DNA Replication

Let’s review DNA!

Watson & Crick called the structure of DNA a ….. Double Helix

DNA is a polymer made up of many monomers called… Nucleotides

Each nucleotide is made up of what three parts? O -P O O Deoxyribose C O Nitrogenous base Phosphate C C C

Chargraff’s base pair rule states that… Adenine always pairs with ….. Thymine Guanine always pairs with ….. Cytosine

According to the Base-Pairing Rule, we can determine the complementary strand of DNA. For example: A T C C A G T A G G T C

You practice base-pairing rule! Figure out the complementary DNA strand: A T T C G C A T T A A G C G T A

G G C A A C T T C C G T T G A A One more! Figure out the complementary DNA strand: G G C A A C T T C C G T T G A A

Some Facts DNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strands copyright cmassengale

Synthesis Phase (S phase) S phase during interphase of the cell cycle Mitosis -prophase -metaphase -anaphase -telophase G1 G2 S phase interphase DNA replication takes place in the S phase. copyright cmassengale

Replication: the process of copying DNA to produce new molecules with the same base sequence.

Read Handout Read the steps of DNA replication. Fill in the complementary bases.

Replication Step 1: DNA unwinds The enzyme Helicase “unzips” and separates the 2 strands. Hydrogen bonds are broken between bases.

DNA Replication The sites where separation and replication occur are called replication forks. Replication Fork Parental DNA Molecule 3’ 5’ Copyright Pearson Prentice Hall

An enzyme unwinds the DNA strands. Talk about the hydrogen bond. 2 strands of nucleotides. 16

Replication Step 2: New bases are added Each strand serves as a template The enzyme DNA Polymerase adds new nitrogen bases. DNA Polymerase proofreads it.

Antiparallel Strands One strand of DNA goes from 5’ to 3‘ The other strand is opposite in direction going 3’ to 5’ copyright cmassengale

Notice that the 3’ and 5’ refer to a numbering system for the carbon atoms that make up the sugar. DNA Nucleotide

Base Pairing Rule Watson and Crick showed that DNA is a double helix A (adenine) pairs with T (thymine) C (cytosine) pairs with G (guanine)

Anti-Parallel Strands of DNA On the left is the DNA double helix. When the helix is unwound, a ladder configuration shows that the uprights are composed of sugar and phosphate molecules and the rungs are complementary bases. Notice that the bases in DNA pair in such a way that the phosphate-sugar groups are oriented in different directions. This means that the strands of DNA end up running antiparallel to one another, with the 3’ end of one strand opposite the 5’ end of the other strand.

Direction of Replication DNA Replication DNA polymerase can only add nucleotides to the 3’ end of the DNA This causes the NEW strand to be built in a 5’ to 3’ direction RNA Primer DNA Polymerase Nucleotide 5’ 3’ Direction of Replication copyright cmassengale

More enzymes add complimentary bases Talk about the hydrogen bond. 2 strands of nucleotides. 24

DNA Replication Original strand New Strand Nitrogen Bases Growth During DNA replication, the DNA molecule produces two new complementary strands. Each strand of the double helix of DNA serves as a template for the new strand. Replication Fork Replication Fork DNA Polymerase Copyright Pearson Prentice Hall

End result: 2 new identical DNA molecules

DNA replication is “semi-conservative”.

Modeling Replication