16_Lecture_Presentation_DNA Part 2.ppt

(1)

Synthesizing a New DNA Strand

• Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork

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

• The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in

human cells

(2)

• Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate

• dATP supplies adenine to DNA and is similar to the ATP of energy metabolism

• The difference is in their sugars: dATP has deoxyribose while ATP has ribose

(3)

Figure 16.14

New strand Template strand

Sugar Phosphate Base Nucleoside triphosphate DNA polymerase Pyrophosphate 5 5 5 5 3 3 3 3 OH OH OH

P _P_i

2 P _i

(4)

Antiparallel Elongation

• The antiparallel structure of the double helix affects replication

(5)

• Along one template strand of DNA, the DNA polymerase synthesizes a leading strand

continuously, moving toward the replication fork

(6)

Leading strand

Lagging strand

Overview

Origin of replication Lagging strand Leading strand Primer Overall directions of replication Origin of replication RNA primer Sliding clamp

(7)

Figure 16.15a

Lagging strand

Overview

Origin of replication Lagging strand

Leading strand Primer

(8)

Origin of replication

RNA primer Sliding clamp

DNA pol III

Parental DNA

3

5

3

5

3

5

3

5

3

(9)

• To elongate the other new strand, called the

lagging strand, DNA polymerase must work in the

direction away from the replication fork

• The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase

(10)

Origin of replication Overview Leading strand Leading strand Lagging strand Lagging strand Overall directions of replication Template strand RNA primer for fragment 1

Okazaki fragment 1

RNA primer for fragment 2

(11)

Figure 16.16a

Origin of replication Overview

Leading strand Lagging strand

Lagging strand

Overall directions of replication

(12)

Template strand

3

5

(13)

Figure 16.16b-2

RNA primer for fragment 1 3

3

5

(14)

Okazaki fragment 1 3

3

5

(15)

Figure 16.16b-4

Okazaki fragment 1 RNA primer

(16)

(17)

Figure 16.16b-6

for fragment 2 Okazaki fragment 2

(18)

Overview Leading strand Origin of replication Lagging strand Leading strand Lagging

strand _{Overall directions} of replication

Leading strand

DNA pol _III

DNA pol III Lagging strand

DNA pol I DNA ligase Primer Primase Parental DNA 5 5 5 5 3 3 3 3 3

3 2 1

(19)

Figure 16.17a Overview Leading strand Origin of replication Lagging strand Leading strand Lagging

strand _{Overall directions} of replication

Leading strand

DNA pol III

(20)

Overview Leading strand Origin of replication Lagging strand Leading strand Lagging

strand Overall directions of replication

Leading strand

Primer

DNA pol III

DNA pol I

Lagging strand DNA ligase 5 5 5 3 3

3 3

4

(21)

The DNA Replication Complex

• The proteins that participate in DNA replication

form a large complex, a “DNA replication machine”

• The DNA replication machine may be stationary during the replication process

• Recent studies support a model in which DNA

polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules

(22)

Parental DNA

DNA pol III

Leading strand

Connecting

protein Helicase

Lagging strand DNA

pol III

Lagging strand template 5 5 5 5 5 5

3 ₃_

3 3

3

(23)

Proofreading and Repairing DNA

• DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides

• In mismatch repair of DNA, repair enzymes

correct errors in base pairing

• DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo

spontaneous changes

• In nucleotide excision repair, a nuclease cuts

out and replaces damaged stretches of DNA

(24)

(25)

Evolutionary Significance of Altered DNA

Nucleotides

• Error rate after proofreading repair is low but not zero

• Sequence changes may become permanent and

can be passed on to the next generation

• These changes (mutations) are the source of the genetic variation upon which natural selection

operates

(26)

Replicating the Ends of DNA Molecules

• Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes

• The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of

replication produce shorter DNA molecules with uneven ends

(27)

Figure 16.20

Ends of parental DNA strands

Leading strand Lagging strand

Last fragment Next-to-last fragment Lagging strand RNA primer

Parental strand _{Removal of primers and} replacement with DNA where a 3 end is available

Second round of replication

Further rounds of replication New leading strand

New lagging strand

(28)

Ends of parental DNA strands

Leading strand Lagging strand

Last fragment Next-to-last fragment

Lagging strand RNA primer

Parental strand _{Removal of primers and}

replacement with DNA

where a 3 end is available

3

5 5

(29)

Figure 16.20b

Second round of replication

Further rounds of replication New leading strand

New lagging strand

Shorter and shorter daughter molecules 3

3

(30)

• Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called

telomeres

• Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules

(31)

Figure 16.21

(32)

• If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce

(33)

• The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions

• There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist

(34)

Concept 16.3 A chromosome consists of a

DNA molecule packed together with proteins

• The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein

• Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein

(35)

• Chromatin

,

a complex of DNA and protein,

is found in the nucleus of eukaryotic cells

• Chromosomes fit into the nucleus through

an elaborate, multilevel system of packing

(36)

DNA double helix (2 nm in diameter)

DNA, the double helix

Nucleosome

(10 nm in diameter)

Histones

Histone tail

H1

(37)

Figure 16.22b

30-nm fiber

Loops Scaffold

300-nm fiber

Chromatid (700 nm)

Replicated chromosome (1,400 nm) Looped domains

(300-nm fiber) Metaphase

(38)

(39)

Figure 16.22d

(40)

(41)

Figure 16.22f

(42)

(43)

• Chromatin undergoes changes in packing during the cell cycle

• At interphase, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping

• Though interphase chromosomes are not highly condensed, they still occupy specific restricted regions in the nucleus

(44)

5



(45)

(46)

(47)

Figure 16.23c

5



(48)

• Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis

• Loosely packed chromatin is called euchromatin

• During interphase a few regions of chromatin (centromeres and telomeres) are highly

condensed into heterochromatin

(49)

• Histones can undergo chemical modifications that result in changes in chromatin organization

(50)

Sugar-phosphate backbone

Nitrogenous bases

Hydrogen bond

G

G G

G

C

C C

C

A

T T

(51)

Figure 16.UN03

DNA pol III synthesizes

leading strand continuously

Parental

DNA _{DNA pol}_III_{starts DNA}

synthesis at 3 end of primer, continues in 5  3 direction

Origin of replication

Helicase

Primase synthesizes a short RNA primer

DNA pol I replaces the RNA primer with DNA nucleotides 3 3 3 5 5 5 5

(52)

(53)

(54)

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