Week 02 Modes of Inheritance Flashcards
What occurs at the G1 phase of the cell cycle? What is the significance of the G1/S checkpoint?
- Interphase begins with G1 (for gap 1).
- Cell grows and proteins necessary for cell division are synthesized
- Near end of G1, a critical checkpoint, (G1/S checkpoint) holds cell in G1 until the cell has all enzymes and proteins necessary for replication of DNA. After this checkpoint has been passed, the cell is committed to divide
- Before reaching the G1/S checkpoint, cells may exit the active cell cycle in response to regulatory signals and pass into a nondividing phase called G0, a stable state during which cells usually maintain a constant size. They can remain in G0 for an extended time, even indefinitely, or they can re-enter G1 and the active cell cycle.
What is the major event that happens at the S phase of the cell cycle?
- After G1, the cell enters the S phase (for DNA synthesis)
- Each chromosome is duplicated
- Although cell is committed to divide after the G1/S checkpoint has been passed, DNA synthesis must take place before the cell can proceed to mitosis.
- If DNA synthesis is blocked (by drugs or by a mutation), the cell will not normally be able to undergo mitosis.
- Before the S phase, each chromosome is un-replicated; after the S phase, each chromosome is composed of two sister chromatids
What happens during the G2 phase of the cell cycle?
What is the importance of the G2/M checkpoint?
- After the S phase, the cell enters G2 (gap 2) and several additional biochemical events necessary for cell division take place.
- The G2/M checkpoint is reached near the end of G2
- This checkpoint is passed only if the cell’s DNA is completely replicated and undamaged. Unreplicated or damaged DNA can inhibit the activation of some proteins that are necessary for mitosis to take place.
- After the G2 /M checkpoint has been passed, the cell is ready to divide and enters the M phase.
What occurs during the M phase of the cell cycle?
What are the stages of the M phase called?
- The M (mitotic) phase is the part of the cell cycle in which the copies of the cell’s chromosomes (sister chromatids) separate, and the cell undergoes division.
- The separation of sister chromatids in the M phase is a critical process that results in a complete set of genetic information for each of the resulting cells. Biologists usually divide the M Phase into six stages the five stages of mitosis (prophase, [prometaphase], metaphase, anaphase, and telophase), and cytokinesis.
- Important to keep in mind that the M phase is a continuous process and that its separation into these stages is somewhat arbitrary
The bottom cell is in which stage of mitosis and why? What is the major event happening here?
Prophase. Chromosomes condense and the mitotic spindle forms.
Below is a picture of earlier in prophase
What stage of mitosis is shown here and why? What is the main event happening at this time?
Metaphase. Chromosomes line up on the metaphase plate.
Below is a polar view of metaphase
What stage of mitosis is shown here and why? What is the main event happening at this time?
Anaphase. Sister chromatids separate and move toward opposite poles
What stage of mitosis is shown here and why? What is the main event happening at this time?
Telophase. Chromosomes arrive at spindle poles. The nuclear membrane re-forms and the chromosomes relax
Chromosomes are normally depicted as they appear in mitosis with two identical sister chromatids joined at the centromere. How is this structure different from the chromosomes in interphase (G1, S and G2)?
During mitosis/meiosis they are in their supercoiled, replicated form. Supercoiled (condensed) chromatin is visible under the microscope but no gene expression is able to take place as the DNA is tightly packed – this is why this only occurs when the cell is dividing (during prophase). Usually in Interphase most of the chromatin (all the euchromatin) is in its relaxed (extended) form. This is required for gene expression, and chromatin in this state is not visible under the microscope.
Replication of the chromosomes produces sister chromatids connected at the centromere, how chromosomes are usually visualised. However, during G1, before the S phase, chromosomes are unduplicated (only one copy is present).
How do the amount of DNA (copy number, C) and the number of chromosomes (n, eg diploid (2n), haploid (1n)) change through the different stages of the cell cycle and mitosis? Remember # chromosomes = # centromeres.
