easily be pulled apart (or “unzipped”) and then put back together.

So if you have one side of DNA that has these bases, Cytosine (C) Adenine (A) Thymine (T) Guanine (G) --(G) Guanine --(T) Thymine --(A) Adenine --(C) Cytosine These bases will make the other side.

DNA and Cell Division

DNA Forms and Function DNA is the instructions for cell. It contains the information for how to make the proteins that carry out homeostasis to stay alive. DNA is a double helix, and that helix is further twisted to create other forms like chromosomes that are easier to move during divisions Some cells contain a LOT of DNA! Organization is key Before each cell division ALL of the DNA must be copied.

DNA in different cells prokaryotes have a SINGLE, circular chromosome in the cytoplasm (no nucleus)

DNA and Chromosomes prokaryotes have a SINGLE, circular chromosome in the cytoplasm containing their DNA eukaryotes have MANY chromosomes contained in a nucleus

Making a Chromosome Before division, the DNA helix is very tightly wound around proteins called histones to make a super coiled structure called a chromosome. Chromosomes cannot be used to make proteins, but they are easier to move around during division. Normally, DNA is found in chromatin form

Before chromosomes, what happens during the S phase of the cell cycle? Think back! Before chromosomes, what happens during the S phase of the cell cycle? DNA replicates!

DNA Replication

DNA Replication C—G G-- --C T-- --A A-- --T G-- --C G-- --C to make a copy of itself, DNA “unzips” the pairs C—G G-- --C T-- --A A-- --T G-- --C G-- --C

Notice these strands are the same. DNA Replication new bases come in to make a new complementary strand C—G G--C G --C T--A T--A A--T A--T G--C G--C G--C G--C Notice these strands are the same.

DNA Replication Enzymes Teams of enzymes copy the DNA very rapidly, within several hours Helicase: unzips the DNA at several locations DNA Polymerase: bring in the new base pairs to build the new strand Ligase: proofread the new DNA

Your turn copy and complete the DNA strand C-- G-- T-- A--

Does it look like this? C--G G--C T--A A--T

“Unzip” and copy it! C—G G-- --C G-- --C T-- --A A-- --T A-- --T

Does it look like this? C—G G--C G--C G--C G--C T--A T--A A--T A--T