Cell Division and Chromosomal heredity Flashcards
What is the difference between homologous chromosomes and sister chromatids? (Hint: think about sequence similarity/identity)
- Homologous chromosomes are paired chromosomes, one from each parent, carry similar (NOT the same) genetic material.
- Exchange material in prophase I
- Sister chromatids are identical halves of a whole chromosome.
How many pairs of chromosomes are found in a typical human somatic cell?
23 pairs
True or false?
a. ( ) Between the interphase and mitosis, cells can enter a stage called G0, in which they
are quiescent.
b. ( ) DNA synthesis occurs during the S phase of the interphase stage.
c. ( )During the G1 phase, the cell checks for DNA damages that might have occurred
during mitosis.
d. ( ) The interphase is characterized by the absence of visible chromosomes.
e. ( ) The transitions from one interphase stage to the other are controlled by cell cycle
checkpoints.
a. true
b. true
c. False, happens during G2
d. true
e. true
What is the principal difference between mitosis and meiosis? (Hint: think about the amount of DNA and the number of chromosomes)
- Mitosis leads to the production of two cells, each with the same number of chromosomes as the parent cell.
- Meiosis converts the diploid number (2n) of chromosomes to the haploid number (n), and leads to the production of gametes.
What is the role of chromosomes in cell division?
Chromosomes carry the genetic material; condense and replicate in cell division.
Explain what a karyotype is and when is the best time to obtain one during mitosis?
Image of a complete set of chromosomes arranged in pairs according to size and centromere location photographed from a metaphase cell
If 2n=6, how many DNA molecules are there for each chromosome in:
a. G1 stage?
b. G2 stage?
c. Mitotic metaphase?
d. Mitotic anaphase?
e. After cytokinesis
a. Chromosomes= 6, DNA= 6
b. Chromosomes= 6, DNA= 12
c. Chromosomes = 6, DNA= 12
d. Chromosomes= 12, DNA= 12
e. Chromosomes= 6, DNA= 6
What is the effect of colchicine on the cell cycle?
Causes damage to the spindle and arrests mitosis
True or false? At the completion of metaphase, the chromosomes align in the metaphase plate, with the spindle fibers attached to their telomeres.
False, spindle fibers are connected to their kinetochores.
What is karyokinesis and what phase does it occur
Formation of 2 new nuclear membranes and occurs in telophase
Whit is cytokineses and which phase does it occur
Formation of 2 new cell membranes and occurs in telophase
Identify the mitotic stage from fluorescent images
Go back to notes and identify or look at pictures or cram
Name the stages of meiotic division and the key event in each
- Prophase I
○ DNA was replicated previously in Interphase
○ Chromosomes condense and become visible
○ Synaptonemal complex - homologs pair and form tetrads
○ Crossing over - exchange of genetic material between homologs - Metaphase I
○ Each tetrad has 2 centromeres
○ Terminal chiasmata hold non-sister chromatids together
○ Tetrad orientation on metaphase plate is random - Anaphase I
○ Separation of homologous chromosomes - disjunction - Telophase I (much shorter than mitotic)
What is a monad and how many DNA molecules contained in each
1 chromatid (1 molecule), mixture at the end of meiosis II
What is a dyad and how many DNA molecules contained in each
2 chromatids (2 molecules), mixture at the end of meiosis I
What is a tetrad and how many DNA molecules contained in each
4 chromatids (4 molecules)
Which cell division is essential for sexual reproduction (mitosis or meiosis)? Why?
Meiosis produces haploid gametes which are used for fertilization.
Which cell division is essential for survival (mitosis or meiosis)? Why?
Mitosis leads to the production of two cells, each with the same number of chromosomes as the parent cells (replication and maintenance)
What happens in Pachynema
- crossing over between non-sister chromatids
- Pairing and synapsis complete
- Formation of tetrads - sister and non-sister chromatids
What happens in Diplonema
- crossing over
- Each bivalent = 4 chromatids = tetrad (4 DNA molecules, 8 DNA strands)
- Chiasmata - reflect sites of recombination
- Non-sister chromatids start to separate - chiasmata keep them together
How many stage of prophase I are there(5)
Leptonema, zygonema, pachynema, diplonema, diakinesis
What are chiasmata and when do they occur?
- Physical link between two chromatids belonging to homologous chromosomes, they reflect the site of recombination
- Occur in diplonema stage of prophase I
What is crossing over? What is its biological importance?
- Synapsed homologs exchange genetic materials
○ Generates new combination of parental genetic material
○ A source of genetic material
○ Corrects mutations
What are recombination hotspots? Coldspots? Give examples.
- Hotspots: regions in a genome that exhibit elevated rates of recombination. Species specific
○ Ex: major histocompatibility complex and the pseudoautosomal region in males (HTT, cag) - Coldspots: regions in a genome that exhibit low frequency of recombination
○ Ex: centromeres
True or false?
a. ( ) In spermatogenesis, the spermatogonia undergo the first meiotic division to produce
the primary spermatocytes.
b. ( ) The secondary spermatocytes undergo meiosis II to produce haploid spermatids.
c. ( ) In oogenesis, the primary oocytes are diploid, as well as the secondary oocytes
d. ( ) Oogenesis is not a continuous process in mammalian females. Explain.
e. ( ) Polar bodies are the result of equal cytoplasm division during meiosis I.
a. False, undergo mitosis
b. Ture
c. False, the secondary oocytes are haploid
d. True, females are born with their lifetime supply of oocytes, oogenesis occurs before birth and finishes during puberty
e. False, they’re the result of unequal cytoplasmic division
What is the Dictyate stage? Know the unique features of meiosis in female mammals.
