Mitosis Cytokinesis and

Why/when are new cells needed?

• Replaces damaged or lost cells (i.e. tissue renewal)

• Permits growth and development of an organism

• Allows for reproduction

Asexual reproduction

Achieved by budding or fragmentation.

• Takes place by

• Binary-fission in prokaryotes

• Mitosis in single-celled eukaryotes

• Results in clones - offspring genetically identical to the parental cell with the exception of mutations (relatively little genetic variation in DNA sequences compared to variation

from sexual reproduction)

Sexual reproduction

• Accomplished by the fusion of two specialized cells, gametes, which are produced by the process of meiosis.

• Results in non-identical daughter cells (and can result in considerable genetic variation)

Reproductive signals

from outside or inside the cell, stimulate cell division

DNA Replication

replication of the genetic material (i.e. the genome)

DNA segregation

one copy of each replicated DNA molecule to each new daughter

cell

Cytokinesis

division of the cytoplasm to form two daughter cells

Bacterial Cell Division: Binary Fission

nutrient concentrations

DNA Replication – begins at the origin of replication (ori)

DNA segregation - active process, prokaryotic

cytoskeleton similar to both actin (in structure) and

tubulin (in function). Ori and associated regions pulled

along this.

Cytokinesis - membrane pinches due to contractile ring

of proteins similar to tubulin (structurally) and actin

(functionally), new cell wall material deposited until

separation completed.

Remember - most prokaryotes have one chromosome, a

single molecule of DNA—usually circular.

Also remember, they can also have one or more small,

circular, extrachromosomal pieces of DNA called

plasmids. These replicate independently of the

chromosome.

Ploidy

the number of Chromosome Sets in a Cell

n =# of chromosomes in a set (1 of each chromosome)

Cells that have two sets of chromosomes,

are diploid (2n) → somatic cells

Cells that only contain a single set of chromosomes, like gametes, are haploid (n)

Meiosis

Takes place in germ cells, giving rise to the gametes

(eggs and sperm), which are

• haploid (n)

• genetically unique (i.e. meiosis contributes to

genetic variation)

Mitosis

• Takes place in somatic cells (non-gamete cells)

• Daughter cells are genetically identical to the

parental cell (i.e. mitosis does not contribute to

genetic variation)

The Organization of DNA in the Eukaryotic Cell

• A chromosome is distinct piece of DNA

• Chromatin consists of fibers of protein and DNA

• DNA associates tightly with proteins called histones

• DNA and protein are packed into discrete units called nucleosomes

Nucleosomes help fold and organize DNA in the nucleus

DNA can be condensed and decondensed

In preparation for cell division DNA will be very

highly condensed

unreplicated chromosome

consists of a single, long DNA double helix wrapped around proteins

replicated chromosome

consists of two copies of the same DNA double helix

condensed replicated chromosomes

consists of DNA condensed around its associated proteins, resulting in a compact chromosome that is 10kx shorter than its og length

SISTER CHROMATIDS

Homologous chromosomes (homologs)

two chromosomes of the same

length, centromere position and staining pattern and which share the same

genes at the same location (loci)

• One is inherited from the father, the other from the mother

Humans have 1 pair of sex\ chromosomes

Presence of the Y chromosome leads to maleness.

Non-sex chromosomes are called autosomes.

The Cell Cycle

• Interphase (G1, S, G2)

• G1: preparation for DNA replication

• non-dividing cells are arrested at this stage of the cell cycle (referred to as G0)

• S phase:replication of genetic material (DNA Synthesis)

• G2: preparation for cell division

• M Phase: cell division

• Mitosis: division of the genetic material

• Cytokinesis: division of the cytoplasm

What does mitosis result in

the division of the cell nucleus

G2 of Interphase

Interphase: G1, S phase & G2

(all events in preparation for

cell division)

prophase

What’s changed?

• Duplicated chromosomes begin to condense

• Mitotic spindle begins to form

• Nucleoli disappear

• Centrosomes move towards opposite pole

prometaphase

• nuclear envelope breaks down

• microtubules penetrate nuclear region, begin to attach to kinetochores of chromosomes

• other microtubules interact from opposite poles

Metaphase

• alignment of duplicated chromosomes along the metaphase plate of the cell

• Centrosomes now at opposite poles

The Mitotic Spindle

Kinetochore microtubules shorten while

non-kinetochore (polar) microtubules

“push” against each other to extend the cell.

At which end to kinetochore microtubules shorten during anaphase?

• microtubules are labeled with a fluorescent dye

• “bleach” region of microtubules (via laser) to mark them

• observe shortening of microtubules relative to mark

What does this mean?

Kineticore microtubules shorten at the kineticore end.

anaphase

• Cohesins connecting sister chromatids cleaved

• microtubules mediate separation of sister chromatids and elongation of the cell

teleophase and cytokinesis

• opposite of prophase

• chromosomes decondense, nuclear envelope reforms spindle fibers disassemble, nucleoli reappear

Cytokinesis in Animal Cells

Actin microfilaments form a contractile ring at the center of the cell inside the plasma membrane

Motor proteins drive the contraction of the ring, forming a cleavage furrow which eventually fuses resulting in 2 separate cells!

Cytokinesis in Plant Cells

Vesicles transport new cell wall material to the

middle of cell

The cell plate begins to form, eventually becoming a complete cell wall separating the 2

daughter cells

1. Three Essential Roles of Cell Division

Growth and Development

Tissue Repair and Renewal

Reproduction

Meiosis – Halving the “genetic deck”

Meiosis I

Unique events in:

• Prophase I – Synapsis & crossing over

• Metaphase I – Alignment of homologs

• Anaphase I – Separation of homologs

Results in two haploid cells

Meiosis II

Separation of sister chromatids

(looks pretty much like mitosis)

Results in four haploid gametes

Prophase I - Crossing Over

Synapsis: homologous chromosomes

are held together along their length

by cohesion proteins to form a tetrad