Lecture 3

Lecture 3

Dr. M. A. Fouad

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ثيللاب ةيحصلا مولعلا ةيلك

ةماعلا ةحصلا مسق

• Cell cycle has four major phases:

– Interphase (G1, S, G2) – Mitosis (M) phase

• The period between two

successive mitoses is known as the interphase of the cell cycle.

G1:

• The Cell grows: The cytoplasm increases in volume (proteins, carbohydrates and lipid synthesized).

G0:

• Cells which have stopped dividing, such as neurons, usually arrest in this phase and are said to have entered a non-cyclic stage

known as G0

• DNA Replication takes place.

• The cell synthesis a complete copy of its genetic material

• Each chromosome now consists of two sister chromatids.

• The cell grows more

• Cell prepares itself for mitosis

• Nuclear division (mitosis) followed by a cell division (cytokinesis)

7. ______ consists of mitosis and interphase:

A. Division

B. The cell cycle C. The life cycle D. Oogenesis

E. Spermatogenesis

The period between successive mitosis is known as _______?

A. Interphase B. Synthesis

C. Gap 0 phase

D. none of the above

Interphase starts with _________?

A. Gap 0 phase.

B. Gap 1 phase.

C. Synthesis phase.

D. Gap 2 phase.

DNA of each chromosome is replicated in _________

A. Gap 0 phase.

B. Gap 1 phase.

C. Mitosis phase.

D. Synthesis phase.

The name of phases (2) is ________

A. Gap 0 phase.

B. Gap 1 phase.

C. Mitosis phase.

D. Synthesis phase.

• DNA replication is the process of

producing two identical copies

from one original DNA molecule.

• Before cell division DNA must be replicated so that every daughter cell receive a complete copy of the genome

• DNA replication is the basis for the transfer of genetic information

from one generation to the next

one (inheritance).

❑ DNA is composed of two complementary strands.

❑ The two strands separate and each strand of the original DNA molecule

serves as template for the production of

a new complementary strand

^.

• The process of DNA replication is termed semiconservative.

• The term semiconservative replication means that:

→ in each of the resultant daughter DNA molecules there is one old and one new strand.

❑ The process of replication

begins in the DNA molecules at multiple points called “origins of replication”.

❑ The copying progresses in both directions, forming bubble-

shaped structures called

“

replication bubbles”.

❑ Eventually, all the bubbles

merge, yielding two double-

stranded DNA molecules.

The Enzyme DNA

Helicase unwinds and separates the 2 parental DNA strands by breaking the weak hydrogen

bonds producing single strand templates.

Single-strand binding proteins (SSBs) keep strands apart.

Double Strand

DNA

Single strand

binding Proteins (SSBs)

Replication Fork

Double Strand

DNA

Single strand binding Proteins (SSBPs)

• The separation of the two strands results in the

formation of the Y-shaped structures known as

replication forks.

• DNA polymerase cannot start a new strand de novo, but must have a

free 3’-end

to

extend.

•

Primase

does not require a free 3`- end to initiate synthesis.

•

Primase

synthesize short RNA primers that serve as a starting points for DNA synthesis.

❑

DNA polymerase

is the enzyme responsible for the synthesis of the new DNA strands.

❑DNA polymerase synthesizes the new DNA strand by

sequentially adding new nucleotides to the 3’ end of the

growing nascent chain via the creation phosphodiester bonds.

DNA Polymerase Template strand

❑The synthesis of both DNA strands occurs in the 5 ' to 3 ' direction.

❑One strand, known as the leading strand, is synthesized as a continuous process.

❑The other strand, known as the lagging strand, is synthesized in

pieces called Okazaki fragments, which are then joined together as a continuous strand by the enzyme DNA ligase

1. Replication proceeds in a ____ to _____

direction:

A. 3' to 3'

B. 3' to 5'

C. 5' to 3'

D. 5' to 5'

Helicase

DNA

Polymerase

Leading strand Lagging strand Synthesized continuously Synthesized

discontinuously DNA polymerase moves

toward the replication fork

DNA polymerase moves away from the replication fork

No Okazaki fragments produced

Produce Okazaki fragments

• Okazaki fragments, are joined together as a continuous

strand by the enzyme DNA ligase.

Quiz?

Test Your Knowledge

2. The process by which DNA is copied to produce two daughter DNA molecules is called?

A. reproduction B. replication

C. translation

D. transcription

3. DNA replication is

A. semiconservative.

B. conservative.

C. dispersive.

D. semidispersive.

4. In replication of DNA, the 2 parental DNA strands are separated and un-winded by:

a) ribase b) ligase

c) primase d) helicase

e) polymerase

5. Okazaki fragments are used to elongate

A. the leading strand B. the lagging strand C. both strands

D. None of the above

6. DNA fragments of the lagging strand are joined by

A.Telomeres

B. Centromeres C.Helicases

D.Ligases

E. Polymerase

7. In DNA replication, which enzyme Catalyze the synthesis of new strands of DNA

A. DNA ligase

B. DNA polymerase C. Helicase

D. Primase

7. Newly synthesized DNA molecule contains:

A. Both new strands B. Both old strands

C. One new and one old strand D. Only one strand

E. One Ribose and one Deoxyribose strand

8.Study the diagram and Fill in the spaces: