Cell division

Cell Division

Cells

Somatic• Regular Cells• Have 46

Chromosomes• 2N Diploid – set of

chromosomes (two genomes)

Gametes• Sex Cells (egg/sperm;

Pollen (sperm)/ovule (eggs)

• Have 23 Chromosomes

• N Haploid – one chromosome (a genome)

Cell Cycle

• The cycle of growth, mitosis and cytokinesis.

Interphase

• The longest phase of the cell cycle

• Gap one- Cell prepares for DNA and chromosome replication

• Synthesis- the DNA is replicated giving two exact copies called sister chromatids.

• Gap two- cell grows for division and organelles duplicate.

Mitosis

Early Prophase

• The DNA and proteins condense and become visible.

• Centrioles begin to move apart.

• Nuclear envelope begins to disappear.

Late Prophase

• Centrioles are on opposite sides of the nucleus.

• Microtubules called spindle fibers begin to form.

• Chromosomes are in homologous pairs.

• Nuclear envelope is gone.

Metaphase

• Homologous Pairs held by a centromere.

• The Kinetochores allow the spindle fibers to attach at the centromere

• The spindle fibers push and pull the homologous pairs to the middle of the cell called the metaphase plate.

Anaphase

• The centromeres split and daughter chromosomes migrate to opposite ends of the poles.

• Centrioles and spindle fibers pull the daughter chromosomes towards the poles.

• Cytokinesis begins

Late anaphase

Telophase

• Nuclear envelope begins to reform around each group of chromosomes.

• Chromosomes begin to become less visible.

• Cytokinesis pinches in and cell membrane or cell plate begins to form.

Animal Cell Plant Cell

Cell Membrane

Cytokinesis

• The division of the cytoplasm.

• This process compartmentalizes the two new nuclei into separate daughter cells.

Animal Cell in Cleavage Furrow

Cell Plate formation

Mitosis

• Maintains a constant amount of genetic material and a constant set of genes from generation to generation.

• No genetic material is lost.

• One copy of each duplicated chromosome segregates into a daughter cell.

• 2 cells at end of Mitosis.

Mitosis Chromosome #

• If a parent cell is diploid, this results in two genetically identical progeny diploid cells, each with two sets of chromosomes.

Or

• If the parent cell was haploid the daughter cells will also be haploid.

Mitosis and Cancer

• Cell division is controlled by specific chemical reactions that act as check points.

• When these signals are ignored by a cell, cancerous cells occur.

• Cancer is a disorder where the body cells lose control of the growth of the cell.

Cells

• Cells come from other cells.• When an organism develops progeny cells

will begin to differentiate.• Cell differentiation means that the cells

become specialized as the fetus develops

or• they express different segments of genes

then other cells and develop into a new cell form.

Cells

• Cells have a lifespan.

• Some cells become terminally differentiated, they will go through Apoptosis or death and will be replace by younger cells.

• The younger cells come from stem cells, which are a cell in the tissue that can self-renew.

• Stem cells change or differentiate into specialized cells.

Cells

• Totipotent- cells that have the ability to differentiate into any cell type. A true totipotent cell is the first cells in an zygote or fertilized egg.

• Pluripotent can develop into many kinds of cells but not all.

• Multipotent cells can produce only the type of cells that are unique to a tissue.

Molecular Control of the Cell cycle – Internal Regulators

• Checkpoints are in place at the different points in the cell cycle.

• Checkpoints look for damaged cells to repair or terminate.

• G1 to S – determines if cell is ready for Synthesis

• G2-M – Checks for all DNA to be replicated and SA:V ratio is correct.

• If environment is not favorable the cycle will stop.

• M checkpoint – determines if the chromosomes are attached to the spindles for chromosome separation.

Proteins are the Checkpoints

• Cyclins (protein) and Kinases (enzyme) are the regulators to the cell cycle.

• The regular chemical reactions between these molecules keeps the cycle active or inactive.

• Complicated signaling system.

External Regulators

• Growth Factor- molecule that stimulates cell division

• Growth Factors can come from hormones or from the cells around the dividing cells.

• Healthy cells will only grow and divide when there is a balance between growth factor and inhibitory signals.

