Studied by 3 people
Asexual life cycle
Offspring are genetically identical to the parent, having the exact same chromosome
Sexual life cycle
Offspring are genetical distinct from each other an from each of the parent. Chromosomes are different, which allows for genetic diversity
Fertilization
Fusion of gametes involved in eukaryotic sexual reproduction
Meiosis
Process in which the nucleus of an eukaryotic cell divides to produce genetically different cells
Why must meiosis occur?
Fertilization doubles the number of chromosome each time it occurs (n + n = 2n). Hence, the chromosome number of the sex cell must be halved to keep it from doubling every generation. Without it, the sexual life cycle of eukaryotes would not occur
When does meiosis occurs in animals?
During the creation of gametes
What occurs before meiosis?
During interphase, all of the DNA in the nucleus of the parent cell is replicated
Each chromosome now consists of two sister chromatids
Sister chromatid are genetically identical because DNA replication is very accurate and the number of mistakes in the copying of DNA is extremely small
What is meiosis divided into?
Meiosis I and II
What are the phases of meiosis?
Prophase, metaphase, anaphase and telophase
Prophase I
Synapsis
Crossing over
Synapsis
Process in which homologous chromosomes pair up with each other
Name of combination formed after <del>synapsis</del>
Bivalent (pair of homologous chromosomes) or tetrad (group of four chromatids)
How is a bivalent also called?
Tetrad
Synaptonemal complex
A protein-based structure that forms between the homologous chromosomes in a bivalent
Crossing over
In a bivalent, two non-sister chromatids extend over each over at homologous sequences. Each sequence breaks apart and rejoins with the other non-sister chromatid at the region of the break
Chiasmata
X-shaped points that connect the non-sister chromatids at the site where crossing over has occurred
Where do crossovers occur?
At random position anywhere along the chromosome
How many crossovers occur?
At least one in each bivalent
What results from crossing over?
Since crossover occurs at the same base sequence on both chromatids, there is a mutual exchange of genes between the chromatids
Recombinant chromosomes
Chromosomes with a new combination of alleles resulting from crossing over
What characteristic of non-sister chromatids allow recombinant chromosomes to form?
Since non-sister chromatids are homologous but not genetically identical, some of the alleles of the exchanged genes are likely to be different
Metaphase I
Nuclear membrane breaks down
Spindle microtubules grow from the poles of the cell and attach to the bivalents, moving them to the equator of the cell
Each of the homologous chromosome in a bivalent is attached to a different pole
Random and independent orientation
Orientation of bivalents
The direction in which the bivalent is facing
Random orientation
The pole faced by a chromosome in a bivalent is random. Thus, each chromosome has an equal chance of attaching to a pole, an eventually being pulled to it
Independent orientation
The orientation of one bivalent does not affect the orientation of other bivalents
Anaphase I
Chiasma in bivalents slide to the end of the chromosomes, allowing them to separate
Disjunction
Disjunction
Process in which the homologous chromosomes in each bivalent separate from each other, moving to opposite poles
Anaphase from mitosis vs Anaphase I
Unlike in mitosis, in which the centromere divides and the chromatids of a chromosome move to opposite poles, in anaphase I, the centromere does not divide and whole chromosomes move to the poles
Telophase I
Chromosomes uncoil
Cytokinesis (division of the cytoplasm) occurs
What is the product of meiosis I?
Two separate nuclei containing one of each type of chromosome (n) → two haploid cells
Why is meiosis called reduction division?
Because the chromosome number is halved
What happens immediately after meiosis I?
After meiosis I, daughter cells enter meiosis II without passing through interphase (i.e. no DNA replication).
Meiosis II
Similar to mitosis in that the replicated chromosome is separated into chromatids
How does meiosis II differ from mitosis?
Separated sister-chromatids are likely to be non-identical sister chromatids due to crossing over
What is the product of meiosis II?
Four haploid nuclei
Nuclei, chromosomes per nucleus and chromatids per chromosome AFTER interphase, meiosis I and meiosis II
Interphase I:
1 nuclei
46 chromosomes, each consisting of 2 chromatids
Meiosis I:
2 nuclei
23 chromosomes, each consisting of 2 chromatids
Meiosis II:
4 nuclei
23 chromosomes, each consisting of 1 chromatid
Alleles in gamete
When the alleles are the same in both copies of the gene (one in each chromatid), each gamete produced by the parent will contain that allele
When the alleles are different, each of the two alleles has an equal chance of being passed on in a gamete
Processes in meiosis that cause genetic variation
Random orientation of bivalents
Crossing over
Random orientation in genetic variation
Combination of alleles that end up in each daughter cell depends on the orientation of the bivalents as they lined up
The different combinations increases genetic variation between the gametes
# of possible chromosome combinations from random assortment
2^n
Crossing over in genetic variation
Without crossing over, the combinations of alleles present in the parent cell would be forever linked together
e.g. if one chromosome carries AB and another carries ab, only these combinations could occur in gametes. With crossing over, Ab and aB are also possible
# of possible chromosome combinations from crossing over
Since crossing over can occur anywhere in the non-sister chromatids, the number of possible combinations is infinite
Fertilization in genetic variation
Allows alleles from to different individual to combine in one new individual
Since any male gamete can fuse with any female gamete, the combination of alleles is unlikely to ever have existed before, promoting genetic variation
# of possible chromosome combinations after fertilization
(2n)^2
Non-disjunction
When chromosomes do not separate correctly: both chromosomes move to one pole and none to another pole
When does it occur?
Anaphase I or Anaphase II
What is the result of non-disjunction?
Gametes with either an extra copy of a chromosome or no copies of a particular chromosome
Chromosome abnormality
When abnormal gametes are fertilized, resulting in a zygote with the incorrect chromosome number
Either 45 or 47 chromosomes
Non-disjunction in Down Syndrome
Occurs during anaphase I, when the 21st bivalent fails to separate
Why aren’t other trisomies in humans common?
Most other trisomies are so serious that the offspring does not survive
Examples of other trisomies
Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) both have very low survival rates with few babies surviving past their first birthday
Klinefelter’s syndrome
When the individual has sex chromosomes XXY
Does not impact life expectancy but may have a negative effect in fertility
Turners syndrome
When the individual has one sex chromosome
Often reduced life expectancy and lack of sexual development during puberty
What is the relationship between parental age and non-disjunction?
Many studies show that as the age of the parents increases, the incidence of non-disjunction increases
In particular, maternal age and Down syndrome
Note 
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