Chapter 12 Notes - DNA Structure and Replication
In 1928, Fredrick Griffith was investigating two forms of the bacterium that causes pneumonia
one form is surround by a coating made of carbohydrates
S form — colonies look smooth
second form does not have smooth coating
R — rough form
Griffith injected the bacteria into mice — the S type killed the mice
when the S bacteria were killed with heat before injection, it didn't kill the mice → only live S bacteria cases the mice to die
he then injected mice with a combination of heat-killed S bacteria and live R bacteria → the mice died
live S bacteria was found in the dead mice
Griffith concluded that some material had been transferred from the heat-killed S bacteria to the live R bacteria
he called this transformation - material transferred to alter another organism
figure: Griffith's Experiments
Oswald Avery worked to find what the transforming principle was that Griffith had discovered
they combined living R bacteria with an extract made from S bacteria
they then developed a process to purify their extract
they then performed a series of tests to find out of the transforming principle was DNA or protein
qualitative tests - standard chemical tests showed that no protein was present; the tests revealed that DNA was present
chemical analysis - the proportions of elements in the extract closely matched those found in DNA; proteins contain almost no phosphorus
enzyme tests - when enzymes were added, the extract still transformed the R bacteria to the S form → transformation only failed to occur when an enzyme was added that specifically destroys DNA
Alfred Hershey and Martha Chase studied viruses that infect bacteria and came up with conclusive evidence for DNA as the genetic material in 1952
bacteriophage - a type of virus that takes over a bacterium's genetic machinery and directs it to make more viruses
phages are relatively simple — are a little more than a DNA molecule surrounded by a protein coat
by discovering if the DNA or protein of a phage enters a bacterium could answer the question
protein contains sulfer but little phosphorus, while DNA contains phosphorus but no sulfer
experiment 1 - bacteria were infected with phages that had radioactive sulfer atoms in their protein molecules
they used a kitchen blender and a centrifuge to separate the bacteria
they found no significant radioactivity
experiment 2 - DNA had radioactive phosphorus → radioactivity was present inside the bacteria
conclusion: the phages' DNA entered the bacteria, but the protein had not
nucleotides - the small units (monomers) that make up DNA
each nucleotide has 3 parts
a phosphate group (one phosphorus with four oxygens)
a ring-shaped sugar called deoxyribose
a nitrogen-containing base (single or double ring built around nitrogen and carbon atoms)
Erwin Chargaff found that the same four bases are in the DNA of all organisms but the proportion of the four bases differs somewhat from organism to organism
A = T and C = G relationships became known as Chargaff's rules
aka base-pairing rules
In 1950 James Watson, an American geneticist, and British physician Francis Crick worked together to understand the structure of DNA
they hypothesized that DNA might be a helix
X-ray evidence: Rosalind Franklin and Maurice Wilkins were studying DNA using x-ray crystallography
when DNA is bombarded with x-rays, the atoms diffract them in a pattern that can be captured on film
Franklin's x-rays showed an X surrounded by a circle
suggested that DNA is a helix consisting of two strands that are a regular, consistent width apart
the double helix: Watson and Crick made models of metal and wood to figure out the structure of DNA
placed the sugar-phosphate backbones on the outside and the bases on the inside
found that if they paired double-ringed nucleotides with single-ringed nucleotides, the bases fit
double helix - two strands of DNA wind around each other like a twisted ladder
The DNA nucleotides of a single strand are joined together by covalent bonds connecting the sugar of one nucleotide to the phosphate of the next
The DNA double helix is held together by hydrogen bonds between the bases in the middle
each hydrogen bond individually is weak, but together maintain the structure
base pairing rule: thymine always pairs with adenine, cytosine always pairs with guanine
due to sizes of the bases and ability to form hydrogen bonds
A & T form 2 hydrogen bonds with each other
C & G form 3 hydrogen bonds with each other
replication - the process by which DNA is copied during the cell cycle
assures that every cell has a complete set of identical genetic information
a single DNA strand can serve as a template, or pattern, for a new complementary strand
from free nucleotides available within the nucleus
completed molecules are identical to the original molecule
DNA can be accurately replicated over and over
Enzymes and other proteins replicate the DNA
DNA does nothing more than store information
helicases - enzymes that start the process by unzipping the double helix to separate the strands of DNA
other proteins hold the strands apart while they serve as templates
nucleotides that float free can then pair up with nucleotides of existing DNA strands