Cancer

• At current rates, over a third of the people who read your textbook will die of cancer

• Tumors or neoplasms are tissue masses caused from non-normal cell growth.

• Benign – not life threatening

• Malignant – life threatening and disruptive to other tissues.

Mitosis Review

• Mitosis is for increasing cell number.• Mitosis is for individual organism growth and

repair.• The loss of control of the Mitotic cycle leads to

cancer.• The result is two identical cells with the same

number of chromosomes as the Parent Cell.• In somatic cells – Nothing to do with heredity.

Meiosis

• Two successive divisions of a diploid nucleus with only one replication of DNA.

• In animals the result is the formation of haploid gamete (gametogenesis)

Meiosis I

• First, interphase and DNA replication

• Meiosis I is the first nuclear division.

• Broken in to stages:– Prophase 1– Metaphase 1– Anaphase 1– Telophase 1– Cytokinesis

Prophase 1 is Different in Meiosis

• Homologous pairs exist (homologous is two sister chromatids connected at a centromere)

Sister Chromatid

Centromere

Prophase 1

• Synapsis: the formation along the length of the chromatids of a zipperlike structure which aligns the two homologs precisely, base pair by base pair.

Prophase 1

• Tetrad: four chromatids in a synapsed set of homologous chromosomes.

Prophase 1

• Crossing-over: the exchange of chromosome segments at corresponding positions along pairs of homologous chromosomes.

Crossing-over

• The exchange of corresponding genetic information produces new gene combinations in a population.

• Usually, no loss of addition to the genetic material because it is a reciprocal exchange along the synapse.

• Chiasma is the site at where the crossing-over occurred and is visible in late prophase 1.

Recombination

• A chromosome that emerges from meiosis with a combination of genes that differ from what the chromosomes started with is a recombinant chromosome.

• This is important in genetic recombination - variety in genetics in a species.

Prophase 1

• After Genetic Recombination occurs the remaining steps in Prophase 1 are the same as Mitosis.

Metaphase 1

• The homologs line up two by two on the equatorial plane versus one by one in Mitosis.

Anaphase 1

• The tetrad separates, so that the homologous pairs disjoin and migrate toward a pole.

Anaphase 1 Difference from Meiosis

• At this time, the segregated sister chromatid pairs remain attached at the centromere.

• Sister Chromatids stay together in meiosis, where as they would separate in Mitosis.

Telophase 1

• The nuclear envelope is formed around the homologous pairs.

• Cytokinesis begins.

Cytokinesis

• Two cells that are 2N.

• Ready to start Meiosis II

Meiosis II

• The second meiotic division similar to Mitosis.

Meiosis II, What is Different?

• The is no Interphase, NO SYNTHESIS of DNA.

• You just go right into prophase II!

• End products are 4 haploid cells from one original diploid cell.

• Each progeny cell has one chromosome for each homologous pair of chromosomes.

Meiosis II, What is the Difference?

• The chromosomes are not exact copies of the original chromosomes due to recombination.

• Recombination generated more variation in genetic combinations.

Independent Assortment

• The factors for different traits assort independently of one another.

• Basically this means, genes on different chromosomes separate independent of each other in Meiosis.

Crossing-over and Independent Assortment.

• If genes are located close together on a chromosome they have less chance to cross-over and assort independently.

• If genes are located farther apart on the chromosomes they have are more likely to independently assort and recombine.

• This can mean that some genes are almost always guaranteed to be on the same chromosome or found together (linkage).

Read Chapter 10

Chromosomes

• Two Types: Autosomal and Sex

• 22 Regular Autosomal Chromosomes and 2 Sex Chromosomes in Humans.

Chromosome Structure

• DNA/ Gene

• Chromatin: sustainable material in a cell nucleus (DNA + Proteins)– Proteins – Histones and Nonhistones– Histones- Protein with positive charge that the

DNA is atracted to and coild around it.– Nonhistone- Proteins that play various roles in

DNA replication and determine physical structure of the chromosome.

Chromosome Structure

• Nucleosome: is the basic unit of the chromatin ( the DNA wrapped around the histone with linkers between histones.

• The interactions between the histones make the DNA coil back onto itself to create a chromosome.