Chapter 15 Flashcards
Cell division
- highly regulated series of events
- 2 types
1. mitosis
2. meiosis
chromosomes
- different species have different numbers of chromosomes
- eukaryotic chromosomes usually found in pairs
- humans have 46 (23 pairs)
haploid
- one
- which cells are haploid?
diploid
- 2
- which are diploid
Cytogenetics (eukaryotic chromosomes)
- involves examination of chromosomes and cell division
- when cells prepare to divide, chromosomes become very compact
- easily viewed with light microscope
- karyotype
homologous chromosomes
- in diploid organisms (including humans), chromosomes come in matched pairs
- one from each parent
- not exact copies, same genes (but maybe different versions
- each pair= homologous chromosomes
- slight differences provide variation in gene function
- red hair vs. blonde hair; brown eyes vs. blue eyes
chromosomes in humans
- 23 pairs (46 total)
- first 22= autosomes
- lower #- larger, more genes
- pair #23= sex chromosomes
- X and Y male, not homologues
cell cycle
- G1- first gap
- S- synthesis of DNA
- G2- second gap
- mitosis and cytokinesis
- first 3 are interphase
- G0- substitute for G1 for cells postponing division or never dividing again (neurons)
G1 phase of cell cycle
- growth phase
- signaling molecules can help cell progress to next phase
- if cell passes restriction point (G1 checkpoint), can move on to S phase
S phase of cell cycle
- replication
- afterward, two copies stay joined as sister chromatids
- human cell in G1 has 46 chromosomes
- same cell in G2 has 46 pairs of sister chromatids or 92 chromatids total
G2 phase of cell cycle
-cell makes proteins necessary for mitosis and cytokinesis
Mitosis and Cytokinesis
- division of one cell nucleus into two, with separation of sister chromatids
- cytokinesis- follows mitosis to divide the cytoplasm into two daughter cells
Checkpoints
- cyclins or cyclin-dependent kinases (cdks) responsible for advancing a cell through the phases of the cell cycle
- after G1, G2 and M phase
mitotic cell division
- end product- two daughter cell
- genetically identical to original mother
- used for:
1. Asexual reproductions
2. Growth and development
preparation for cell division
- DNA replicated (what phase)
- sister chromatids
- tightly associated at centromere
- used as attachment site for kinetochore
mitotic spindle
- apparatus responsible for organizing and sorting the chromosomes
- composed of microtubules
Centrosomes (mitotic spindle)
- microtubule organizing centers (MTOCs)
- duplicate at beginning of M phase
- each composed of two centrioles
- each defines a pole
- plants and fungi- use different MTOCs
Astral microtubules
position spindle in cell
polar microtubules
separate 2 poles
kinetochore microtubules
attached to kinetochore bound to centromeres of each chromosome
phases of mitosis
prophase -> pro metaphase -> metaphase -> anaphase -> telophase
prophase of mitosis
- sister chromatids condense
- mitotic spindle starts to form
- nuclear envelope begins to dissociate
- nucleolus is no longer visible
prometaphase of mitosis
- nuclear envelope completely dissociates into vesicles
- mitotic spindle is fully formed
- sister chromatids attach to the spindle via kinetochore microtubules
metaphase of mitosis
sister chromatids align along the metaphase plate
anaphase of mitosis
- sister chromatids separate
- individual chromosomes move toward the poles as kinetochore microtubules shorten
- polar microtubules lengthen and push the poles apart
Telophase and Cytokinesis of mitosis
- chromosomes decondense
- nuclear envelope re-forms
- cytokinesis separates mother cell into two daughter cells, which begins with a cleavage furrow in animal cells
when the haploid sperm and haploid egg fuse…
a diploid fertilized egg (zygote) is produced
meiosis.. produces sperm and eggs??
