Mitosis, Meiosis, & Non-disjunction Flashcards
Describe cell cycle
-Time from birth of new somatic cell till it divides to two new daughter cells
TWO broad stages:
- interphase (period between cell division )
- maximum time spent
- varies between cell types
- cell grows and ready for mitosis at end of interphase
- Mitosis (period of actual cell division)
- cell divides to form two daughter cells
- 2-4 hours and is constant
Summarize the stages of the cell cycle
Interphase (time between cell division )
- G1 phase (Go phase) - S phase - G2 phase
Mitosis (stages of cell division)
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis(cell division )
Summarize interphase
G1- 4 unreplicated chromosomes (chromosomes are shown partially condensed to make them visible)
S phase
G2 phase-4 replicated chromosomes, each considering of two sister chromatids
Summarize M phase
At start of mitosis! Replicated chromosomes condense
During mitosis, sister chromatids separate. Two daughter cells are formed by cytokinesis
What checkpoint checks for cell size and DNA replication ?
G2 checkpoint
What checkpoint, Checks for chromosome attachment to spindle?
M checkpoint
What checkpoint, Checks for cell size, nutrients, growth factors and DNA damage?
G1 checkpoint
What occurs at G2?
One large cell with TWO SISTER CHROMATIDS PER CHROMOSOME (2n; 4d). Cell is ready for division
What occurs in M phase?
Includes mitosis and cytokinesis, new cell enters cycle
What occurs in Go?
Occurs in non- dividing cells, it is an active process, not just a stall in G1. Occurs in cells that have terminally differentiated into tissue cells eg, neurons.
ONE CHROMATID PER CHROMOSOME (2n;2d)
Exit from cell cycle (non-dividing cells)
What occurs in G1?
Most variable in length, differences in rates of division are primarily a result of differences in G1 length
DIPLOID (two copies of each chromosome, mom+dad copies) ONE CHROMATID PER CHROMOSOME
Homologous chromosomes
What occurs in the S phase?
S phase; DNA synthesis occurs, chromatids are copied and joined at centromere. TWO SUSTER CHROMATIDS PER CHROMOSOME (2n; 4d)
Summarize in Great detail the events of G1?
G1 (gap 1 or growth 1) phase: Cell size increases. Protein synthesis active. Differentiation and performs function. End of G1, cell checks whether ready for division.
Some cells in G1 enter Go phase (quiscent). Perform their function, but unable to divide. Permanent and stable cells are in Go . Stable cells renter cell cycle in presence of stimuli (eg: Growth factors)
Give the S phase in great detail
DNA synthesis/ DNA replication. Enzymes of replication active. DNA proofreading activity of DNA polymerase is active. Entire chromosome replicates. At end of S phase, DNA content doubles. Each chromosome has two sister chromatids (2n; 4d)
What happens in G2 phase(summary)
DNA content is double (2n;4d). Cell /cytosol size increases. Genome scanned for mistakes. Post replication DNA repair mechanisms active
Summarize the roles of p53
DNA damage results in increased levels of p53 protein which causes:
- cell cycle arrest or activation of G1-S checkpoint
- Activates DNA repair systems
- If damage is extensive, may cause Apoptosis
Loss of p53 tumor-suppressor activity allows proliferation of cells with DNA damage. Uncontrolled proliferation can lead to tumor development
What happens to chromatid number/ DNA content at the end of S phase?
Chromatid number/ DNA content (4d) doubles at the end of S phase( also in G2) - replication takes place in S phase-each chromosome contains 2 suster chromatid
How does mitosis form two daughter cells ?
Segregation/ separation of sister chromatids and later cytoplasmic division to form two daughter cells
How identical or different are daughter cells to their parent cells?
Genetic composition of daughter cells identical to parent (2n)
What happens in Prophase of mitosis?
Nuclear envelope dissolve
- Mitotic spindle forms
- Chromosomes condense and visible
- Chromosomes bind to spindle
Homologous chromosomes (chromosomes of the same number - one paternal and one maternal ) and sister chromatids (are copies joined at the centromere)
What happens in metaphase of mitosis?
