Cell reproduction - C&M

2 functions of cell division

1. multicellular organisms — growth, maintenance, & repair of cells/tissues

2. single-celled organisms — reproduction

prokaryotic genome

one, double-stranded, circular DNA molecule residing in the nucleoid

• additional smaller loops of DNA—plasmids—may be present

  • exchange of plasmids w/ other cells allows gene transfer

eukaryotic genome

several double-stranded DNA molecules in the form of chromosomes

chromosomes

• the # of these in the nucleus varies among species

  • within a species, # is consistent, but may vary w/ developmental stage or specific cell type

somatic

these cells typically have 2 matched sets of chromosomes which make them a diploid (2n)

gametes

these are egg & sperm cells, and they have half the number of chromosomes which makes them haploid (n)

karyotype

arranging the chromosomes by size produces a…

homologous

chromosomes (homologs) that pair in reproduction of diploid cells are…

heterologous

some genomes have pairs that don’t match (X & Y chromosomes in humans) , and they are…

condensed, compact

eukaryotic DNA must be ___ into ___ chromosomes to fit into the nucleus

histone proteins

packaging is accomplished by short stretches of DNA that wrap around a core of 8 _____ _____ (like a string of beads):

• histone-DNA complex (the bead) is called a nucleosome & the connecting DNA (string) is called linker DNA

  • coils to form a chromatin fiber

  • fibrous proteins further pack each chromosome

the cell cycle

an ordered series of events in the life of a cell

2 phases of the cell cycle

interphase

• normal growth & preparation for cell division

mitotic phase

• replicated DNA & cytoplasm are split—cell divides

G1 phase

change is not evident (“first gap”), but the cell is biochemically active

S phase

DNA synthesis:

• identical copies of DNA molecules (sister chromatids) are joined at centromere

• centrosomes produce mitotic spindles to move chromosomes

• animal cells — centrosomes are associated w/ centrioles which organize cell division

G2 phase

energy is replenished, organelles reproduce, & cytoskeleton breaks down (“second gap”)

2 mitotic phases

1. mitosis — aka karyokinesis; nuclear division

2. cytokinesis — cytoplasmic components physically separate into 2 daughter cells

prophase

• nuclear envelope breaks down

• membranous organelles (Golgi, ER) disperse toward edges of cell

• nucleolus disappears

• centrosomes begin migration to poles

• microtubules of spindle form

• sister chromatids coil tighter w/ aid of condensing proteins

prometaphase

• sister chromatids develop a protein kinetochore in the centromere region

• attaches the chromatids to the spindle microtubules

metaphase

• chromosomes line up along metaphase plate

• sister chromatids remain attached by cohesion proteins

anaphase

• cohesion proteins degenerate allowing chromatids to separate

• separated sister chromatids move in opposite directions

  • towards centrosomes where microtubules are attached

• cell elongates

telophase

• chromosomes reach opposite poles & begin to unravel

• spindles depolymerize into tubulin monomers that will form cytoskeletal components for daughter cells

• nuclear envelopes form around the chromosomes & nucleosomes appear w/in the near area

cytokinesis

cell division is completed via the physical separation of the cytoplasmic components into 2 daughter cells

cell cycle checkpoints

1. near the end of G1

2. at the G2 to mitosis transition

3. in metaphase of mitosis

control of the cell cycle

external triggers can initiate OR inhibit the cycle (internal ones can also do this):

• death of nearby cells

• release of growth hormones

• cell crowding

G1 checkpoint

are conditions favorable for cell division?

• important — external influences, adequate cell reserves, & size

  • check for DNA damage!

if conditions aren’t met…

• …cell won’t be allowed to enter S phase

• …stop the cycle & try to fix problem

• …enter G0 & wait to get better

G2 checkpoint

prevents entry into the mitotic phase if conditions aren’t met:

• size & protein reserves are checked again

• ensure all chromosomes have been replicated & no DNA damage

• problems? — cycle is halted while cell tries to complete DNA replication OR repair DNA

M checkpoint

aka “spindle checkpoint”; determines whether sister chromatids are correctly attached to spindle microtubules

• near end of metaphase

• problem? — cycle won’t proceed until kinetochores of each pair of sister chromatids are firmly anchored to at least 2 spindle fibers

  • failure = non-disjunction of chromatids

cyclins & cyclin-dependent kinases (Cdks)

• levels of these proteins fluctuate predictably

• internal & external signals can trigger increases

cancer

• many different diseases characterized by uncontrolled cell growth

• begins w/ a gene mutation that results in a faulty protein regulating cell production

• tumors result when reproduction of mutated cells surpass growth of normal cells

proto-oncogenes

normal genes that code for positive cell cycle regulators:

• mutated = oncogenes

tumor suppressor genes

segments of DNA that code for negative regulator proteins:

• when activated = prevent uncontrolled division

• mutated neg. reg. = cannot stop cycle if there’s a problem

binary fission

prokaryotic cell division happens by…