View
214
Download
0
Category
Preview:
Citation preview
When and why do cells divide?
• Cells divide when there is a chemical signal to do so.
• Skin cells may divide in response to crowding. Certain cells send out a chemical signal that tells neighboring cells to divide.
• Cells may divide in response to an injury, to mend damaged tissue.
• Growth factors can signal cell division in children to lengthen bones and add other tissues.
Cdk’s are alwayspresent in the cell.
growth
factor
cyclin-dependentkinase
growth factorreceptor
(cytoplasm)
cyclin
(plasmamembrane)
Growth factorbinds to receptorand stimulatescyclin synthesis.
Cyclin activatesCdk; active Cdkstimulates DNAreplication.
Specialized cell membrane proteins signal cell division when growth factors are present.
Mitosis
• One-celled eukaryotic organisms, and individual cells in a multi-cellular organisms, reproduce by mitosis followed by cytokinesis.
Overview of Mitosis
• After DNA is _________, it is condensed into chromosomes and identical copies are sorted in the process of mitosis.
• Mitosis assures that the ___ daughter cells have _________________ DNA.
Warning: Confusing terminology ahead!
Before cell division, a strand
of DNA is a chromosome.
(Think of it as a one-chromatid chromosome.)
During cell division, two
identical copies of a DNA strand
link together into a two-chromatid
chromosome.
After cell division, the
single strand is a chromosome again. (Again, think of it as a one-chromatid chromosome.)
G0: nondividing
Under certaincircumstances, cellmay return tocell cycle.
G1: cellgrowth anddifferentiation
G2: cellgrowth
S: synthesisof DNA;chromosomesare duplicated
interphase
anaphaseprophasem
etaphase
telophase andcytokinesis
mitotic cell
division
Animated cell cycle at http://cellsalive.com
Normal cell processes,
including protein synthesis
(transcription and translation).
The cell prepares for division,
which includes making
specialized proteins (more
transcription and translation).
DNA is replicated ONLY during S-
phase. Once replication has
happened, the cell MUST divide.
Prior to Mitosis, DNA is replicated during the S-phase of the cell cycle.
Chromosomes appear late in G2 phase, just prior to mitosis.
How many cells in this slide of an onion root tip are actually dividing? How can you tell?
INTERPHASE
nuclearenvelope
centriolepairs
nucleolus
chromatin
Late Interphase
Can we tell if a cell in Interphase is in G1, S, or G2 of the cell cycle?
DNA (2 nm diameter)
histone proteins
nucleosome: DNA wrappedaround histone proteins(10 nm diameter)
coiled nucleosomes(30 nm diameter)
DNA coils
proteinscaffold
chromosome:coils gathered ontoprotein scaffold(200 nm diameter)
A strand (double helix) of DNA wraps around _________ _______ to form _____________. This
protects DNA from damage during cell division.
gene 1
different allelessame alleles
gene 2
Homologous chromosomes are those that carry the same genes but may have slightly different
information (such as dominant or recessive versions of a gene). Homologous chromosomes do not pair
together. Chromosomes only pair with their identical sister chromatids.
sister chromatids centromereIdentical (sister) chromatids pair up during
Prophase, and join at a pinched-in point called the centromere.
sisterchromatids
duplicatedchromosome(2 DNA doublehelices)
The chromosome at the end of Prophase consists of two strands of condensed DNA. Each ______ ________
carries exactly the same information.
MITOSIS: Prophase
beginning ofspindle formation
condensingchromosomes
Notice that these cells in prophase have barely visible
chromosomes as DNA begins to condense.
MITOSIS: Metaphase
spindlemicrotubules
Chromosomes, with their paired identical chromatids, move to the center of the cell.
MITOSIS: Anaphase"free" spindlefibers
Identical chromatids separate from one another and migrate to opposite poles of the cell.
MITOSIS: Telophasenuclear envelopere-forming
chromosomesextending
Telophase completes Mitosis. Both poles of the cell now have identical DNA, and the cell can
divide in half.
