DNA Replication

							     DNA Replication

& The structure of a Chromosome
    The Nucleotides

     Purines            Pyrimidines
Adenine   Guanine    Cytosine     Thymine




   Phosphate
   Group            Deoxyribose
                  Building a DNA
              Nucleotide

                                   Hydrogen Bonds




Sugar-Phosphate
Backbone
The Chromosome




  Chromatin
            The Chromosome
• Both Eukaryotes and Prokaryotes have DNA,
  eukaryotes have a nucleus to hold the DNA
• Eukaryotes have 100 x the amount of DNA as
  prokaryotes
• The eukaryotic chromosome consists of DNA is tightly
  coiled around proteins to create a substance called
  chromatin
• Chromatin consists of DNA that is tightly coiled around
  proteins called histones
• Together, the DNA and histones molecules form
  beadlike structures called a nucleosome. The
  nucleosome allows the DNA tightly coiled and pack
  inside of the nucleus
The Chromosome




  Chromatin
          The Chromosome
• The number of chromosomes varies from 1
  species to the next
   – Fruit flies have 8
   – Humans have 46
   – Sequoia trees have 22

  • The chromosome is only visible when the cell
    becomes ready to divide. The chromatin
    fibers are drawn together to create the
    chromosome structure that is visible in a light
    microscope
    Section 12-1
                         Order! Order!

•     Genes are made of DNA, a large, complex molecule. DNA is
      composed of individual units called nucleotides. Three of these
      units form a code. The order, or sequence, of a code and the type
      of code determine the meaning of the message.



1. On a sheet of paper, write the word cats. List the letters or units that
   make up the word cats.
2. Try rearranging the units to form other words. Remember that each
   new word can have only three units. Write each word on your paper,
   and then add a definition for each word.
3. Did any of the codes you formed have the same meaning?
4. How do you think changing the order of the nucleotides in the
   DNA codon changes the codon’s message?
                    Interest Grabber Answers




1. On a sheet of paper, write the word cats. List the letters or units that make
   up the word cats.
   The units that make up cats are c, a, t, and s.
2. Try rearranging the units to form other words. Remember that each
   new word can have only three units. Write each word on your paper, and
   then add a definition for each word.
   Student codes may include: Act; Sat; Cat
3. Did any of the codes you formed have the same meaning?
   No
4. How do you think changing the order of the nucleotides in the DNA codon
   changes the codon’s message?
   Changing the order of the nucleotides changes the meaning of the codon.
                  A Perfect Copy
•   When a cell divides, each daughter cell receives a complete set of
    chromosomes. This means that each new cell has a complete set of
    the DNA code. Before a cell can divide, the DNA must be copied so
    that there are two sets ready to be distributed to the new cells.

1. On a sheet of paper, draw a curving or zig-zagging line that divides
   the paper into two halves. Vary the bends in the line as you draw it.
   Without tracing, copy the line on a second sheet of paper.
2. Hold the papers side by side, and compare the lines. Do they look
   the same?
3. Now, stack the papers, one on top of the other, and hold the papers
   up to the light. Are the lines the same?
4. How could you use the original paper to draw exact copies of the
   line without tracing it?
5. Why is it important that the copies of DNA that are given to new
   daughter cells be exact copies of the original?
                   Interest Grabber Answers




1. On a sheet of paper, draw a curving or zig-zagging line that divides the paper
   into two halves. Vary the bends in the line as you draw it. Without tracing,
   copy the line on a second sheet of paper.
2. Hold the papers side by side, and compare the lines. Do they look the same?
   Lines will likely look similar.
3. Now, stack the papers, one on top of the other, and hold the papers up to the
   light. Are the lines the same?
   Overlaying the papers will show variations in the lines.
4. How could you use the original paper to draw exact copies of the line without
   tracing it?
   Possible answer: Cut along the line and use it as a template to draw the line
   on another sheet of paper.
5. Why is it important that the copies of DNA that are given to new daughter
   cells be exact copies of the original?
   Each cell must have the correct DNA, or the cell will not
   have the correct characteristics.
           DNA Replication
• During DNA Replication, a DNA molecule
  separates into two strands, then produces
  two new complementary strands following
  the rules of base pairing. Each strand of
  the double helix of DNA serves as a
  template, or model, for the new strand.
• Base Pairing-
  – A attaches to T
  – G attaches to C
• In prokaryotes replication occurs in 1 place and
  continues in both direction
• In eukaryotes replication occurs in 100’s of places along
  1 DNA strand and continues in both directions until entire
  strand is copied
• Each strand can be used to make the other strand, the
  strands are said to be complimentary
• Places where separation occurs along the DNA are
  called replication forks
• Many enzymes are needed (ex: one to unzip the DNA)
• DNA polymerase will connect newly created base pairs
  (the DNA polymer) and will also proofread the molecule
  to make sure its an exact replica of the original strand
           DNA Replication
• Before a cell divides, it needs to copy it’s
  DNA in a process called DNA Replication
• During DNA replication, the DNA molecule
  separates into 2 strands, then produces 2
  new complimentary strands following the
  rules of base pairing. Each strand of the
  double helix of DNA serves as a template,
  or model, for the new strand
DNA Replication Video




        Click Here
DNA Replication
            HW Questions
• Explain the structure of a chromosome
  down to the structure of a nucleotide.
• Explain why DNA needs to replicate.

Due Tomorrow