Lesson Overview DNA Replication

Transcription

1 12.3

2 THINK ABOUT IT Before a cell divides, its DNA must first be copied. How might the double-helix structure of DNA make that possible?

3 Review Question! At what stage of the cell cycle do cells duplicate their DNA? DNA replication takes place in the S phase. S phase G 1 interphase G 2 Mitosis -prophase -metaphase -anaphase -telophase

4 Copying the Code Base pairing in the double helix explained how DNA could be copied, or replicated, because each base on one strand pairs with only one base on the opposite strand.

5 Copying the Code Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary.

6 Copying the Code What role does DNA polymerase play in copying DNA?

7 Copying the Code What role does DNA polymerase play in copying DNA? DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA.

8 The Replication Process Before a cell divides, it duplicates its DNA in a copying process called replication. Why? This process ensures that each resulting cell has the same complete set of DNA molecules.

9 The Replication Process During replication DNA molecule separates into two strands Then produces two new complementary strands following base pairing rules

10 The Replication Process Each strand of the double helix of DNA serves as a template, or model, for the new strand. Animation

11 Let s Model this with our paper! Fold your paper in half hamburger style. On each side of the paper, fold back the flaps to meet at the fold (this hides the middle section of your paper) On the top portion of your paper, record the following DNA sequence: ACT GGG ATT TGC CCC ATG GTA AAA CCC TTT On the bottom portion of your paper, write the complementary strand. They should line up when the bottom of your paper is hidden.

12 Now, mimic DNA replication: Divide the two strands and match up the complementary base pairs. Use a different color to do this Identify the parent strands Identify the complementary strands Are the two resulting copies of DNA identical?

13 The Replication Process (Skip) The two strands of the double helix separate, or unzip, allowing two replication forks to form.

14 The Replication Process (Skip) As each new strand forms, new bases are added following the rules of base pairing. Review: What would pair with Adenine? Guanine?

15 The Replication Process Result of replication is two DNA molecules identical to each other and to the original molecule. Each DNA molecule resulting from replication has one original strand and one new strand. DNA Template Parental DNA New DNA

16 The Role of Enzymes DNA replication is carried out by a series of enzymes. They first unzip a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands of the molecule. Each strand then serves as a template for the attachment of complementary bases.

17 The Role of Enzymes Animation with Helicase & DNA Polymerase The principal enzyme involved in DNA replication is called DNA polymerase. Joins individual nucleotides to produce a new strand of DNA. Proofreads each new DNA strand, ensuring that each molecule is a perfect copy of the original.

18 Telomeres The tips of chromosomes are known as telomeres. The ends of DNA molecules, located at the telomeres, are particularly difficult to copy. Over time, DNA may actually be lost from telomeres each time a chromosome is replicated.

19 Telomeres An enzyme called telomerase compensates for this problem by adding short, repeated DNA sequences to telomeres Lengthening the chromosomes slightly Makes it less likely that important gene sequences will be lost from the telomeres during replication.

20 Replication in Living Cells How does DNA replication differ in prokaryotic cells and eukaryotic cells?

21 Replication in Living Cells How does DNA replication differ in prokaryotic cells and eukaryotic cells? Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

22 Replication in Living Cells The cells of most prokaryotes have a single, circular DNA molecule in the cytoplasm, containing nearly all the cell s genetic information. Eukaryotic cells, on the other hand, can have up to 1000 times more DNA. Nearly all of the DNA of eukaryotic cells is found in the nucleus.

23 Prokaryotic In most prokaryotes, DNA replication does not start until regulatory proteins bind to a single starting point on the chromosome. Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.

24 Prokaryotic Often, the two chromosomes produced by replication are attached to different points inside the cell membrane and are separated when the cell splits to form two new cells.

25 Eukaryotic Eukaryotic chromosomes are generally much bigger than those of prokaryotes.

26 Eukaryotic In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

27 Eukaryotic The two copies of DNA produced by replication in each chromosome remain closely associated until the cell enters prophase of mitosis. At that point, the chromosomes condense, and the two chromatids in each chromosome become clearly visible.

28 Eukaryotic They separate from each other in anaphase of mitosis, producing two cells, each with a complete set of genes coded in DNA.

29 What do BOTH prokaryotes & eukaryotes have in common with? Hydrogen bonds are broken, strands unwind & separate Each add complementary base pairs using parent strand as a template Both result in 2 identical copies of DNA, each with one parent strand and one complementary strand