MITOSIS: Cytokinesis
After Mitosis has finished sorting the
chromosomes, cytokinesis takes
place, dividing the cell into two new
cells.
INTERPHASE
Before S phase, the cell was diploid (two
copies of each chromosome).
After cytokinesis, are the cells
diploid or haploid?
2 The microfilamentring contracts,pinching in thecell's “waist.”
1 Microfilaments forma ring around thecell's equator.
3 The waistcompletely pinchesoff, forming twodaughter cells.
The process of cytokinesis
1 Carbohydrate-filledvesicles bud off theGolgi apparatus andmove to the equatorof the cell.
3 Completeseparation ofdaughter cells.
2 Vesicles fuse toform a new cellwall (red) andplasma membrane(yellow) betweendaughter cells.
Golgi apparatus
carbohydrate-filled vesicles
plasmamembrane
cell wall
Cytokinesis in plant cells
The problem:
• When diploid organisms reproduce sexually, two cells must fuse and share genetic information.
• The end result of sexual reproduction is a new diploid organism that has genetic information from both parents.
meioticcell division
haploidgametes
diploidfertilizedegg
diploidparentalcells
fertilization
The cells from the parents must be haploid if their offspring is to be diploid.
While diploid cells hold two copies of each chromosome (one from each parent), haploid sex
cells hold one copy of each chromosome.
sisterchromatidshomologous
chromosomes
Meiosis is reduction division. It begins with a diploid cell
and produces haploid cells.
Why does it produce four haploid cells?
G0: nondividing
Under certaincircumstances, cellmay return tocell cycle.
G1: cellgrowth anddifferentiation
G2: cellgrowth
S: synthesisof DNA;chromosomesare duplicated
interphase
mitotic cell
division
Meiosis also involves the
cell cycle, and takes place
after S phase of the cell
cycle. DNA is replicated
before meiosis.
MEIOSIS IHomologous chromosomesmove to opposite poles.
chiasma spindlemicrotubule
recombinedchromosomes
paired homologouschromosomes
Homologous chromosomespair and cross over.
Homologous chromosomesline up in pairs.
(a) Prophase I (b) Metaphase I (c) Anaphase I (d) Telophase I
First half of meiosis: separation of homologous chromosomes.
Prophase I
chiasmaspindlemicrotubule
paired homologouschromosomes
Homologous chromosomespair and cross over.
Notice that four strands — maternal and paternal chromosomes and their identical sister
chromatids — join into a single unit, called a tetrad.
direction of“zipper”formation
protein strandsjoining duplicatedchromosomes
Protein strands “zip” the homologous chromosomestogether.
chiasma
Recombination enzymes snip chromatids apart and reattach the free ends. Chiasmata (the sites of crossing over) form when one end of the paternal chromatid (yellow) attaches to the other end of a maternal chromatid(purple).
Recombinationenzymes bindto the joinedchromosomes.
Recombinationenzymes and proteinzippers leave.chiasmata remain,helping to holdhomologouschromosomestogether.
recombinationenzymes
chiasma
While in tetrads, homologous chromosomes often swap ends, further mixing up genetic information.
Metaphase I
recombinedchromosomes
Homologous chromosomesline up in pairs.
Tetrads line up in the center of the cell.
Anaphase I
Homologous chromosomesmove to opposite poles.
Because homologous chromosomes
separate (instead of identical sister
chromatids), each pole of the cell gets a full set of chromosomes but different genetic
information.
(e) Prophase II
MEIOSIS II
(f) Metaphase II (g) Anaphase II (h) Telophase II(i) Fourhaploidcells
Meiosis II begins immediately after Meiosis I, with a short rest in between (no interphase in
between). In Meiosis II, sister chromatids separate from one another.
Metaphase II
In both cells, chromosomes line up in Metaphase II
so that sister chromatids can
separate in Anaphase II.
Anaphase II
Telophase II End
The result of meiosis is four haploid cells.
Each has one copy of each chromosome, which may carry
different versions of the same genes. Each gamete (sex cell) can have different genetic
information.
Recommended