DNA Structure DNA Structure and Replicationand Replication

(Ch. 12-1, 12-2)(Ch. 12-1, 12-2)

DNADNA DNA is one of the DNA is one of the

4 types of 4 types of macromolecules macromolecules known as a known as a nucleic acid. nucleic acid.

DNA stands for DNA stands for deoxyribonucleic deoxyribonucleic acid.acid.

Components and Structure Components and Structure of DNAof DNA

All living things use DNA to carry All living things use DNA to carry genetic information from one genetic information from one generation to the next (genes are generation to the next (genes are made of DNA)made of DNA)

be easily and accurately copied be easily and accurately copied before cell division.before cell division.

NucleotidesNucleotides The monomer, repeating The monomer, repeating

subunits that make up nucleic subunits that make up nucleic acids, like DNA, are called acids, like DNA, are called nucleotides.nucleotides.

3 parts of a Nucleotide3 parts of a Nucleotide one 5-carbon one 5-carbon

sugar called sugar called deoxyribosedeoxyribose

one phosphate one phosphate groupgroup

one nitrogenous one nitrogenous (nitrogen-(nitrogen-containing) basecontaining) base

Open your notes and label the nucleotide!

2 types of Nucleotides2 types of Nucleotides

1) 1) purinespurines= = ((two ringstwo rings))

adenine (adenine (AA))

Draw Adenine in Draw Adenine in the first box the first box ( upper Right) ( upper Right) on your noteson your notes

Another PurineAnother Purine

guanine (guanine (GG))

Draw guanine in Draw guanine in the 2the 2ndnd purine purine box (middle box (middle Right) in your Right) in your notesnotes

2) 2) pyrimidinepyrimidiness ( (oneone ring) ring)

•thymine (thymine (TT))

•Draw thymine Draw thymine across from across from adenineadenine

2) 2) pyrimidinepyrimidiness ( (oneone ring) ring)

•cytosine (cytosine (CC))

Each strand of DNA has Each strand of DNA has directionality. directionality.

Each molecule will Each molecule will have a phosphate, or a have a phosphate, or a 5’ end and a sugar 5’ end and a sugar molecule, or a 3’ end molecule, or a 3’ end (3 prime). (3 prime).

DNA looks like DNA looks like a twisted a twisted ladder. The ladder. The strands run in strands run in opposite opposite directions; directions; called called antiparallel antiparallel (like opposite (like opposite flows of traffic)flows of traffic)

One strand runs 5’ to 3’ while One strand runs 5’ to 3’ while the other strand runs 3’ to 5’.the other strand runs 3’ to 5’.

5’5’ 3’3’

3’3’ 5’5’

Each base pair is made of 1 purine Each base pair is made of 1 purine and 1 pyrimidine. and 1 pyrimidine.

1. Adenine from one strand and thymine 1. Adenine from one strand and thymine from the other always pair together from the other always pair together with two hydrogen bonds,with two hydrogen bonds,

2. guanine on one strand and 2. guanine on one strand and cytosine on the other strand cytosine on the other strand always bond together with always bond together with three hydrogen bonds.three hydrogen bonds.

DNA and chromosomesDNA and chromosomes DNA molecules are extremely long. DNA molecules are extremely long.

In order to fit into cells, they must In order to fit into cells, they must fold upfold up as much as possible. as much as possible.

In humans, the amount of In humans, the amount of DNA in the nucleus is more DNA in the nucleus is more than 1 meter long. than 1 meter long.

In Eukaryotic cells the DNA In Eukaryotic cells the DNA is bound to histone proteins is bound to histone proteins forming chromatin in the forming chromatin in the nucleus. nucleus.

To fit the DNA into the To fit the DNA into the nucleus, the DNA and histone nucleus, the DNA and histone proteins are packed tightly a proteins are packed tightly a process called supercoiling.process called supercoiling.

Chromosome

Histone Proteins

DNA

Label the picture inside your notes!

Super Coiling!!!

Nucleosomes

DNA ReplicationDNA Replication:: How is How is DNA copied?DNA copied?