DNA polymerases - a group of enzymes that bond the new nucleotides together
results in in two complete molecules of DNA, each identical to the original strand
also carry out a proofreading step that quickly removes nucleotides that have base-paired incorrectly during replication
DNA polymerases and DNA ligase are also involved in repairing DNA damaged by harmful radiation or toxic chemicals in the environment
Replication Process: (in eukaryotes and is similar to prokaryotes)
helicase enzymes begin to unzip the double helix at numerous places along the chromosome — called origins of replication
origins of replication - short stretches of DNA that have a specific sequence of nucleotides
hydrogen bonds connecting base pairs are broken
original molecule separates
bases on each strand are exposed
the process of unzipping DNA proceeds in two directions at the same time
the DNA molecule of a eukaryotic chromosome has many origins where replication can start simultaneously, shortening the total time needed for replication
the sugar-phosphate backbones run in opposite directions and are said to be "antiparallel"
one by one, free nucleotides pair with the bases exposed as the template strands unzip between what is known as replication forks
DNA polymerases bond the nucleotides together and form new strands complementary to each template
on one template, DNA replication occurs in a smooth, continuous way in one direction — called the leading strand
on the other template, replication occurs in a discontinuous, piece-by-piece way in the opposite direction — lagging strand
these pieces are called Okazaki fragments
DNA ligase - an enzyme which then links the pieces together into a single DNA strand
two identical molecules of DNA result, each with one strand from the original molecule and one new strand
DNA replication is called semiconservative because one old strand is conserved, and one new strand is made
DNA replication varies in prokaryotes and eukaryotes — prokaryotes have a single, circular piece of DNA (plasmid) in their cytoplasm
replication begins when regulatory proteins bind to a certain point on the plasmid and proceeds in two directions until the whole plasmid is copied
the two copies are then put into different cells during cell division
Eukaryotes have about 1000 times more DNA than prokaryotes — chromosomes are linear and tightly packed with proteins in the nucleus
replication may begin at dozens are even hundreds of places at the same time and proceed in both directions until the whole chromosome is copied
remain close to each other (sister chromatids) until they are separated during mitosis or meiosis (anaphase/anaphase II)
In 1928, Fredrick Griffith was investigating two forms of the bacterium that causes pneumonia
one form is surround by a coating made of carbohydrates
S form — colonies look smooth
second form does not have smooth coating
R — rough form
Griffith injected the bacteria into mice — the S type killed the mice
when the S bacteria were killed with heat before injection, it didn't kill the mice → only live S bacteria cases the mice to die
he then injected mice with a combination of heat-killed S bacteria and live R bacteria → the mice died
live S bacteria was found in the dead mice
Griffith concluded that some material had been transferred from the heat-killed S bacteria to the live R bacteria
he called this transformation - material transferred to alter another organism
figure: Griffith's Experiments
Oswald Avery worked to find what the transforming principle was that Griffith had discovered
they combined living R bacteria with an extract made from S bacteria
they then developed a process to purify their extract
they then performed a series of tests to find out of the transforming principle was DNA or protein
qualitative tests - standard chemical tests showed that no protein was present; the tests revealed that DNA was present
chemical analysis - the proportions of elements in the extract closely matched those found in DNA; proteins contain almost no phosphorus
enzyme tests - when enzymes were added, the extract still transformed the R bacteria to the S form → transformation only failed to occur when an enzyme was added that specifically destroys DNA
Alfred Hershey and Martha Chase studied viruses that infect bacteria and came up with conclusive evidence for DNA as the genetic material in 1952
bacteriophage - a type of virus that takes over a bacterium's genetic machinery and directs it to make more viruses
phages are relatively simple — are a little more than a DNA molecule surrounded by a protein coat
by discovering if the DNA or protein of a phage enters a bacterium could answer the question
protein contains sulfer but little phosphorus, while DNA contains phosphorus but no sulfer
experiment 1 - bacteria were infected with phages that had radioactive sulfer atoms in their protein molecules
they used a kitchen blender and a centrifuge to separate the bacteria
they found no significant radioactivity
experiment 2 - DNA had radioactive phosphorus → radioactivity was present inside the bacteria
conclusion: the phages' DNA entered the bacteria, but the protein had not
nucleotides - the small units (monomers) that make up DNA
each nucleotide has 3 parts
