Forensic DNA Analysis DNA Replication. Lesson Overview Lesson Overview Identifying the Substance of...

Forensic DNA Analysis

DNA Replication

Lesson Overview Identifying the Substance of Genes

The Double-Helix Model• The double-helix model

explains Chargaff’s rule of base pairing and how the two strands of DNA are held together

• The nucleotides can be joined together in any order, meaning that any sequence of bases is possible


Antiparallel Strands• In the double-helix

model, the two strands of DNA are “antiparallel”—they run in opposite directions, (5’-3’ direction)

• It allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.

Hydrogen bond 3 end

5 end

3 end

5 end

(b) Partial chemical structure


Hydrogen Bonding• Hydrogen bonds

between bases hold the two strands of DNA together

• Because Hydrogen bonds can be broken easily, two strands of DNA can separate to be copied


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.


The Replication Process• Before a cell divides, it duplicates its DNA in a copying

process called replication.• This process ensures that each resulting cell has the

same complete set of DNA molecules.

A T

GC

T A

TA

G C

(a) Parent molecule

A T

GC

T A

TAG C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

GC

T A

TA

G C

A T

GC

T A

TAG C


The Replication Process• During replication, the 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.


• The result of replication is two DNA molecules identical to each other and to the original molecule.

• Each DNA molecule resulting from replication has 1. one original strand 2. one new strand.

The Replication Process

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

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1. Replication begins at special sites called origins of replication,2. The two DNA strands are separated, opening up a replication “bubble” and proceeds in both directions from each origin, until the entire molecule is copied

DNA Replication Steps: Getting Started

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Lesson Overview Identifying the Substance of GenesThe Role of Enzymes

DNA replication is carried out by a series of enzymes. • At the end of each replication bubble is a replication fork,

a Y-shaped region where new DNA strands are elongating• Helicases are enzymes that untwist the double helix at the

replication forks• An enzyme called primase can start an RNA chain from

scratch and will be used as the starting point (primer) for the new DNA strand• The primer is short (5–10 nucleotides long), and the 3

end• DNA polymerase III: Joins individual nucleotides to

produce a new strand of DNA and proofreads new strand.• DNA Ligase: joins DNA nucleotides together (“glue”)


Enzymes in DNA Replication

Helicase

Primase

RNA primer

55

5 3

3

3

3. Enzymes called DNA polymerases start the elongation of new DNA at a replication fork

• Most DNA polymerases require a primer and a DNA template strand

4. DNA Polymerase adds nucleotides in a 5’-3’ direction (A pairs with T, G pairs with C).

DNA Replication Steps:Synthesizing a New DNA Strand

DNA Replication: Antiparallel Elongation5. The antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication

• Leading strand: DNA polymerases add nucleotides only to the free 3end of a growing strand and elongates in the 5to3direction on fork)

6. Lagging strand: DNA polymerase must work in the direction away from the replication fork and is synthesized as Okazaki Fragments, which are joined together by DNA ligase


Telomeres• The tips of chromosomes are known as

telomeres.• The ends of DNA molecules are particularly

difficult to copy and DNA may be lost from telomeres each time a chromosome is replicated.

• An enzyme called telomerase compensates for this problem by adding short, repeated DNA sequences to telomeres, lengthening the chromosomes slightly.

• It is less likely important gene sequences are lost from the telomeres during replication.


• Replication may begin at dozens or even hundreds of places on the DNA molecule

• Proceeds in both directions until each chromosome is completely copied.

Eukaryotic DNA Replication






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Lesson Overview Identifying the Substance of GenesEukaryotic DNA Replication

• The two copies of DNA produced by replication in each chromosome remain closely associated until the cell enters prophase of mitosis.

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


GTCA Song…• https://www.youtube.com/watch?v=ID6KY1QBR5s

https://www.youtube.com/watch?v=ID6KY1QBR5s

https://www.youtube.com/watch?v=ID6KY1QBR5s

Forensic DNA Analysis DNA Replication. Lesson Overview Lesson Overview Identifying the Substance of...

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