Chapter 2

Mitosis and Meiosis

Genetic Material

• For almost all organisms it is DNA with the exception of a few viruses that use RNA

• To transfer the genetic material to new cells or offspring, eukaryotes use 2 processes:– Mitosis – 2 cells with same # chromosomes– Meiosis – 4 cells with ½ the # chromosomes

• Both use similar mechanisms but have different outcomes

Cell Structure Tied to Genetic Function

• Many cellular components play a role in cell division– mitochondria and chloroplasts are unique in

they have their own DNA

• Eukaryotes and prokaryotes are different in how they replicate

• Gene expression is what allows for specific variation in a given cell

Cell Structure • Plasma membrane• Plants have cell wall

and animals have cell coats that aid in biochemical recognition

• Nucleus – chromatin and chromosomes

• Nucleolus – rRNA synthesis and ribosome formation– nucleolus organizing

region (NOR)

• Other membranous organelles

Bacterial Structure

• No membranous organelles

• Nucleoid – region that DNA is in the cell – may be attached to the

plasma membrane– DNA does not undergo

compaction into chromosomes

• No nucleolus but does have genes for rRNA

Additional Structures

• From the plasma membrane to nuclear membrane is the cytoplasm

• Remove the membrane bound organelles from the cytoplasm – cytosol– colloidal material that contains proteins, ribosomes and

cytoskeleton• Cytoskeleton consists of system of tubules and filaments

– microtubules, actin and intermediate filaments– aids in cell shape, motility and anchoring various organelles

• Endoplasmic reticulum – compartmentalizes the cytoplasm and increases surface area– SER – fatty acid synthesis and phospholipids– RER – studded with ribosomes for protein synthesis

More Additional Structures

• Mitochondria and chloroplasts – cellular respiration and photosynthesis, respectively– both can duplicate themselves – have DNA a little

different than in nucleus– Endosymbiont Hypothesis

• Centrioles – found in centromere and associated with spindle fibers that function in mitosis and meiosis– derived from basal body – spindle fibers play a role in the movement of

chromosomes as thy separate - microtubules

Chromosomes

• Distinctive shapes and lengths when visible

• Each has a condensed or constricted region called the centromere which helps establish the general appearance

• Centromeres are placed at different points along the chromosome – depending on the placement, create arms of different lengths

Shorter arm is always placed above the centromere and called the p (petite) armLonger arm is always placed below the centromere and called the q armMust know name and location of the centromere

Observations of Mitosis

• Somatic cells (all but sex cells) have identical numbers of chromosomes – diploid or 2n

• Nearly all chromosomes exist in pairs – pairs are homologous – have identical features– exception is in human males – have one of each sex

chromosomes, females have 2 Xs, X and Y act as homologous pairs during meiosis

• Exceptions:– bacteria and viruses usually have 1 chromosome– some organisms live in a haploid state (1n)

Karyotype

• Pairing of chromosomes to look at structure• See as a double structure – sister chromatids

that are connected at centromere– will divide during mitosis

Haploid Numbers

• n = genetic information that constitutes the genome – genes and non-coding DNA

• Contain identical genes on chromosome pair– genes are present at a locus (loci) – identical in genetic potential

• 1 from ♂ and 1 from ♀ - called biparental inheritance

• In the population, many different alternative forms called alleles

• During meiosis, only 1 from each homologous pair is placed in the gamete or spore– fertilization forms a diploid zygote

Mitosis

• Critical to all eukaryote cells, in protozoans and some fungi and algae use as the basis of asexual reproduction

• Partitions chromosomes during nuclear division or karyokinesis – follows exact copying of DNA

• Karyokinesis is followed by cytokinesis which causes the cell to divide into 2 new daughter cells– half the size of parent cell and needs to grow but the

nucleus is about the same as parent

Cell Cycle • Occurs from the completion of one division to the beginning of the next division

• Interphase is the step between cell division that gets the cell ready to divide– includes the S phase –

DNA replication

– 2 gap phases (G1 and G2) – one before and after DNA replication

• time of intense metabolic activity to get ready for cell division

Time Line

• Mitosis occupies a small portion of the cell cycle

• S phase and G2 are fairly constant

• G1 is the most variable as cells can stay here until signals for cell division comes– may actually spin out to G0

which is a metabolically active stage without cell division

Phases of the Cell Cycle

Interphase

• No visible chromosomes

• Mitosis happens after G1, S and G2 finished

• Not really individual steps but subdivided into discrete stages for ease of learning– M phase consists of prophase,

prometaphase, metaphase, anaphase and telophase

Prophase

• ½ the time of mitosis • 2 centrioles move to opposite ends of cells – outside the

nuclear membrane – 1 new and 1 old• Spindle fibers organized by centriole and run between

the centrioles – animals have but not in plants, fungi or some algae

• Nuclear membrane disintegrates• Chromosomes become visible – double structure joined

at the centromere – each called a chromatid, genetically identical, randomly dispersed where nucleus was

