Download Structure of DNA

10/12/2014
Some Review!
Nucleic Acids
Date _________
Discovery of the Genetic Material
• Jons Jakob Berzelius, 1838: coined the term “protein”, though its
existence was known since the 1680s.
• Johann Miescher, 1860s: studied human pus and fish sperm, and
isolated a substance he called “nuclein”, which was just DNA.
• Frederick Griffith, 1928: used two strains of the bacterium
Steptococcus pneumoniae to find that genetic material can be
passed between organisms.
• Oswald Avery, 1944: pinpointed DNA as the genetic material by
repeating Griffith’s experiment with DNA, proteins, and lipids. He
found that the bacteria were passing DNA as their genetic material.
This conclusion was not widely accepted.
• Alfred Hershey and Martha Chase, 1952: pinpointed DNA as the
genetic material by using radioactive-labeled proteins and DNA in
bacteriophages (viruses that infect bacteria). They found that the
labeled DNA was injected into the bacteria, providing instructions
for making new viruses. This was the powerful evidence needed for
the scientific community to accept DNA as the genetic material.
The Model
• Double helix (twisted ladder)
• Sugar-phosphate backbones
• Nitrogenous bases
– Adenine – Thymine
– Guanine – Cytosine
• Hydrogen bonds connect
nitrogenous bases
– 2 between A—T pairs
– 3 between G—C pairs
• There are 4 major macromolecules
– Carbohydrates
— Lipids
– Proteins
— Nucleic acids
• Nucleic acids serve as the blueprint for proteins
– Proteins give every living thing its unique
traits
• Ex: eye color, blood type, height, digestive
enzymes, skin color, etc.
DNA is the Genetic Material. Now
What?
• Erwin Chargaff, 1950: analyzed the amounts of nitrogenous
bases in DNA of various species. He knew there are four
nitrogenous bases – adenine, guanine, cytosine, and
thymine. He found that the amount of guanine nearly
equals the amount of cytosine, and the amount of adenine
nearly equals the amount of thymine within a species.
– This is known as Chargaff’s rule: A = T and C = G
• Rosalind Franklin, Maurice Wilkins, James Watson, Francis
Crick, 1953: all were involved in discovering the structure of
DNA. Franklin and Wilkins worked in an x-ray
crystallography lab, where molecules are crystallized and an
x-ray image of it can be taken. Franklin was one of the first
to be able to crystallize DNA, and captured the photo that
gave Watson and Crick the information they needed to
finalize their research and build a model replica of DNA.
– Their findings showed that there are two outside strands of
alternating deoxyribose and phosphate in DNA. Cytosine and
guanine bases pair to each other with 3 hydrogen bonds, and
adenine and thymine bases pair to each other with 2 hydrogen
bonds in between the outside strands.
Nucleic Acids
• There are two types of nucleic acids
– DNA = deoxyribonucleic acid
– RNA = ribonucleic acid
• Both types of nucleic acids are composed of
building blocks called nucleotides (monomers)
• Nucleotides have 3 parts:
nitrogen-containing
phosphate group
base
– Nitrogenous base
– Sugar
– Phosphate group
sugar
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DNA v. RNA
Shape
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Double helix
Single helix
Nitrogenous Adenine, thymine, guanine,
Bases
cytosine (ATGC)
Adenine, uracil, guanine,
cytosine (AUGC)
Sugar
Deoxyribose
Ribose
Location
Nucleus only
Nucleus and cytoplasm
Purpose
Stores genetic information,
replicates
Stores genetic information,
used to make proteins
Types
Only one – DNA
Three – messenger RNA
(mRNA), transfer RNA
(tRNA), ribosomal RNA
(rRNA)
A Little More About Structure
• Nitrogenous Bases
– Purines: double-ringed nucleotides, like adenine and
guanine
– Pyrimidines: single-ringed nucleotides, like thymine,
cytosine, and uracil
– Purine and pyrimidine bases pair, but not two purines
together or two pyrimidines.
• Orientation
– DNA is antiparallel – each strand of the molecule runs in
opposite directions.
• Carbon molecules are numbered in organic compounds. One side
of the DNA molecule is oriented where the 3’ (three prime) sugar
is on the left end and the 5’ (five prime) sugar is on the right end.
The other strand is the opposite, and runs 5’  3’.
Organization of DNA
• DNA: double helix, made up of
nucleotides, and is a recipe for
making proteins.
• Proteins are determined by the order of
nitrogenous bases (ACTG), and are made
up of amino acids.
• Genes: specific location on a
chromosome, a segment of DNA that
codes for a particular protein.
– Proteins determine an organism’s
characteristics.
– Each chromosome contains hundreds
to thousands of genes.
• Chromosome: a long thread of tightly
coiled DNA contained within the
nucleus of a cell.
Prokaryotic v. Eukaryotic Chromosome
Structure
• Prokaryotes: DNA molecule is in the cytoplasm,
consisting mainly of a single ring of DNA and
associated proteins.
• Eukaryotic: DNA is linear, and organized into individual
chromosomes.
– The length of a single chromosome ranges from 51 million
– 245 million base pairs. All the DNA in the nucleus of
EACH cell is about 3 – 6 ft. long, if laid out in a straight line.
– How does all the DNA fit into the nucleus of a eukaryotic
cell?
• DNA tightly coils around a group of beadlike proteins called
histones. When DNA associates with histones, it wraps around
them and forms a structure called a nucleosome. The
nucleosomes then group together into chromatin fibers, which
supercoil to make up the DNA structure recognized as a
chromosome.
Human Chromosomes
• Humans have a total of 46 chromosomes, or 23 pairs of
chromosomes.
• There are two types:
– Autosomes: autosomal chromosomes are chromosomes that are not
involved in sex determination. In humans, these are chromosome
pairs 1 – 22.
• Humans have 44 autosomes.
– Sex Chromosomes: determine the sex of an organism.
• Humans have 2 sex chromosomes.
• In females, they are XX. In males, they are XY.
• All cells except eggs and sperm contain two of each chromosome.
Each chromosome of a pair contains the same types of genes on
them, but not the same exact traits.
• Your chromosomes came from your parents, and contain genes
that code for your proteins, which in turn make you a unique
individual.
• The closer related two people, or even two species are, the more
genes they have in common.
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DNA is the Code of Life
• Every organism uses the exact
same building blocks – A, T, C,
and G. The methods for
replicating DNA, transcription,
and translation are also the
same in most organisms on the
planet.
• DNA contains the code for every
protein an organism needs, and
every cell in an organism’s body
contains a complete set of
chromosomes.
Trivia!
• Do all organisms have the same amount of chromosomes?
• What eukaryotic organisms do you think have more
chromosomes: potatoes, dogs, or humans?
– Humans: 46 chromosomes (23 pairs)
– Dogs: 78 chromosomes (39 pairs)
– Potatoes: 48 chromosomes (24 pairs)
• Which eukaryotic organism has the most chromosomes?
– Adder’s tongue fern with 1200 – 1260 chromosomes (600 – 630
pairs).
• Which eukaryotic organism has the least chromosomes?
– Jack jumper ant with 2 chromosomes (1 pair).
• How many genes do humans have?
– About 20,000
– How do we know?
• Human Genome Project, 1990 – 2003, was a project dedicated to mapping
every single human gene. It was a success! Now we’re working on the human
brain…
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