Mitosis and DNA Replication

SBI 3U

AGENDA● Lesson Learning Goals● Kahoot● Take Up “Cell division & Genetic

material” Worksheet● DNA Replication

Lesson Learning GoalsBy the end of this lesson you will be able to:● Use appropriate vocabulary terms related to

this unit, including mitosis and DNA replication

● explain the different phases of mitosis● Understand how DNA replicates

Cell division & Genetic materialGenetics is the field of biology that involves the study of how genetic information is passed from one

generation of organisms or cells to the next generation.

Cell DivisionThere are a number of different types of cell division, depending on the type of cell that you start with.

● Prokaryotic cells (no nucleus) divide by process called binary fission.

● Somatic (body) eukaryotic cells divide by a process called mitosis○ All somatic cells within an organism have the same number

of chromosomes (humans have 46 chromosomes)● Reproductive eukaryotic cells divide to produce sex cells (gametes)

by a process called meiosis.

Binary Fission● In binary fission , the fully grown parent cell splits into two halves ,

producing two new cells.● Bacteria, cyanobacteria, and most single celled organisms reproduce by binary

fission

Asexual Reproduction

The Cellular TheoryThe Cellular Theory states that:

1. All living things are composed of one or more cells2. Cells are the smallest units of living organisms3. New cells come only from pre-existing cells by cell

division● Daughter cells: cells that form from cell division

and contain the genetic information, DNA, from the parent cell

The Cell Cycle● Cells reproduce through controlled growth and division

in a process called the cell cycle ● All somatic cells (body cells of plants and animals) go

through cell cycles.● Three functions of cell division are:

○ Growth of the organism○ Repair of tissues and organs that have been damaged○ Maintenance to replace dying or dead cells

Stages of the Cell Cycle● Interphase is the stage during which a cell carries out its normal

function, grows, and makes copies if its genetic material in preparation for the next stage of the cycle.

● Mitosis is the stage during which a cell’s nucleus and genetic material divide.

● Cytokinesis begins near the end of mitosis and involves the division of the cell cytoplasm and creation of new cell.○ Cancer is an example of something that interferes with signals

that results in uncontrolled growth and rapid cell division.

Chromosomes

● A chromosome is made up of a DNA and histone protein.● Chromatin is a long thin fiber that is folded and coiled

to form chromosomes.DNA double helix

Histone Protein

Replicated Chromosomes● Consists of two strands of identical chromosomal material called

Chromatids (sister chromatids)● A chromatid is a chromatid as long as it is held in association with a

sister chromatid at the centromere

S phase: chromosomes replicate

Chromatid

Chromatid

Interphase● The cell grows and develops into a mature, functioning cell,

copies its DNA and prepares for division. ● Three subphases of Interphase:

1. Growth 1 (G1) Phase: the major period of growth for a cell. During this phase in the cell cycle, the cell is synthesizing many new molecules in preparation for the next phase in the cell cycle

2. Synthesis (S) Phase: cellular DNA is copied, or replicated. During this phase, DNA exists as uncondensed fibres called chromatin.

3. Growth 2 (G2) Phase: the final phase of interphase. During this phase, the cell synthesizes more molecules prior to mitosis and cell division.

Mitosis● The key purpose of mitosis is the accurate

separation of the cell’s replicated DNA.● The four stages of mitosis are Prophase,

Metaphase, Anaphase, and Telophase

Prophase

● Longest phase● Chromosomes contain the DNA of a cell● Each chromosome in prophase exists as two copies of one

chromosome.● Two chromosome arms are called sister chromatids and

they are joined in the middle by a centromere.● The nuclear membrane breaks down, and the nucleolus

disappears● Spindle fibres are formed from the centrosomes as they

move apart to opposite poles of the cell.● Together, the fibres and the centrosomes are called the

spindle apparatus, which moves and organizes the chromosomes during mitosis

Prophase

Metaphase

● Shortest phase● Spindle fibres guide the

chromosomes to the equator (centre line) of the cell

● Spindle fibres from opposite poles attach to the centromere of each chromosome

Metaphase

Anaphase

● Each centromere splits apart and the sister chromatids separate from each other

○ Chromatids are now considered chromosomes

● Spindle fibers shorten, pulling the chromosomes to opposite poles of the cell

● The cell begins to elongate● At the end of anaphase, one complete set of

chromosomes has been gathered at each pole of the cell

AnaphaseChromosomes

Telophase

● Nuclear membrane reappears ● Chromosomes start to unwind into

strands of less-visible chromatin.● Spindle fibers break down, the nuclear

membrane forms around the new set of chromosomes and a nucleolus forms within each new nucleus.

● Two genetically identical nuclei are present

Cytokinesis● Is the division of the cytoplasm to complete

the creation of two new daughter cells● The daughter cells are now in G1 of Interphase,

and the cycle begins again

Mitosis Review

DNA Replication

Discovery of DNA● The structure of DNA was discovered in the 1950’s when a

group of scientists working at Cambridge University in England came up with a model of its structure

Rosalind FranklinRosalind Franklin worked in the x-ray lab taking pictures of DNA which helped James Watson and Francis Crick discover the overall spiral structure of the DNA molecule.

James Watson and Francis CrickDiscovered the double helix model of DNA (deoxyribonucleic acid)

The macromolecule is composed of two strands that connect in the middle on the sides, forming a two-sided spiral.

The dised of the molecule are composed of phosphate and sugar (deoxyribose), and the “rungs” are 4 nitrogen bases- adenine, thymine, cytosine, and guanine.

Deoxyribonucleic AcidMade up of Nucleotides (DNA molecule) in a DNA double helix.

Nucleotide:

1. Phosphate group2. 5-carbon sugar3. Nitrogen Base

The bases vary depending on if they are purines (like adenine and guanine) or pyrimidines (like thymine and cytosine)

OO = P-O O

AdenineGuanine

Cytosine Thymine

Deoxyribonucleic Acid

The nitrogen bases in a DNA molecule attach together with hydrogen bonds. These bonds hold the bases together in a certain order, so that adenine binds to thymine, and cytosine binds to guanine.

This is known as the “base-pairing rule”

Base-Pairings

Purines Pyrimidines Base Pairs # of H-Bonds

Adenine (A) Thymine (T) A=T 2

Guanine (G) Cytosine (C) C=G 3

Base-Pairings

Deoxyribonucleic Acid

A strand of DNA on one side of the double helix is said to have a “complementary strand”- one where the base sequence can be determined using the base-pairing rule (Chargaff’s Rule).

Eg. if DNA strand has the sequenceAGCTCGS

Then its complementary strand will be TCGAGCT

Where and when does DNA replication take place?

Replication occurs in the S (synthesis) phase in interphase of the cell cycle. This only occurs in the nucleus of eukaryotes.

DNA ReplicationDNA replication is known as a semi-conservative replication, where new DNA strands are formed from pre-existing strands that are already found in the cell.

A double strand DNA molecule “unzips” free floating nucleotides attach to the exposed strands that results, and these then build complementary strands on the exposed strands.

Origins of Replication

1. Replication forks: hundreds of Y-shaped regions of replicating DNA molecules where new strands are growing.

2. Replication Bubbles:

a. Hundred of replicating bubbles (eukaryotes).b. Single replication fork (bacteria)

DNA Replication

Necessity of DNA Replication

● DNA replication ensures that the DNA of a cell is copied exactly before cell division occurs.

● Daughter cells produced at the end of cell division will have an identical set of DNA to each other, and the cell that produced them.

● This ensures genetic continuity.