Cell Cycle and Cell Division

Cell Division• Creates two identical “daughter” cells

roughly ½ the size of the parent cell

• Before the cell divides: – DNA of the cell (chromosomes) must be

copied in interphase– chromosomes must be separated in

mitosis

• Actual cell division is cytokinesis.

Why do cells have to divide

• Surface area/volume ratio

• Too much demand placed on DNALarger the cell, more proteins need

to be made!Only one copy of each DNA

molecule in a cell!• Signals from outside the cell

Cell Division• In multicellular organisms, the somatic (body cells) cells undergo mitosis.

• Germ cells (cells destined to become sperm eggs) divide by a related process called meiosis.

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

Forms of DNA• DNA + Proteins = Chromosomes

• Most of the time, DNA is “unspooled” into loose strands – called chromatin– can be used to provide instructions in this

form.

• Before a cell divides, the DNA winds around histone proteins and becomes visible as “chromosomes”

• The chromosomes can be counted in this form.

Structure of DNA

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Chromosome number

• Eukaryotic organisms have a specific number of different types of chromosomes.

• They have two of each type – so chromosomes come in pairs.

• Cells with chromosomes present in pairs are said to be diploid (2N)– one of each is haploid

Chromosome identity

• Each chromosome type can be identified by shape and size.

• If stained, characteristic patterns of bands can be seen as well.

• A karyotype is a display of all of an individual’s chromosomes arranged by type.– Can be used to identify major genetic

disorders.

Chromosome number

• Each organism has a different number of chromosomes:Camel 70 Chicken 78 Opossum 22 Housefly 22 Bat 44 Corn 24Lentil 14 Rice 24Goat 60 Barley 14 Apple 34 Lettuce 12

HUMANS HAVE 46! 23 different pairs

How many chromosomes?• In a diploid cell, one set of chromosomes

comes from the mother and another from the father.

• Each human gets 23 from each parent (46 total)– 44 are autosomes (general body

characteristics) and two are sex chromosomes (determine sex and carry general characteristics)

Being Diploid

• The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical.– For example, a gene for eye color at one

locus (location) on the father chromosome may code for green eyes, while the same locus on the mother chromosome may code for brown.

Before Cell Division, each chromosome must replicate!

• Individual chromosomes replicate and form sister chromatids.

• Each sister chromatid is destined for one of the two resulting daughter cells

• Sister chromatids together are considered one chromosome.

• After separation, the two independent copies are sister chromosomes.

• Each sister chromosome goes to a different cell.

After Replication Chromosomes

The Cell Cycle• Sequence of events a

cell goes through as it grows and divides

• (G1 Phase) Cell grows and synthesizes proteins and new organelles

• (S Phase) Chromosomes replicate

• (G2 Phase) Organelles and molecules used in cell division are produced

• (M Phase) mitosis (chromosomes) and cytokinesis (cytoplasm) are divided.

Cell Division M-Phase• Consists of TWO steps (Mitosis and

Cytokinesis)

• Mitosis process by which a cell separates its duplicated genome into two identical halves. Mitosis only separates the newly replicated chromosomes; DNA replication does not occur during mitosis. – broken down into five phases: (PMAT)

Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis which divides the cytoplasm and cell membrane.

Mitosis Prophase• Longest phase of mitosis• Chromosomes condense

(become visible)• Centrioles (in cytoplasm)

separate and move to opposite sides of cell

• Nuclear membrane breaks-down

• Microtubule structure called the spindle develops (attaches from centrioles to chromosomes

Chromosome Structure

• Prior to separation, the two sister chromatids are attached together in a specialized region of the chromosome known as the centromere.

Mitosis Metaphase• Chromosomes line-

up along center of cell (metaphase plate)

• Each chromosome is connected to its centromere by a spindle fiber

Mitosis Anaphase

• Sister chromatids separate into separate chromosomes

• Separated chromosomes pulled to opposite sides

Mitosis Telophase• Chromosomes move

together at opposite ends of the cell and become less condensed

• Spindle breaks apart• Two new nuclear

membrane form• Result is one cell

with 2 nuclei!