- State where oogenesis is arrested at diplonema and remains for months, years, or decades depending on the species.
○ Primary oocyte meiotic division 1 starts in embryogenesis.
Stops in prophase I - diplonema (crossing over has already occurred), this stop can
last for years.
○ Oogenesis resumes in puberty and completes 1st meiotic division:
▪ Secondary oocyte and first polar body
▪ Meiosis II takes place after fertilization
How do you explain the fact that only females produce milk in mammals? What kind of trait is milk production?
Sex-limited inheritance (similar to beard growth in males)
What is sex linked traits and give an example
transmission of genes on sex chromosomes
○ Ex: color blindness and congenital hypertrichosis
What is sex limited traits and give an example
expression of a phenotype is absolutely limited to one sex
○ Ex: breast development and beard growth
What is sex influenced traits and give an example
sex of an individual influences the expression of a phenotype that is not limited to one sex or another
○ Ex: pattern baldness, dominant in males and recessive in females
What is the PAR and how does it allow the X and Y chromosomes to find each other?
- Pseudoautosomal region, these actually crossover for proper disjunction in male
meiosis. - There are two on X and Y chromosomes.
In humans, females have two X chromosomes, while males only one. What is the mechanism by which the expression levels of genes present in this chromosome is balanced between genders?
- X chromosome-inactivation dosage compensation
○ Dosage compensation prevents excessive expression of X-linked genes in mammals, and balances the dose of X chromosome gene expression in females and males
○ Inactivation of X chromosome is random
○ Occurs in somatic cells at an early stage of embryonic development and is then passed on to progeny cells by mitosis
▪ Calico cats and fur color/patterns
What are Barr bodies?
- Condense inactive X chromosomes
- Found only in females
- are visible in interphase
Which group of genes escape X inactivation?
PAR genes and X-Y gametologs
Which feature of X inactivation causes large patches of orange and black color to appear in tortoiseshell cats?
- Lyon hypothesis: random X inactivation
- Fur pigmentation is X-linked, color of the coat (orange or black) depend on which X is inactivated.
What is the role of the XIST mechanism in the X inactivation process?
- X-inactivation-specific-transcript - gene that produces large RNA molecules that spread
out and cover/paint the chromosome to be inactivated
~ Can only act on the chromosome from which it is being transcribed and not the homolog
What kind of gene is XIST? (protein coding? Non-protein coding? RNA-gene? Small RNA gene? Long non-coding RNA gene?)
Large non-coding RNA gene
What is the importance of the SRY gene?
- sex-determining region Y - determination of sex depends on the presence or absence of this gene; produces a transcription factor for male specific genes
○ Codes for the testis-determining factor (TDF) that triggers testes formation
What are sex chromosomes and how do they differ from the autosomes?
- Characteristic to one sex or another
- Heteromorphic (dissimilar)
- Different in gene content
- Do not recombine (at least partially)
Name the two main sex chromosome systems in nature and explain their difference.
XY system -males heterogametic
○ Male gametes = with X or Y
○ Female gametes = with X only
ZW system - females heterogametic (birds)
○ Male gametes = with Z only
○ Female gametes = with Z or W
Understand the Drosophila sex determination A:X ratio.
Sex is determined by the ratio of X chromosomes to autosomes
○ XX and AA (1:1) is female
○ XY and AA (2:1) is male
○ XXX and AA (2:3) is metafemale
Prophase (50%)
- Sister chromatids become visible - DNA decatenation
- Nucleolus disintegrates
- Nuclear membrane breakdown
- Centrioli divide and move apart
Prometaphase
- Chromosomes move towards cell midline
- Chromosome condensation continues
- DNA decatenated - sister chromosomes visible
- Formation of spindles
- No nuclear membrane
- Chromosomes attach to spindles
Metaphase
- Chromosome condensation maximum
- Chromosomes attach to spindles from both poles
- Kinetochores must be under tension
Anaphase
- Split of centromeres
- Separation of sister chromatids to poles of the cells
▪ Movement of microtubules and spindles poles - After disjunction, sister chromatids are called daughter chromosomes
Telophase
- Sister chromatids/daughter chromosomes arrive at the poles
- Karyokinesis - formation of 2 new nuclear membranes
- De-condensation of chromosomes
- Cytokinesis - formation of 2 new cell membranes
What is chromosomal nondisjunction and how does it affect ploidy?
When chromosomes/sister chromatids do not separate properly, leading to abnormal
numbers of chromosomes in offspring
Can sex chromosomes be referred to as ‘homologs’? Why or why not?
No, because each sex chromosome contains different information (X and Y are very different, they are only able to synapse because of the PARs)
