Biology 30 AP - Cell Division

Why do cells divide?

growth/differentiation: mitosis enables organisms to grow from a single-celled zygote into a mature organism that might contain hundreds of trillions of specialized cells

maintenance: new cells produced to replace worn out/dead cells

repair: can regenerate damaged tissues (finger cut = new skin)

• some organisms can regenerate entire body parts

Asexual reproduction

no new combination of cellular material occurs

all new cells contain same DNA as original cell

occurs in all somatic (body) cells, unicellular organisms, and simple multicellular organisms (budding, runners)

Chromosomes

small

sausage-like

may be found as individual chromatids (late stage of cell division) or as paired chromatids (sisters) connected at the centromere

all the DNA in a cell constitutes the cell’s genome

Homologous pairs (chromosomes)

all somatic cells contain homologous pairs of chromosomes

• one from mother’s egg (maternal)

• one from father’s sperm (paternal)

humans: 23 pairs (46 chromosomes total)

each homologous pair is similar in shape and length and is responsible for the same types of characteristics

Chromatin

long, thin threadlike material

present in this state during interphase

Sister chromatids

connected at the centromere

identical exact copies of each other

Stages of the cell cycle

interphase (G1, S, & G2)

mitosis

cytokinesis

continuous (does not start and/or stop) → different cells may cycle at different pace

Interphase

growth stage

90% of cell cycle

consists of G1, S, & G2

G1

cell growth (organelle replication)

“first gap”

DNA = 46 single strands of unreplicated chromatin

S - ynthesis

“synthesis” phase

DNA replicated

46 single → 46 double strands of chromatin

G2

growth stage

“second gap”

rebuild energy reserves

preparation for division

Mitosis

division of genetic material and nucleus

in somatic (body) cells

all cells produced are IDENTICAL in genetic makeup to the original cells

(particularly important that the chromosome # does not change)

Cytokinesis

division of the cytoplasm and organelles

Differentiation

unique appearance and functionality found is due to difference in the way that content is expressed

Prophase

contents of the nucleus become visible

DNA strands shorten and thicken

chromatin → chromosomes

centrioles (organelles during cell division) separate and move to opposite poles of cell

spindle fibres start to appear

nuclear envelope and nucleolus disappear

Metaphase

chromosomes move to center of cell

centromeres on align on equator

spindle fibres attach to the centromeres

Anaphase

chromatids separate at centromeres

chromatids move to opposite poles of cell

same number of single-copy chromosomes should be at each pole

Telophase

chromosomes at opposite ends of cell

chromosomes un-condense to form chromatin

nuclear envelope and nucleolus reappears

Cytokinesis

cytoplasm division → cell membrane pinches in to form two distinct daughter cells

plants: cell plate forms first which separates two cells by forming cell wall

animals: cell membrane pinches in at the cleavage furrow

Steps of mitosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

5. Cytokinesis

PMAT

Somatic (body) cells

DNA from maternal and paternal sides combined

diploid = 2n

e.g. humans have 23 diff. chromosomes → 2(n) = 2(23) = 46

Gametes

sperm or eggs

hold half the DNA from somatic cells from which they came

haploid = n

e.g. humans have 23 different chromosomes → n=23

How is gender determined

last “pair” of chromosomes (#23) determines gender (sex chromosomes)

homologous pair → XX → female

XY = male

Zygote

ovum fertilized by sperm

original number of chromosomes (46 = 2n) restored

What does the life cycle alternate between?

haploid and diploid

Meiosis

creates gametes

reduces chromosome number from 2n → n by copying chromosomes once but dividing twice

First division (meiosis I)

separates homologous chromosomes

Second division (meiosis II)

separates sister chromatids

Prophase I

same as mitosis

chromatin → chromosomes

centrioles move to opposite poles

spindle fibres appear

nuclear envelope and nucleolus disappears

homologous chromosomes pair up side by side (synapsis) by corresponding genes → tetrad (4 chromatids)

homologous chromosomes overlap and occasionally break and exchange identical sized segments → crossing over = more genetic variation

Chiasmata

location of crossing over

Metaphase I

homologous pairs move to center → centromeres on either side of equator

spindle fibres attach to centromeres only on exposed sides

Anaphase I

homologous pairs separate (not sister chromatids) at the centromere

chromosomes move to opposite poles = segregation

23 double chromosomes at each pole (sister chromatids remain intact)

Telophase 1

chromosomes at opposite poles

do not uncoil to form chromatin

nuclear envelope occasionally reappears (in some cells)

Step 5 of first division (meiosis I)

cytokinesis

Prophase II

centrioles move to opposite poles

new spindle fibres form

Metaphase II

cell moves directly to metaphase → no DNA replication & no formal organization of nucelus

chromosomes move to center

centromeres align on equator

spindle fibres attach to centromeres

Anaphase II

spindle fibres shorten → chromatids separate at centromeres

chromatids move to opposite poles

23 single stranded chromosomes/chromatids at each pole

Telophase II

chromosomes at opposite ends un-condense to form chromatin

nuclear envelope reappears

Step 10 of second division (meiosis II)

cytokinesis

Gametogenesis

formation of ova & sperm follow the process of meiosis

specialization depends on function

Sperm specialization

movement (little cytoplasm)

lots of cell division

produce 4 small sperm

determine gender of the child

Egg specialization

nourish the zygote

only one ovum is produced per oocyte → the other 3 polar bodies sacrifice their cytoplasm (and other organelles) to produce one large egg

