5 - Meiosis, mitosis and the cell cycle Flashcards
What is the locus
A gene’s specific location on a chromosome
Explain diploid
Each diploid organism has two of each chromosome and therefore 2 alleles of each gene
Describe homo/heterozygous alleles
Homozygous (AA): alleles are the same for that gene
Heterozygous (Aa): different alleles for that gene
Compare and contrast mitosis and meiosis
Explain how humans are diploid
- Two copies of each of 22 autosomes and 1 pair sex chromosomes = XX, XY, X0 etc
- Each is a pair of homologous chromosomes
- One copy is via egg (maternal line) and other is from sperm (paternal line)
Define mitosis
a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus
What are the stages of the cell cycle
- Interphase: G1, S, G2
- Mitosis: prophase, metaphase, anaphase, telophase
- Cytokinesis
- Resting phase (G0)
Explain interphase
G1: growth and metabolic roles
S: replication of DNA
G2: Growth and more preperation
G roles: Increases mass of cells, growth and preperation for chromosome seperation
Nucleolus and nuclear envelope are distinct
Chromosomes are in the form of threadlike chromatin
Explain prophase
- Chromosomes begin to condense and become visible
- Nuclear envelope breaks down
- Centrioles move to opposite sides of cell
- Mitotic spindle begins to form (microtubules)
Explain prometaphase
- Chromosomes continue to condense
- Chromosomes begin to attach to the spingle at centromeres via kinetochores
- Centrosomes form the spindle poles
What are kinetochores
Connect chromosomes to microtubules
Explain metaphase
- Chromosomes are now tightly coiled
- Each has 2 chromatids that are lined up on the metaphase plate
- Each sister chromatid is attached to a spindle fibre from opposite pole
Explain Anaphase
- Centromeres split in 2
- Sister chromatids (chromosomes) are pulled towards opposite poles
Explain telophase and cytokinesis
Telophase
- Chromosomes are at the poles and decondense (become more diffuse)
- Nuclear envelope and nucleolus reform
- Mitotic spindle breaks down
- Spindle fibres continue to push the poles apart
- Cytoplasm begins to divide
Cytokinesis
- Cleavage furrow seperates daughter cells
- Division into 2 identical daughter cells is completed
How is the cell cycle regulated?
- Timing of events are controlled (each process can be turned on and off at certain times)
- Events occur in the correct order in a linear, irreversible direction
- Each event is triggered only once per cycle
- Redundancy (back ups) to ensure the cycle functions properly even when some parts are malfunctioning
- Adaptable systems so that it can be modified for different cell types and environmental conditions
Why is the cell cycle highly regulated?
ensures that a dividing cell’s DNA is copied properly, any errors in the DNA are repaired, and each daughter cell receives a full set of chromosomes.
What are the 3 important cell cycle checkpoints?
G1 checkpoint
- cell size
- nutrients
- growth factors
- DNA damage
- If cells don’t meet the conditions, move to G0 (resting phase)
G2 Checkpoint
- Cell size
- DNA replication
- DNA damage
Metaphase checkpoint
- Chromosome attachment to spindle
Why do we need cell cycle checkpoints?
- ensure that the processes have occurred, to enable cell cycle to move to next stage
- Unregulated cell cycles usually result in cancerous cells
What proteins help regulate cell cycle?
- Cyclins
- Cyclin-dependent kinases (CDKs)
What are cyclins?
- different ones increase during the cell cycle
- Activate CDKs
Describe how CDKs control the cell cycle
- CDKs are always there, but most are inactive, waiting for cyclins
- Kinases are proteins that activate/inactivate via phosphorylation
- Attached phosphate group acts like a switch (making the target protein more or less active)
- Target proteins control functions like: proteins that make nuclear membrane break down or promote DNA replication
- Threshold of cyclin-CDK activation = cell moves past “restriction point”
What is cancer and how is the cell cycle related to it?
- Cancer: disease of abnormal gene expression that allows for inappropriate cell division
- Cancer cells are in an unregulated cell cycle (loss of control)
- Develops in a multi-step process (multiple mechanisms must fail for cells to become cancerous)
- Most cancers arise as cells acquire a series DNA mutations that make them divide more quickly, escape internal and external controls on division, and avoid programmed cell death
Describe homologous chromosomes
- Sister chromatid from each parent
Not identical but have: - Genes controlling the same specific characteristics
- Genes are in the same order
- Gene sequences may change through mutation (=alleles)
- Diploid cells have 2 alleles for each gene
What is a karyotype?
- Display of homologous chromosomes for visual comparison
- Cell must be in metaphase of meiosis to see diploid human chromosomes
- Each chromosome has been copied into 2 sister chromatids and condensed so its ready for division
How are different alleles created
- Mutations in genes create different alleles
New alleles can: - Can be passed onto offspring
- Introduce genetic variation to population
- May increase ability of population to adapt to environmental change
How do genes show different genetic characteristics
Locus: position of gene on chromosome
Gene: confers trait (eye colour)
Allele: form of trait (specific colour)
First stage of meiosis
Interphase (G1 - cell grows; S - copies all of its chromosomes; G2 - prepares for division)
Describe prophase 1
- Occupies 90% of meiosis
- Chromosomes condense and become visible
- Homologous chromosomes (tetrads) pair up at the metaphase plates
- Synapsis and crossing over occur (1 sister chromatid of each pair of homologous chromosomes exchanges DNA)
Describe crossing over
- Produces genetic variation in offspring
- Produces new chromosomes that contain genes from both parents
- Homologous portions of 2 non-sister chromatids in a tetrad trade places
- 2-3 crossover events per chromosome pair in humans
- Increases genetic diversity of gametes
Describe Anaphase I
- Homologous chromosomes move apart to
opposite sides of the cell - Guided by spindle apparatus
- Sister chromatidsr emain attached at the centromere
Describe telophase I and cytokinesis I
- New nuclei form
- Cytoplasm of cell divides
- Haploid chromosome number (n)
- Chromosomes still as sister chromatids
- cytokinesis
Explain prophase II
- Each chromosome consists of 2 sister chromatids
- Chromosomes move towards metaphase plate
Explain metaphase II
chromosomes align on metaphase plate
Explain anaphase II
- Replicated chromosomes seperate
- Sister chromatids move towards opposite poles of cell
Explain telophase II
- Nuclei form at opposite poles of the cel
- Chromosomes unravel to form chromatin
Explain cytokinesis II
- Both cells split into 2
- Result: 4 daughter cells with haploid chromosome complement
- males: 4 sperm develop
- females: 1 egg + 3 polar bodies
What is independent assortment
What is reshuffling of genes?
