8.3 dna replication teks 3e, 5a, 9c the student is expected to: 3e evaluate models according to...

8.3 DNA Replication TEKS 3E, 5A, 9C

The student is expected to:

3E evaluate models according to their limitations in representing biological objects or events; 5A describe the stages of the cell cycle, includingdeoxyribonucleic acid (DNA)replication and mitosis, and the importance of the cell cycle to the growth of organisms; 9C identify and investigate the role of enzymes


KEY CONCEPT DNA replication copies the genetic information of a cell.


Replication copies the genetic information.

• A single strand of DNA serves as a template for a new strand.

• The rules of base pairing directreplication.

• DNA is replicated during theS (synthesis) stage of thecell cycle.

• Each body cell gets acomplete set ofidentical DNA.


During DNA replication:During DNA replication:

1.1. the DNA is unwound and unzipped by the the DNA is unwound and unzipped by the enzyme Helicase.enzyme Helicase.

• The strands are held apart by single-stranded The strands are held apart by single-stranded binding proteins (also known as ssbps)binding proteins (also known as ssbps)


2. each original DNA strand is used as a 2. each original DNA strand is used as a template (or model) to make a new template (or model) to make a new DNA strand with base pairingDNA strand with base pairing

New DNA strand

New DNA strand


3. The enzyme Primase lays down an 3. The enzyme Primase lays down an RNA Primer a few base pairs long to RNA Primer a few base pairs long to which the new DNA can be added.which the new DNA can be added.


4. Another enzyme, called DNA 4. Another enzyme, called DNA Polymerase, adds new Polymerase, adds new nucleotides to the RNA nucleotides to the RNA Primer. Primer.

– It always reads 3’ to 5’ and It always reads 3’ to 5’ and synthesizes the new strand synthesizes the new strand from 5’ to 3’. from 5’ to 3’.

– This occurs in the direction This occurs in the direction following Helicase opening up following Helicase opening up the “replication fork”. For this the “replication fork”. For this reason, the new strand is reason, the new strand is called the leading strand.called the leading strand.


5. Another DNA Polymerase also 5. Another DNA Polymerase also “proofreads” the new DNA to “proofreads” the new DNA to check for errors.check for errors.

6. Meanwhile, on the other strand, 6. Meanwhile, on the other strand, known as the lagging strand, known as the lagging strand, Primase and DNA Primase and DNA Polymerase synthesize DNA Polymerase synthesize DNA from 5’ to 3’ away from the from 5’ to 3’ away from the replication fork. replication fork.

• These small spurts of These small spurts of replication form what are replication form what are known as known as Okazaki fragments.Okazaki fragments.


** DNA Polymerase only works in one direction. One ** DNA Polymerase only works in one direction. One strand is read and synthesized continuously while the strand is read and synthesized continuously while the other is synthesized in fragments.other is synthesized in fragments.

Growth

Growth

Replication fork

DNA polymerase

New strand Original strand DNA polymerase

Nitrogenous bases

Replication fork

Original strandNew strand


7. The Okazaki fragments are joined together by the 7. The Okazaki fragments are joined together by the enzyme Ligase.enzyme Ligase.

8. On both strands, the RNA Primer is replaced with DNA 8. On both strands, the RNA Primer is replaced with DNA nucleotides by the enzyme DNA Polymerase.nucleotides by the enzyme DNA Polymerase.


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

9. The new DNA molecules are 9. The new DNA molecules are rewound by the enzyme Gyrase.rewound by the enzyme Gyrase.

Original DNA

New DNA(one old strand, one new strand)


There are many origins of replication in eukaryotic chromosomes.

• DNA replication starts at many points in eukaryotic chromosomes.

Replication is fast and accurate.

• DNA polymerases can find and correct errors.


• DNA replication is semiconservative.

original strand new strand

Two molecules of DNA

• Two new molecules of DNA are formed, each with an original strand and a newly formed strand.

8.3 dna replication teks 3e, 5a, 9c the student is expected to: 3e evaluate models according to...

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