- Chromosoges condense and MAXIMALLY VISIBLE
- chromosomes align at EQUATOR
- Visuslized by karyotyping
- Microtubules attach to centromere kinetochore
When are chromosomes maximally visible?
After they condense in metaphase, can be seen with karyotyping
What occurs in anaphase of mitosis?
- Sister chromatids move to centrioles
- separation of sister chromatids
- Non-disjunction during anaphase is mitotic non-disjunction
What is the purpose of Telophase of mitosis?
- sister chromatids move to opposite poles
- nuclear envelope starts forming
- Chromosomes less condensed
- Two new daughter cell (cytokinesis)
Daughter cells have same genetic composition as parent cell (2n; 46 chromosomes; 23 pairs of chromosomes; 2d)
What occurs in non-disjunction?
Non-disjunction during mitotic divisions during embryogenesis(after fertilization)
- Sister chromatidsvof chromosome fail to separate —> Aneuploidy in daughter cells
- As a result, the fetus is mosaic
Karyotype shows a mixture of cells with aneuploidy
What can mitotic non-disjunction during embryogenesis?
Post-zygotic event during embryonic development (fertilization of normal ovum and normal sperm)
May lead to 2 cell types:
- Normal karyotype
- Cells with trisomy 21
What is Mosaicism?
Two or more populations of cells with different genotypes in an individual
Mixed somatic/germline mosaic
-Karyotype shows mixture of cells: normal cells (46,XX); and cells with aneuploidy (47, XX, +21)
What does “Confined germline mosaic “ mean?
- All somatic cells are of one genotype
- Gametes have a different genotype (Risk of having children with genetic disorder)
Gametes are …
Haploid (n;23 chromosomes)
Each gametes are genetically unique
When does meiosis occur?
Takes place only in gamete forming cells of ovary or testis (germline cells), final stages of gametogenesis
Differentiate meiosis in males and females
- Meiosis in males forms four gametes, each has 23 chromosomes: one of each kind of autosome and either an X or a Y (23X or 23Y)
- Meiosis in females results in formation of one ovum (23X) and 3 polar bodies
Are sister chromatids formed during meiosis?
Chromosomes duplicate before meiosis to form two sister chromatids (2n;4d)
When do homologous chromosomes exchange genetic material in meiosis?
Homologous recombination (meiotic crossover) in Prophase 1 of meiosis
Genetic diversity in gametes occurs in two stages…
Meiosis 1: Homologous chromosomes separate (reduction division)(n;2d)
Meiosis 2: sister chromatids separate (n;d)
Crossing over ensures…
Genetic variation
Make germ cell is…
2n….
Mother’s chromosomes are 1n
Father’s chromosome ‘s are 1n
What are the crucial roles of meiosis?
-Formation of haploid gamete (n). After fertilization, diploid set of chromosomes derived from other parent
- Gene recombination a/d diversity (gametes not alike)
- Recombination/crossover during Prophase 1 of meiosis
- Random segregation of homologous chromosomes during anaphase 1
Very briefly state what is nondisjunction in meiosis 1 and meiosis 2?
Meiosis 1: pair of homologous chromosomes fails to separate
All are abnormal gametes(n+1 n+1 n-1 n-1)(
Meiosis 2: pair of sister chromatids fails to separate (n+1, n-1, n, n) two abnormal gametes
What are the effects of non-disjunction during meiosis?