Prometaphase and Metaphase

• Prometaphase is a period of chromosome movement

• Metaphase is strictly configuration after migration

• Migration of chromosomes to equatorial plate – metaphase plate – perpendicular to axis formed by the spindle fibers– movement caused by spindle fibers attaching to

kinetochore which is associated with centromere– kinetochore is multi-layer plates of proteins that

microtubules attach to and pull chromosomes to the pole

Anaphase

• Shortest stage• Sister chromatids disjoin and migrate to opposite

poles – centromere must split to create the daughter chromosome– dependent on centromere-spindle fiber attachment– use motor proteins to move – uses ATP hydrolysis and

are called molecular motors

• Migration leads with centromere• Critical step to make sure that each new cell gets

a full complement of chromosomes

Telophase

• Final stage• Cytokinesis is the significant event – partitions

the cytoplasm – plants make a cell plate that forms at the metaphase

plate which eventually becomes a cell wall– animals have a constriction in the center making a

cell furrow

• In late telophase, the reversal of prophase takes place

• Cell re-enters interphase

Meiosis

• Process by which we reduce the number of chromosomes from 2n to n– sexual reproduction will reconstitute the 2n number

• Each gamete or spore will have only 1 copy of each chromosome– allows for genetic diversity as each time a sex cell

goes thru meiosis will result in a different assortment of chromosomes

– also can add diversity by crossing over – swapping of portions of adjacent chromosomes in the homologous pair

• it is part mom and part dad - mosaic

• Mitosis maintains the 2n quantity of genetic material

Mitosis vs.

Meiosis

Overview of Meiosis

• 2 divisions with several stages• Homologous chromosomes form pairs –

synapse– initially called a bivalent which eventually becomes a

tetrad (4 chromatids as both chromosomes have been duplicated)

• Division 1 – reductional division – go from 2n to n number of chromosomes– tetrads separate forming dyads

• Division 2 – equational division – chromosome number stays equal but the sister chromatids split to make monad– potentially forms 4 cells

Prophase I

• Chromatin condenses into chromosomes

• Homologous chromosomes undergo synapsis

• Crossing-over of synapsed homologous chromosomes

• Broken into 5 additional stages for ease of discussion

5 Stages of Prophase I

• Leptonema – leptotene – chromatin condenses, still linear, have chromomeres that is thought to aid in homolog recognition

• Zygonema – zygotene – chromosomes continue to shorten and thicken – undergo “rough-pairing”, synaptonemal complex forms to create bivalent (= n number)

• Pachynema – pachytene – continue to shorten and coil, closer pairing and formation of the tetrad

• Diplonema – diplotene – can see the 2 sister chromatids, chromosomes intertwine at the chiasma – get crossing-over between non-sister chromatids of a tetrad

• Diakinesis – chromosomes are pulled further apart but chiasma stays connected, nucleolus and nuclear membrane disappear, centromeres attach to spindle fiber

Remainder of Meiosis I

• Use similar mechanisms as seen in mitosis • Metaphase I – thickened, short chromosomes, terminal

chiasmata visible, tetrads interact with spindle fibers, alignment is random at metaphase plate

• Anaphase I – sister chromatids held by centromeres and tetrad is separated and moved to the poles in process called disjunction, each gets n number of chromosomes– occasionally non-disjunction occurs with an abnormal splitting

of the tetrad

• Telophase I – nuclear membrane forms around the dyads, undergo a short interphase without DNA replication– some cells enter Meiosis II after anaphase I

Meiosis II

• Separate sister chromatids to gamete or spore• Prophase II – sister chromatids held by

centromere• Metaphase II – centromeres are positioned at

the metaphase plate• Anaphase II – chromatids pulled to separate

poles• Telophase II – chromosome is a monad and

after cytokinesis you have 4 haploid cells that may have increased genetic diversity

Meiosis

Spermatogenesis

• Makes ♂ gametes, in the testes• Spermatogonium – large

undifferentiated germ cell that enlarges to primary spermatocyte

• Meiosis I make the secondary spermatocytes that have haploid number of dyads

• Meiosis II makes 2 spermatids from each 2° spermatocyte

• Spermatids undergo developmental changes (spermatogenesis) to become spermatozoa or sperm– all are equal size

Oogenesis• Make ♀ gamete or spore in the

ovary, each receives equal amounts of chromosomes but unequal amounts of cytoplasm– almost all cytoplasm from primary

oocyte goes to secondary oocyte– used to nourish growing zygote– cell with little to no cytoplasm is the 1st

polar body which may or may not divide again

• Mature ovum is produced after meiosis II, ootid (differentiates to ovum) and 2nd polar body

• Meiosis I and II may not be continuous – in humans, meiosis I occurs in embryonic ovary and arrests in prophase I and completes at ovulation, second division occurs after fertilization

Sexual Reproduction

• Meiosis is critical for this and allows for genetic diversity

• Plants produce haploid spores that will become gametes

• Yeast spend most of life as haploid and divide by mitosis

• Plants alternate between sporophyte and gametophyte – alternation of generations

Photomicrographs of Chromosomes