DNA copies/cell (C)
STAGE OF CELL CYCLE/MITOSIS
COPY NUMBER
NUBMER OF CHROMOSOMES/CELL
G1
2C
2n
S
2C 4C
2n
G2
4C
2n
Prophase
4C
2n
Metaphase
4C
2n
Anaphase
4C
4n
Telophase
4C2C
4n 2n
Cytokinesis
2C
2n
How does meiosis differ from mitosis?
- Mitosis consists of a single nuclear division and is usually accompanied by a single cell division, while meiosis, on the other hand, consists of two divisions
- After mitosis, the chromosome number in the newly formed cells is the same as that in the original cell, whereas meiosis causes the chromosome number in the newly formed cells to be reduced by half
- Mitosis produces genetically identical cells, whereas meiosis produces genetically variable cells
What are the major events at each stage of Meiosis I?
Chromosomes replicate before meiosis (interphase)
Chromosomes begin to condense and pair together (prophase I)
Chromosome pairs (homologues) line up together on the metaphase plate (metaphase I)
Chromosome pairs separate (anaphase I)
Nuclear membranes re-form (telophase I)
What are the major events at each stage of Meiosis II?
No further replication occurs.
New spindles form (prophase II).
Chromosomes line up again (metaphase II).
Sister chromatids separate at the centromere (anaphase II).
Nuclear membrane re-forms (telophase II)
https://youtu.be/nMEyeKQClqI
What is crossing over? When does it occur?
During bivalent pairing at meiotic prophase, the maternal and paternal homologues usually swap one or more sections of chromatid in a process called crossing over.
This process results in a recombination event. Recombination can only occur in prophase I of meiosis.
What is a dominant trait/allele?
Dominant Trait/Allele: the allele/trait that is expressed in a heterozygous individual, ‘overriding’ the recessive allele/trait
What is Mendel’s first law of independent segregation?
“For any single gene, the alleles segregate independently into gametes, such that any gamete has an equally likely chance of inheriting either allele”
What is Mendel’s second law of random assortment?
“For any pair of genes, the alleles of one gene will sort into gametes independently of the alleles of the second gene.”
That is, which allele of gene A a gamete receives, does not affect which allele of gene B that same gamete receives.
What is a monohybrid cross?
A monohybrid cross is a cross between two organisms with different variations at one genetic locus of interest.
Types of Cross are defined by what we do with the F1! Monohybrid = single gene F1 x F1
What is the genotypic and phenotypic ratio in a monohybrid cross?
Genotypic Ratio: 1 AA : 2 Aa : 1 aa
Phenotypic Ratio: 3 Dom : 1 Rec
What is a dihybrid cross?
A dihybrid cross is a cross between two individuals who differ in two observed traits that are controlled by two distinct genes.
Types of Cross are defined by what we do with the F1! Dihybrid cross = 2 gene F1 x F1
What are the genotypic and phenotypic ratios for a dihybrid cross?
Don’t need to know exact genotypic ratios as complicated and difficult to analyse!
Dominant traits will appear in the phenotype whether homo or heterozygous with dom, so:
Genotypic & Phenotypic Ratio = 9 : 3 : 3 : 1
9 A_B_ Dom A Dom B
3 A_bb Dom A Rec b
3 aaB_ Rec a Dom B
1aabb Rec a Rec b
What are the genotypic and phenotypic progeny ratios for a one gene test cross?
Genotypic & Phenotypic Ratio: 1 : 1
Progeny:
- ½ Aa - Dom
- ½ aa - Rec
What is the genotypic and phenotypic ratio for a two gene test gross?
Genotypic & Phenotypic Ratio: 1 : 1 : 1 : 1
Progeny:
- ¼ AaBb Dom A Dom B
- ¼ Aabb Dom A Rec b
- ¼ aaBb Rec a Dom B
- ¼ aabb Rec a Rec b