-Increased maternal age increases meiotic non-disjunction risk
- Aneuploidy (Chromosome number not multiple of 23)
- Trisomy of specific chromosome (3 copies of specific chromosome )- chromosome number 47
Karyotype shows ALL cells with abnormal karyotype -differentiate mitotic non-disjunction
Differentiate between non-disjunction during meiosis 1 or meiosis 2
- Marker analysis can identify whether non-disjunction occurred in meiosis-1 or meiosis-2
- For example, a chromosome has markers 1 (paternal) and 2 (maternal)
- If child has copy of 1 and copy of 2 ( 1,2), it is meiosis 1 non-disjunction (homologous chromosomes failed to separate; but sister chromatids separate)
- If child has two copies of marker 1 (1,1), it is meiosis-2 non-disjunction (sister chromatids failed to separate)
Describe uniparental disomy
In pair of homologous chromosome, one copy derived from dad(paternal) and 2nd copy is derived from mom (maternal)
Both copies from mom maternal uniparental disomy ); no dads copy
Both copies from dad (paternal uniparental disomy) and no moms copy
Describe imprinting
- When paternal copy of gene is methylated and inactive; Paternally imprinted gene; Only maternal copy of the gene is active
- When maternal copy of gene is methylated and inactive; Maternally imprinted gene; Only paternal copy of this gene is active
Imprinting on chromosome 15and disorders discussed in Patterns of inheritance
What does meiosis 1 nondisjunction in males result in?
Homologous chromosomes don’t separate
2 sperm cells without sex chromosomes but all autosomes
2 sperm cells with 2 sex chromosomes
How can someone get Turner syndrome?
No sex chromosomes but all the autosomes(sperm cells 1 and 2 of nondisjunction in meiosis 1)- ovum with one X chromosome leads to Turner syndrome X,45
How can someone get Klinefelter syndrome, 47,XXY?
Sperm cell 3 and 4 in nondisjunction of meiosis 1 have two sex chromosomes And the X chromosome from an egg
What are the results of nondisjunction in males in meiosis 2?
Homologous chromosomes separate
Sperm cell 1 and 3 only have autiosomes
Sperm cells 2 and 4 have sisteer chromatids, since they didn’t separate in meiosis 2
What are the effects of nondisjunction in meiosis 2 in males?
Turner syndrome sperm cells 1 and 3 that only have autosomes, fertilize with ovum with only X chromosome form individual with Turner syndrome, 45,X
47, XYY is formed with sperm cell 2 when nondisjunction occurs in sperm cells in meiosis 2(has two y cells) and gains X chromosome with egg
47, XXX is formed when nondisjunction in meiosis 2 in sperm cell, resulting in two XX chromosome in sperm cells
Briefly compare male and female gametogenesis
Overall gametogenesis is the same in males and females but there are some very significant differences
- Male meiosis produces four sperm cells per round
- Female meiosis produces one egg and three polar bodies
- Mitotic division of cells occur at different times in males and females. Females have undergone all mitotic division by birth
Summarize the timeline of gametogenesis in females
Meiosis—> Oogenesis begins during prenatal development—> Primary oocytes (several million) arrest at Prophase 1 of meiosis at birth—> most degenerate, and about 400 remain to mature and ovulate—> ovulation, on average once per month (completion of meiosis 1)—> meiosis 2 is completed only after fertillization
What are the differences between gametogenesis in males & females ?
Males- gametogenesis commences in puberty, while females have it commence in early embryonic life (Oocytes are arrested in Prophase 1 of meiosis until ovulation)
of mitosis in gamete formation= 30-100 in males whereas females have 20-30
Gamete production per meiosis= 4 spermatids in males, 1 ovum, 3 polar bodies in females
Gamete production= 100-200 million per ejaculate in males whereas 1 ovum per menstrual cycle
What are defects if maternal or paternal age is increased?
- Higher risk of non-disjunction (non-separation of homologous chromosomes/sister chromatids) in meiotic divisions I or II in a female greater than 35 years, resulting in higher risk of trisomy (chromosomal abnormality) in fetus
- Higher the paternal age, greater is risk of development of new dominant single gene mutations (Replication errors)
What are the differences between meiosis and mitosis ?
- Mitosis- occurs in all dividing somatic cells, during embryogenesis and initial stages of gametogenesis
Meiosis- takes place only in germline cells during gametogenesis
- Genetic composition of daughter cells is same as parent (46,XX or 46,XY) in mitosis whereas meiosis sees gametes formed have a unique genetic composition due to meiotic crossover (homologous recombination)
- Gametes are haploid in meiosis while cells are diploid in mitosis