The process of copying DNA is called The process of copying DNA is called DNA replication. DNA replication.

Because DNA is double stranded, we Because DNA is double stranded, we can separate the 2 strands and create can separate the 2 strands and create a new strand using 1 strand as a a new strand using 1 strand as a template. template.

Our end product is 2 identical double Our end product is 2 identical double stranded DNA molecules. stranded DNA molecules.

Because of this, the two strands are Because of this, the two strands are called complementary (think about the called complementary (think about the angles in geometry). angles in geometry).

During DNA replication:During DNA replication: the DNA is unwound and unzipped the DNA is unwound and unzipped

(the two strands of DNA separate)(the two strands of DNA separate) each original DNA strand is used each original DNA strand is used

as a template (or model) to make a as a template (or model) to make a new DNA strand with base pairingnew DNA strand with base pairing

New DNA strand

New DNA strand

Remember: What do Remember: What do enzymes do?enzymes do?

Enzymes are biological catalysts that Enzymes are biological catalysts that help chemical reactions happen help chemical reactions happen faster. faster.

Forming and breaking chemical Forming and breaking chemical bonds takes energy.bonds takes energy.

Enzymes lower the amount of energy Enzymes lower the amount of energy needed to do make and break those needed to do make and break those bonds. bonds.

Helicase unzips or separates the two Helicase unzips or separates the two DNA strands by breaking the hydrogen DNA strands by breaking the hydrogen bonds between the nitrogenous basesbonds between the nitrogenous bases

The enzyme Primase lays down an RNA The enzyme Primase lays down an RNA Primer a few base pairs long to which Primer a few base pairs long to which the new DNA can be added.the new DNA can be added.

Another enzyme, called DNA Another enzyme, called DNA polymerase, adds new bases to the new polymerase, adds new bases to the new DNA strandDNA strand

DNA polymerase also “proofreads” the DNA polymerase also “proofreads” the new DNA to check for errorsnew DNA to check for errors

The new DNA molecules are rewound by The new DNA molecules are rewound by the enzyme Gyrase.the enzyme Gyrase.

ENZYMES USED IN DNA REPLICATION

When replication is When replication is complete, each complete, each DNA molecule is DNA molecule is made of one old made of one old strand and one new strand and one new strand. This is strand. This is described as the described as the semi-conservative semi-conservative model of model of replication.replication.

Original DNA

New DNA(one old strand, one new strand)

Chromosome Chromosome ArrangementArrangement

Prokarytoic cells (bacteria)Prokarytoic cells (bacteria) Prokaryotic cells have one Prokaryotic cells have one

circular chromosome that is circular chromosome that is free-floating in the cytoplasm. free-floating in the cytoplasm. Remember, prokaryotic cells Remember, prokaryotic cells do not have a nucleus.do not have a nucleus.

When prokaryotic cells copy When prokaryotic cells copy their DNA, the process begins their DNA, the process begins at one point in the chromosome at one point in the chromosome and moves around the circle in and moves around the circle in both directions until complete.both directions until complete.

Write this and draw this picture in your notes under “Prokaryotic cells”

Eukaryotic cellsEukaryotic cells Eukaryotic cells Eukaryotic cells

have more have more chromosomes than chromosomes than prokaryotic cells, prokaryotic cells, and DNA replication and DNA replication begins at hundreds begins at hundreds of places and of places and continues in both continues in both directions until directions until each chromosome is each chromosome is completely copied. completely copied.

The point in the DNA where the two The point in the DNA where the two DNA strands are separating and DNA strands are separating and replication is occurring is called the replication is occurring is called the replication fork.replication fork.

Growth

Growth

Replication fork

DNA polymerase

New strand Original strand DNA polymerase

Nitrogenous bases

Replication fork

Original strandNew strand

DNA Polymerase only works in one DNA Polymerase only works in one direction (5’ to 3’). One strand is read direction (5’ to 3’). One strand is read and synthesized continuously while the and synthesized continuously while the other is synthesized in fragments.other is synthesized in fragments.

New DNA strand

New DNA strand

Replication Fork

Draw this picture in your notes under “Eukaryotic cells”