- if we produced sperm and eggs by mitosis, each gamete would have 46 chromosomes
- sperm (46) + egg (46) -> offspring with 92 chromosomes
- grandchildren would have 184 chromosomes
- sperm (92) + egg (92)
- clearly does not happen
meiosis
- haploid cells produced from originally diploid cells
- humans: in testes and ovaires
- replicate DNA once
- divide the cell twice
Meiosis- key differences
- homologous pairs form a bivalent or tetrad
- crossing over -> “gene swapping”
- independent assortment of chromosomes
- overall effect- increase genetic possibilities in offspring
- bivalent or tetrad
- homologous pairs of sister chromatids associate, form a bivalent or tetrad
- process called synapsis
- allows crossing over to take place
- crossing over
- overlapping sections of tetrad can exchange genetic material
- increases the genetic variation of a species
- chiasma- arms of the chromosomes tend to separate but remain adhered at a crossover site
- independent assortment
- random movement of chromosomes into daughter cells
- metaphase I of Meiosis I
- homologous chromosomes randomly assort to either side of metaphase plate
- humans have 23 pairs of chromosomes
- 2^23 different ways chromosomes could line up in meiosis
- 8 mill different ways
- 8 mill genetically diff eggs
- 8 mill genetically diff sperm
- 64 trillion genetically diff offspring
- NOT including variation due to crossing over
Prophase I of meiosis
- homologous chromosomes synapse; crossing over occurs
- chromosomes condense
- nuclear envelope begins to dissociate into vesicles
Prometaphase I of meiosis
- the nuclear envelope completely dissociates into vesicles
- bivalents become attached to kinetochore microtubules
metaphase I of meiosis
- bivalents randomly align along the metaphase plate
- each pair of sister chromatids is attached to one pole
anaphase I of meiosis
-homologous chromosomes separate and move toward opposite poles
telophase I and cytokinesis of meiosis
- the chromosomes decondense, and the nuclear envelope re-forms
- the 2 daughter cells are separated by a cleavage furrow
Prophase II of meiosis
- sister chromatids condense, and the spindle starts to form
- the nuclear envelope begins to dissociate into vesicles
prometaphase II or meiosis
- the nuclear envelope completely dissociates into vesicles
- sister chromatids attach to the spindle via kinetochore microtubules
metaphase II of meiosis
- sister chromatids align along the metaphase plate
- each pair of sister chromatids is attached to both poles
anaphase II of meiosis
- sister chromatids separate, and chromosomes move toward poles as kinetochore microtubules shorten
- polar microtubules lengthen and push the poles apart
telophase II and cytokinesis of meiosis
- chromosomes decondense, and nuclear envelope re-forms
- cleavage furrows separate the 2 cels into 4 cells
life cycles
- most animals are diploid-dominant
- are there haploid-dominant eukaryotes?
- many fungi, some plants and algae
- haploid cells fuse to form diploid zygote, immediately use meiosis to make 4 haploid spores
chromosome variation
- variations of chromosome structure and/or number can have major effects on organisms
- several human diseases
- important in evolution of new species
Deletions; chromosomal mutations
- segment or whole
- example: Cri-du-chat syndrome
- “cry of the cat”
- deletion on chromosome 5
- abnormal brain development, small head, abnormal larynx
duplications; chromosomal mutations
-section occurs 2 or more times in a row
inversions; chromosomal mutations
change in direction along a single chromosome
Simple translocations; chromosomal mutations
-one segment becomes attached to another chromosome
reciprocal translocations; chromosomal mutations
-exchange pieces between two different chromosomes
Aneuploidy
- alterations in number if a particular chromosome pair
- total number not an exact multiple of a set
- usually has detrimental consequences
- often results from non-disjunction
nondisjunction
- common cause of aneuploidy
- chromosomes do not sort properly during cell divisions
- gametes with too many or too few chromosomes
- if used during reproduction, effects every cell of offspring
trisomy
- third copy of a chromosome
- 2n+1 (47 in humans)
- due to non-disjunction: 10% in sperm; 90% in egg
- down syndrome: three copies of chromosome 21
- 0.1% of all live births
- various morphological and behavioral effects
- short stature, reduced life span, infertility in males, heart defects
Trisomy: Sex chromosomes
- XXY (Klinefelter Syndrome)
- male external appearance
- dysfunctional testicles
- tall stature
- some feminine features
- normal intelligence; some verbal cognitive difficulties
- reduced fertility
- may require hormonal treatment at puberty
monosomy
- missing one of normal copies of a chromosome
- 2n-1; (45 total in humans)
- ex: Turner’s Syndrome:
- XO genotype
- random error (not heritable) due to non-disjunction
- normal female external appearance, but fertility may be affected by abnormal ovaries
- short stature with folds of skin at neck
- normal intelligence, with some spatial and memory deficits
trisomy and monosomy
- trisomic and monosomic individuals have imbalance in the level of genes
- increased or decreased expression of these genes can interfere with proper cell function
Aneuploidy in humans
- 5-10% of all fertilized human- abnormal chromosome number
- approx. 50% of all spontaneous abortions (miscarriage) due to chromosomal abnormality
- we can survive some abnormalities
- trisomies or abnormalities in sex chromosome number
euploid
- chromosome number that is viewed as normal
- in a diploid organisms, 2 sets is normal; 2n
polypoid
- 3 or more sets of chromosomes
- triploid 3n
- tetraploid 4n
- not compatible with life in humans