Why are mules sterile?

cannot form gametes

no homologous pairs to synapse during prophase I (horse 2n = 64, donkey 2n = 62, mule 2n = 63

Karyotyping

method of identifying chromosomes

pictures of chromosomes are taken as the cells undergoes mitosis → picture is enlarged

individual chromosomes are cut up

What are chromosomes matched up on?

size (largest to smallest)

centromere position

g-banding

Nondisjunction

chromosomes don’t separate properly during anaphase I or II

one daughter cell produced during separation will be lacking information, one will have too much

occurs quite often among humans

impact so severe to zygote that miscarriage occurs very early

Trisomy

one too many chromosomes

one pair will be a triplet

Monosomy

one too few chromosomes

one pair will be a singlet

Syndrome

set of traits if baby survives nondisjunction

Down’s Syndrome

trisomy 21

most commonly known

common facial features

short stature & short fingers and toes

large tongue - speech difficult

mental disability

Patau’s Syndrome

trisomy 13

most fetuses die before term

5% live to age 3

45% die within first month

serious eye, brain, and circulatory defects, malformations, kidney/heart defects

Edward’s Syndrome

trisomy 18

only 10% survive past one year

all die early in infancy with many complications

What is special about trisomy 21, 13, and 18?

only known trisonomic autosomal genetic disorders that result in surviving offspring

Nondisjunction of the sex chromosomes

can be fatal

most survive just fine

Klinefelter’s Syndrome

XXY

tall, sterile males

female characteristics

normal intelligence

Jacob’s Syndrome

XYY

super male

somewhat taller than average

slightly below normal intelligence

extra testosterone

Super female

XXX

normal intelligence

fertile

no physical problems

each increasing X = lesser intelligence

Turner’s Syndrome

only surviving monosomy

XO

live normal lives but do not mature sexually at puberty = sterile

short stature

short broad neck

broad chest

Polyploidy

nondisjunction desired characteristic in development of large luscious fruit

big strawberries = 4n or 6n

estimated 30-80% of living plant species are polyploidy

Cloning

process in which identical offspring are formed from a single cell or tissue (clone = cutting)

all cells are identical (some small variations due to mutations are expected)

used in some plants and animals for reproduction

Asexual reproductive strategies

binary fission

budding

sporulation

regeneration

vegetative propagation

embryo splitting

Binary fission

equal division of the cytoplasm and nucleus of an organism resulting in two new organisms

e.g. ameba, paramecium, euglena

Budding

nucleus of an organism’s cell divides equally but the cytoplasm divides unequally

new cells formed may live as individuals or as colonies

e.g. yeast, hydra

Sporulation

production of spores

e.g. molds

Spores

single, specialized cells released from the parent

enclosed in a protective case and develop when environmental conditions are favourable

Regeneration

development of entire new organism from part of an original organism

e.g. starfish: one ray and part of central body can develop into an entire new organism OR restoration fo lost body parts

Vegetative propagation

regeneration in plants

complete new plants develop from part of the original plant

Bulbs

natural

enlarged underground stems surrounded by leaves & containing stored food

e.g. onions, tulips

Tubers

natural

enlarged underground stem with buds or “eyes” that contain stored food

new plants develop from the bud

e.g. potato

Runners

natural

stems that grow along the ground

at intervals, roots form and penetrate the soil and new plants develop at these points

e.g. strawberries

Rhizomes

natural

underground stems from which new plants develop at intervals

e.g. quackgrass

Layering

occurs when part of an old plant is bent and covered with soil

a new plant develops from the covered plant

e.g. blackberry

Cutting (slips)

piece of plant is placed in moist soil or water and complete plant develops

Grafting

stem of one plant to be propagated is attached to cut end of another growing plant

Cancer

uncontrolled cell growth → too much life = mitosis

abnormal growth (unlike normal controlled growth replacing dying and dead cells) without the signals of the body directing growth

lost the ability to differentiate and carry out cell processes

prevention focuses on elimination of carcinogenic/mutagenic substances (tobacco, diet, sun)

“Fast-forward” cell cycle

cancer cells show growth more rapid than fetal growth outside of the human body

Metastasis

spreading of cancer cells through the body

Life cycle

based on regular pattern of meiosis and mitosis

alternation of generation refers to the alternation of diploid and haploid generations

Binary fission (expanded)

bacteria and other prokaryotes - single, circular chromosome and no nucleus

replicates via binary fission

favourable conditions = exponential division rate (huge populations in little time)

Life cycle of a fern

diploid generation of a plant = sporophyte (spore-making body)

meiosis → sporophyte produces a haploid spore → grows into a plant body called gametophyte (gamete-making body)

gametophytes = produce male and female gametes → fuse to make a sporophyte

Advantages of sexual reproduction

offers population a way to adapt to changing environment

competition among siblings may be reduced if genetically diverse

increases variation

Advantages of asexual reproduction

proceeds quickly and does not require second organism

less energy

maximizes the chance that individuals will survive