Cytokinesis• Remember, NOT part of mitosis• Animals

– Cell membrane pinches off cytoplasm into two equal parts at a region called the cleavage furrow

• Plants– Cell Plate develops between two

new nuclei which grows into a separating membrane and ultimately a separating cell wall

Mitosis Animation

Cell Cycle Animation

• http://www.cellsalive.com/mitosis.htm

Limits to Division?

• Problem with eukaryotes is that they have to replicate linear chromosomes. The polymerase enzyme can’t work all the way to the end, so the chromosome gets shorter with each round of replication.

• Solution: use special ends called telomeres that don’t contain genes. “Expendable” DNA.

• When cell runs out of telomere, can’t divide any more.

• May act as “replication counter” for cell.

Regulation of Cell Cycle• Not all cells move through cell cycle at same

rate

– Bone marrow cells/skin cells continuous division

– Nerve and muscle cells seldom or never

Cycle Regulators• The cell cycle is regulated by special proteins called

cyclins and cyclin-dependent kinases. • High concentrations of cyclin influences a cell to

divide.• Internal Regulators proteins that respond to

internal stimuli: cell cycle checkpoints!– Ex. Cell will not enter mitosis until all chromosomes are

replicated

• External Regulators proteins that respond to external stimuli– Ex. Cell will begin to divide rapidly after injury– Ex. When dividing cells come in contact with adjacent cells,

division will slow

WHEN CELLS GO BAD!• Cell loses the ability to

control growth; cancer is the result.

• Cells do not respond to chemical signals that tell them to stop growing.

• Do not differentiate.• Form masses of abnormal

cells called tumors that damage surrounding tissues. •May rebuild telomeres

and become “immortal”

Meiosis• We know that regular somatic (body)

cells contain TWO sets of chromosomes (diploid/ 2N)

• When a sexually reproducing organism produces gametes (sex cells) they must somehow separate these pairs of chromosomes so gametes only get one set.

• WHY?

Ex. Humans• Normal Diploid (2N) somatic cell

contains _____ chromosomes (_____ pairs)

• Gametes (sperm and egg cells) need to contain _________ chromosomes.

• We generate these HAPLOID (N) cells through the process of meiosis!

Steps of Meiosis

• Divided into two distinct stages

–Meiosis I

–Meiosis II

• Starts with one diploid cell and ends with 4 haploid daughter cells

• Before meiosis begins, DNA undergoes replication just like in mitosis!

Meiosis I: Prophase I• Appearance of the

chromosomes, the development of the spindle, and the breakdown of the nuclear membrane (envelope).

• Each replicated chromosome pairs up with its corresponding homologous chromosome

• Paired chromosomes (4 chromatids) form a tetrad

Tetrads and crossing over

• It is during this alignment that chromatid arms may overlap and temporarily fuse (chiasmata, or synapsis), resulting in crossovers

• Segments of homologous chromosomes may switch places where overlap occurs.

What is Crossing Over?

• Paired-up homologous chromosomes, may exchange portions of their chromatids

• Advantage?

Meiosis I: Metaphase I

• Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis. In the latter, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate.

Meiosis I: Anaphase I

• During Anaphase I the homologous pairs separate from each other and move along the spindle fibers to each pole of the cell.

End of Meiosis I

• At the end, each daughter cell has a single set of chromosomes, half the total number in the original cell where the chromosomes were present in pairs.

• While the original cell was diploid, the daughter cells are now haploid. This is why Meiosis I is often called reduction division.

Meiosis II

• Meiosis II is quite simple in that it is simply a mitotic division of each of the haploid cells produced in Meiosis I.

• There is no Interphase between Meiosis I and Meiosis II

Meiosis II: Prophase II

• A new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell.

Meiosis II: Metaphase II

• All the chromosomes in the two cells align with the metaphase plate.

Meiosis II: Anaphase II

• Sister chromatids separate as they are pulled by spindle fibers

Meiosis II: Telophase II

• A cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane (envelope).

• When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.

Meiosis in Males and Females• In male animals,

meiosis results in the formation of 4 ___________ cells

• In female animals, meiosis results in the formation of one _______ cell and three small polar bodies which die.

Advantages/Disadvantages of sexual reproduction?

• Recombination of maternal and paternal chromosomes in the gamete results in genetic variation among the offspring. In an environment which changes, this allows the process of natural selection to occur.