Forensic DNA Analysis DNA Replication Lesson Overview Identifying

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

Lesson Overview Identifying the Substance of Genes

Lesson Overview Identifying the Substance of Genes 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. 5 end Hydrogen bond 5 end 3 end (b) Partial chemical structure 3 end

Lesson Overview Identifying the Substance of Genes 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

Lesson Overview Identifying the Substance of Genes 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.

Lesson Overview Identifying the Substance of Genes 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 A T C G C G T A T A T G C G C (a) Parent molecule (b) Separation of strands (c) “Daughter” DNA molecules, each consisting of one parental strand one new strand

Lesson Overview Identifying the Substance of Genes 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.

Lesson Overview Identifying the Substance of Genes The Replication Process • 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. Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0. 25 µm Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes

DNA Replication Steps: Getting Started 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 Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0. 25 µm Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Lesson Overview Identifying the Substance of Genes The 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”)

Lesson Overview Identifying the Substance of Genes Enzymes in DNA Replication Primase 3 5 Helicase 5 RNA primer 3

DNA Replication Steps: Synthesizing a New DNA Strand 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: Antiparallel Elongation 5. 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 3 end of a growing strand elongates in the 5 to 3 direction 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

Lesson Overview Identifying the Substance of Genes 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.

Lesson Overview Identifying the Substance of Genes Eukaryotic DNA 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. Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0. 25 µm Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes

Lesson Overview Identifying the Substance of Genes Eukaryotic 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.

Lesson Overview Identifying the Substance of Genes

Lesson Overview Identifying the Substance of Genes GTCA Song… • https: //www. youtube. com/watch? v=ID 6 KY 1 QBR 5 s