DNA - Lecture notes 5 PDF

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Dr. Major...

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Biology 106-A02

1/30/18 DNA is the genetic material.

The x-ray diffraction pattern of DNA   

Shoot x-rays at a DNA sample X-rays hit a piece of film The pattern on the film allows one to determine structure of the molecule

The x-ray diffraction pattern on DNA   

Franklin’s x-ray diffraction pattern for DNA **Rosalind Franklin ** James Watson and Francis Crick (the bright one) discovered the structure of DNA

The “Double Helix” (twisted)    

DNA contains two strands that are bound together The molecule is twisted One strand is oriented in one direction The other strand is oriented in the opposite direction (anti-parallel)

Two long “anti-parallel” strands Nucleic Acids are made up of many nucleotides 1. It has a sugar (deoxyribose) (green) 2. It has a phosphate (yellow circle) 3. It has a nitrogenous base (blue) Two long “anti-parallel” strands  

Sugars and Phosphates form a “backbone” The bases bind in the middle

Watson and Crick knew of “Chargaff’s Rules”  

******The number of A’s always equal the number of T’s****** ******The number of C’s always equal the number of G’s******

Very Important!!!  

A’s will always bind to T’s C’s will always bind to G’s

DNA Replication  

1953 Watson and Crick DNA Structure Paper “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

Why would a cell want to replicate its DNA? 

A parental cell will need to copy its DNA before undergoing cell division.

DNA is zipped apart during replication 

1 old strand is used a template for making a new strand

Both old strands are templates for a new strand 

Two DNA molecules come from one.

Nucleotides are linked together to form a new strand    

New nucleotide being added New strand Template strand The base on the template strand is used to determine the correct nucleotide to add

DNA synthesis only occurs from 5’ to 3’ 

Meaning the new strand is being synthesized

For the new strand, what should the next 5 bases be?   

CCAGT A-C-G-G-T-A-C-H-T-G-C-A  T-G-C-A-C-G-T-A-C-C-G-T RIGHT TO LEFT T-G-C-C-A-T-G-C-A-C-G-T  LEFT TO RIGHT

DNA replication requires multiple enzymes #1-DNA strands must be separated 

Helicase is the enzyme that pulls the double helix apart

#2-Nucleotides need to be added 

DNA Polymerase is the enzyme that puts nucleotides on the ends of the growing strand.

DNA Polymerase has “proofreading” abilities 

Polymerase can sense if it makes a mistake. It rips out the wrong base before continuing.

#3-Fragments need to be fused together 

Ligase is the enzyme that glues the fragments together....