D2.1 Cell and nuclear division SL

processes in a living organism requiring cell division

• growth

• maintenance

• reproduction tissue repair

mother cell

cell that divides in cell division

daughter cells

cells produced from mother cell in cell division

interphase

DNA replication in preparation for cell division

• a single chromosome is duplicated to form two identical chromatids

• DNA is in chromatin form

chromatin

uncoiled and loosely packed form of DNA

interphase duration

hours to years

sister chromatids

two identical chromosomes created after DNA replication in interphase, separated when the cell divides into two daughter cells

cohesin

protein complex whose loops hold together sister chromatids before cell division

centromere

constricted region that joins the two sister chromatids

requirements for the functional separation of sister chromatids

1. supercoiling

2. microtubule and spindle fiber organization

3. movement of chromosomes by kinetochores

supercoiling

condensation of chromatin into chromosome

• wrapping double helix of DNA around histone proteins to produce nucleosomes

• nucleosomes coil further to produce chromosome structure

reason for supercoiling

allows DNA to be moved during cell division without knots, tangles, or breaks

spindle fibers

move chromosomes to opposite sides of cells during cell division

• made up of microtubules

microtubules

hollow cylinders composed of tubulin which make up spindle fibers

different functions of microtubulin fibers

• pull apart sister chomatids during mitosis or homologous pairs during meiosis

• push spindle poles apart, contributing to cell elongation

• anchor microtubule organizing center to the poles

centrosome/microtubule organizing center (MTOC)

site of assembly of microtubule spindle fibers, contains a pair of centrioles

centriole

cylindrical organelle in animal cells composed of microtubules which help to anchor growing microtubules and organize spindle tubules during the separation of sister chromatids in mitosis and meiosis

kinetochore

protein complex associated with centromere, acts as microtubule motor, causing shortening of microtubule fibers and movement of sister chromatids to opposite sides of the cell

types of cell division (M-phase)

mitosis and meiosis

mitosis

maintains chromosome number and genome of cells, producing diploid cells

4 main phases of mitosis

PMAT

1. prophase

2. metaphase

3. anaphase

4. telophase

prophase (mitosis)

• chromatin supercoils into chromosomes

• chromosomes are now visible as sister chromatids

• nuclear membrane begins to break down and the nucleolus disappears

• centrosomes move to opposite sides of the cell

metaphase (mitosis)

• microtubules continue to grow and attach to the kinetochores on the centromeres of each sister chromatid

• movement of the microtubule spindle fibers cause the sister chromatids to line up along the center (metaphase plate/equator) of the cell

• sister chromatids within each chromosome become attached to opposite poles

• spindle microtubules are under tension but the chromosomes cannot yet be pulled to either side due to cohesin loops

anaphase (mitosis)

• cohesin loops are cut, allowing chromatids to become separate chromosomes

• kinetochores shorten the microtubules, pulling the chromosome to the poles

• non-kinetochore microtubules elongate, stretching the cell

telophase (mitosis)

• chromosomes arrive at opposite poles of the cell

• chromosomes uncoil into chromatin

• nucleoli reappear

• microtubule spindle fibers disappear

• cytokinesis

• plasma membrane is pulled in around the equator

• two new daughter cells are formed

cytokinesis

division of cytoplasm which occurs to separate a daughter cell from its parent cell

cytokinesis in a plant cell

• vesicles derived from the Golgi apparatus filled with sugars and complex carbohydrates migrate to the center of the cell

• vesicles fuse together and form a cell plate (two layers of plasma membrane)

• cell plate grows to both sides until connected to existing cell’s plasma membrane, complete separation occurs

• daughters cells secrete cellulose to form cell walls, creating two separate daughter cells

cytokinesis in an animal cell

• a ring of contractile actin and myosin filaments forms around the center of the cell

• filaments constrict, pinching in the cell and forming a cleavage furrow

• cleavage furrow reaches the center of the cells, cells are further pinched until full separation into daughter cells

unequal cytokinesis

daughter cells do not receive the same amount of cytoplasm

• small cells produced can still survive if they receive a nucleus and at least one organelle which cannot be assembled by itself

oogenesis as an example of unequal cytokinesis

polar bodies received after two cell divisions contain much less cytoplasm and degenerate, while the larger cell continues to mature into an ovum with enough stored food in its cytoplasm

meiosis

halves the chromosome number, producing haploid cells during sex cell formation, generating genetic diversity

meiosis as reduction division

reduces diploid cell (2n, 46 chromosomes, 23 chromosome pairs) into four haploid cells (n, 23 chromosomes)

• maternal and paternal chromosomes carrying the same genes join together to form homologous pairs

• one chromosome from each homologous pair is passed on, creating haploid gamete cells

diploid nucleus

contains two chromosomes (one homologous pair) of each type

• two copies of every gene with different versions of genes

haploid nucleus

contains only one set of genes, half of genetic material

homologous chromosome

a pair of chromosomes which have the same genes in the same order with different alleles

allele

variation of a gene