BIO 103 JMU McMullen Exam 2 - topic 8

all living things reproduce, either asexually or sexually (involving gametes)

process will involve either cell division (mitosis) or meiosis or both

mitosis

every eukaryotic cell has a true nucleus bound by a nuclear envelop

chromatin: unorganized mass of DNA and histone proteins seen within the nucleus

during cell division, DNA wraps itself around histone proteins, DNA and proteins fold to form chromosomes

each species has a characteristic number of chromosome, called diploid (2n) number

eukaryotic cell division consists of

• karyokinesis

• cytokinesis

before division, each chromosome is composed of two identical sister chromatids, attached

mitosis produces daughter cells that are genetically identical

then go through cell cycle

cell cycle

process by which cells divide by mitosis and restore their DNA to the original level

G1 phase: cell grows in size, organelles double in number

S phase: DNA replication

G2 phase: cell growth, production of enzymes and other proteins

mitosis: karyokinesis and cytokinesis

karyokinesis

Prophase, metaphase, anaphase, telophase

prophase

chromatin begins to condense and form chromosomes

in an animal cell, two pairs of centrioles can be seen outside of the nucleus in the centrosomes (microtubule organizing centers)

• the centrosome begin to move apart, and between them form polar spindle fibers, made up of microtubules

• reach from pole to pole

  • kinetochore fibers attach to protein structures called kinetochores within the centromere of each chromatid pair

metaphase

the kinetochore and its fibers move the chromosomes to the equatorial plate of the cell where they form a line

anaphase

the chromatids separate and move to opposite poles

centromeres move first and arms drag behind

most rapid phase of mitosis

telophase

the spindle fibers break down

nuclear membranes, nucleoli, and chromatin reappear, new centrioles may appear

cytokinesis

typically begins during telophase

in plants, a cell plate is formed from the inside out

plate is formed from vesicles produced by the dictyosomes, eventually becomes the middle lamella

asexual reproduction in plants

rhizomes: horizontal underground stems (grasses, irises)

stolons: horizontal above ground stem (strawberries)

plantlets: formed at the margins of the leaves (air plant)

asexual reproduction in animals

fragmentation: a portion of an individual can break free and generate an entire organism (sponge)

budding: a mini offspring forms on the parent and then breaks off (hydra, sea anemone)

fission: the separation of a parent into two or more individuals of roughly equal size (sea anemone)

advantages of asexual reproduction

create numerous offspring quickly

quickly colonize a new habitat

no need to find a mate

perpetuates successful combinations of genes

mitosis recap

results in growth of multicellular organisms

results in replacement of cells in multicellular organisms

is a means of asexual reproduction in many unicellular organisms and some multicellular

useful when fast, efficient reproduction without variation is acceptable

meiosis

most eukaryotes do this

involves gamete formation and syngamy

meiosis process

chromosome number is reduced from the diploid number 2n to the haploid umber n

each daughter cell possess haploid number of chromosomes, aka gametes

humans: sperm and egg cells

nucleus of a diploid cell undergoes two divisions

• result in production of 4 daughter cells (gametes)

• each contains ½ the number of chromosomes of the original cell

syngamy

process by which the two gametes fuse to form a diploid zygote

sperm and egg comes together

Meiosis I - homologous chromosomes separate

interphase: chromosomes are replicated, each consists of two identical chromatids held together at the centromere

prophase: the chromosomes become visible and group in pairs, pairs synapse and are called tetrads

metaphase: these are called homologous chromosomes, pairs line up at the equatorial plane of the cell

anaphase: homologous separate and move to opposite poles

telophase: homologous reach the poles, depending on organism nuclear envelopes form/cytokinesis occurs

Meiosis II - chromatids separate

interkinesis: similar to interphase but no DNA replication occurs

prophase: if present, nuclear envelope breaks down again and spindle fibers reform

metaphase: chromosomes in each cell line up on the equatorial plane

anaphase: sister chromatids separate, each resulting daughter chromosome moves toward one of the poles

telophase: spindles disappear, nuclear envelope forms around each set of chromosomes, cytokinesis takes place

importance of meiosis

potential for genetic variability in sexually produced individuals is enormous

8,388,608 different combinations of chromosomes are possible in humans

64 trillion different combinations in two people

does not even take into account crossing over variation