a phosphate group (one phosphorus with four oxygens)
a ring-shaped sugar called deoxyribose
a nitrogen-containing base (single or double ring built around nitrogen and carbon atoms)
Erwin Chargaff found that the same four bases are in the DNA of all organisms but the proportion of the four bases differs somewhat from organism to organism
A = T and C = G relationships became known as Chargaff's rules
aka base-pairing rules
In 1950 James Watson, an American geneticist, and British physician Francis Crick worked together to understand the structure of DNA
they hypothesized that DNA might be a helix
X-ray evidence: Rosalind Franklin and Maurice Wilkins were studying DNA using x-ray crystallography
when DNA is bombarded with x-rays, the atoms diffract them in a pattern that can be captured on film
Franklin's x-rays showed an X surrounded by a circle
suggested that DNA is a helix consisting of two strands that are a regular, consistent width apart
the double helix: Watson and Crick made models of metal and wood to figure out the structure of DNA
placed the sugar-phosphate backbones on the outside and the bases on the inside
found that if they paired double-ringed nucleotides with single-ringed nucleotides, the bases fit
double helix - two strands of DNA wind around each other like a twisted ladder
The DNA nucleotides of a single strand are joined together by covalent bonds connecting the sugar of one nucleotide to the phosphate of the next
The DNA double helix is held together by hydrogen bonds between the bases in the middle
each hydrogen bond individually is weak, but together maintain the structure
base pairing rule: thymine always pairs with adenine, cytosine always pairs with guanine
due to sizes of the bases and ability to form hydrogen bonds
A & T form 2 hydrogen bonds with each other
C & G form 3 hydrogen bonds with each other
replication - the process by which DNA is copied during the cell cycle
assures that every cell has a complete set of identical genetic information
a single DNA strand can serve as a template, or pattern, for a new complementary strand
from free nucleotides available within the nucleus
completed molecules are identical to the original molecule
DNA can be accurately replicated over and over
Enzymes and other proteins replicate the DNA
DNA does nothing more than store information
helicases - enzymes that start the process by unzipping the double helix to separate the strands of DNA
other proteins hold the strands apart while they serve as templates
nucleotides that float free can then pair up with nucleotides of existing DNA strands
DNA polymerases - a group of enzymes that bond the new nucleotides together
results in in two complete molecules of DNA, each identical to the original strand
also carry out a proofreading step that quickly removes nucleotides that have base-paired incorrectly during replication
DNA polymerases and DNA ligase are also involved in repairing DNA damaged by harmful radiation or toxic chemicals in the environment
Replication Process: (in eukaryotes and is similar to prokaryotes)
helicase enzymes begin to unzip the double helix at numerous places along the chromosome — called origins of replication
origins of replication - short stretches of DNA that have a specific sequence of nucleotides
hydrogen bonds connecting base pairs are broken
original molecule separates
bases on each strand are exposed
the process of unzipping DNA proceeds in two directions at the same time
the DNA molecule of a eukaryotic chromosome has many origins where replication can start simultaneously, shortening the total time needed for replication
the sugar-phosphate backbones run in opposite directions and are said to be "antiparallel"
one by one, free nucleotides pair with the bases exposed as the template strands unzip between what is known as replication forks
DNA polymerases bond the nucleotides together and form new strands complementary to each template
on one template, DNA replication occurs in a smooth, continuous way in one direction — called the leading strand
on the other template, replication occurs in a discontinuous, piece-by-piece way in the opposite direction — lagging strand
these pieces are called Okazaki fragments
DNA ligase - an enzyme which then links the pieces together into a single DNA strand
two identical molecules of DNA result, each with one strand from the original molecule and one new strand
DNA replication is called semiconservative because one old strand is conserved, and one new strand is made
DNA replication varies in prokaryotes and eukaryotes — prokaryotes have a single, circular piece of DNA (plasmid) in their cytoplasm
replication begins when regulatory proteins bind to a certain point on the plasmid and proceeds in two directions until the whole plasmid is copied
the two copies are then put into different cells during cell division
Eukaryotes have about 1000 times more DNA than prokaryotes — chromosomes are linear and tightly packed with proteins in the nucleus
replication may begin at dozens are even hundreds of places at the same time and proceed in both directions until the whole chromosome is copied
remain close to each other (sister chromatids) until they are separated during mitosis or meiosis (anaphase/anaphase II)
