Chapter 9 Cell Division

Unicellular organisms 

Functions of Cell Division

reproduction

Multicellular organisms

Functions of Cell Division

- produce progeny 

- embryonic development

- growth 

renewal and repair

mitosis 

- somatic cells

• 1 cell, 2 sets of chromosomes

→ 2 nuclei, each with 2 identical sets of chromosomes

meiosis 

(produces gametes)

- 1 cell, 2 sets of chromosomes

→ 4 cells, each with 1 set of chromosomes, cells do not divide until fertilization

cytokinesis 

the division of the cytoplasm

binary fission

Prokaryotes

chromosome replicates and daughter chromosomes actively move apart while plasma membrane pinches inward

Genome 

the total supply of DNA in a cell

chromosomes

DNA molecules in a cell are packaged into —-

chromatin

chromosomes consist of —-, a complex of DNA and protein that condenses during cell division (chromatin is a lower order of DNA organization than chromosome)

Chromosomes of prokaryotes

usually one circular chromosome

Chromosomes of eukaryotes

multiple different linear chromosomes 

Ploidy

every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

haploids 

(n) = one set of chromosomes

ex) human gametes- (reproductive cells): n=23

diploids 

(2n) = two sets of chromosomes

ex) human somatic cells (nonreproductive cells): 2n=46

kinetochores, centromeres

— are protein complexes associated with —-, the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

cohesins 

sister chromatids are joined at their centromeres or all along their lengths by 

separase 

at anaphase, cohesins are cleaved by 

Centrosome 

- subcellular region containing material to assemble microtubules

- replicates during interphase and migrates to opposite ends of the cell

- contains centrioles that seed the growth of microtubules 

The Mitotic Spindle 

microtubular structure that controls chromosome movement during mitosis

Spindle Microtubules 

kinetochore / nonkinetochore 

Asters 

- radial array of short microtubules

- involved in correct positioning/orientation of mitotic spindle apparatus and determining site of cleavage furrow 

The Cell Cycle Control System

a cyclically operating set of molecules that triggers and coordinates cell cycle events

Checkpoints

- control points where stop and go-ahead signals regulate the cell cycle

- signals registered at checkpoints report whether important cellular processes have been correctly completed

cyclins and cyclin-dependent kinases (Cdks)

two types of regulatory proteins are involved in cell cycle control

MPF (maturation promoting-factor)

a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

Stop and Go Signs

Internal and External Signals at the Checkpoints

G0 phase

if a cell receives a go-ahead signal at the G1 checkpoint (: likely most important checkpoint), it will usually complete the cycle and divide, if not, it will exit the cycle, switching into a nondividing state called the —-

density-dependent inhibition 

crowded cells stop dividing

anchorage dependence 

cells must be attached to a substratum in order to divide

transformation 

the process of a normal cell being converted to a cancerous cell

tumors 

masses of abnormal cells within otherwise normal tissue

benign 

when abnormal cells remain only at original site

malignant 

when abnormal cells invade surrounding tissues and metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors

Cancer treatments

- localized tumors may be treated with high-energy radiation, which damages the DNA in the cancer cells (usually repaired in normal cells)

- chemotherapy targeting rapidly growing cells

- recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment

- coupled with the ability to sequence the DNA of cells in a particular tumor, treatments are becoming more “personalized”

variation 

demonstrated by the differences in appearance that offspring show from parents and siblings

genes 

the units of heredity, and are made up of segments of DNA

gametes 

(sperm and eggs) : reproductive cells that carry genes that are passed to the next generation

somatic cells 

all cells of the body except gametes and their precursors

locus 

the specific location of each gene on a certain chromosome

homologous chromosomes

pair of chromosomes inherited from each parent

allele 

an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome (=locus)

sexual reproduction 

two parents give rise to offspring that have unique combinations of genes inherited from the two parents

asexual reproduction 

a single individual passes genes to its offspring without the fusion of gametes

clone

a group of genetically identical individuals from the same parent

human somatic cells 

diploid

have 23 pairs of chromosomes homologous chromosomes (2n = 46)

each set of 23 consists of 22 pairs of autosomes and a single pair of sex chromosomes

human gametes 

(sperm or egg, haploids)

- have a single set of chromosomes (n = 23)

- 22 autosomes + 1 sex chromosome ~ egg (ovum) : X, sperm : X or Y

karyotype 

an ordered display of the pairs of chromosomes from a cell 

life cycle 

the generation-to-generation sequence of stages in the reproductive history of an organism

fertilization 

the union of gametes)

zygote 

fertilized egg)

produces somatic cells by mitosis and develops into an adult

meiosis I 

(reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes

meiosis II

(equational division): sister chromatids separate

➔ The result is four haploid daughter cells with unreplicated chromosomes

crossing over

nonsister chromatids exchanging DNA segments (~3 crossing over events/chromosome pair)

Genetic recombination during prophase I

in early prophase I each chromosome pairs with its homolog and crossing over occurs

chiasmata 

X-shaped regions called —- are sites of crossover

synapsis 

a state where duplicated homologs pair up and become physically connected to each other along their length.

synaptonemal complex 

a zipper-like structure called the —- holds the homologs together tightly

synapsis

during —-, DNA breaks are repaired, joining DNA from one nonsister chromatid to the corresponding segment of another

Events unique to meiosis 

(all occur in meiosis l)

i) synapsis and crossing over in prophase I

ii) at the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

iii) at anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

arms, centromeres

in meiosis, cohesins are cleaved along the chromosome —- in anaphase I (separation of homologs) and at the —- in anaphase II (separation of sister chromatids)

recombinant chromosomes

crossing over produces —-, which combine DNA inherited from each parent

Random fertilization 

adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)