Meiosis and Sexual Life Cycles Flashcards
What does heredity mean?
Transmission of traits from one generation to the next
What does variation mean?
It’s differences between members of a species
What is genetics?
Scientific study of heredity and hereditary variation
What is a gene?
A stretch of DNA encoding a specific RNA or protein
Locus (loci)
The specific location of a gene on a chromosome
What is asexual reproduction?
- Single parents produces offspring with identical genomes
- Mitosis
What is sexual reproduction?
- Parents produce offspring with unique gene combinations
- Involves meiosis and fertilization
- Produces genetic variation
What are gametes?
Reproductive cells
What are somatic cells?
Any body cell except sperm or egg cells
How many copies of each chromosome do somatic cells have?
- Two copies
- Diploid
How many copies of each chromosome do gamete cells have?
- One copy each
- Haploid
What are homologous chromosomes?
- Chromosomes with the same genes at the same loci
- May carry different alleles
Karyotype
A display of the chromosome pairs of a cell arranged by size and shape
How many pairs of chromosomes do humans have?
- 23 pairs
- 22 pairs of autosomes and 1 pair of sex chromosomes
Sexual life cycle - animals
- Gametes are the only haploid cells
- A zygote divides through mitosis to grow into a multicellular adult human
- Adult human germ cells (2n) produce gametes (n) through meiosis
What is fertilization?
Gametes (n) join and their nuclei fuse, producing a zygote (2n)
Sexual life cycles - plants and some algae
- Meiosis produces haploid spores that undergo mitosis to produce a multi-cellular haploid organism
- Gametes are produced by mitosis
- Fertilization produces a diploid zygote that undergoes mitosis to produce a multicellular diploid organism
What is the alternation of generations?
Includes both diploid and haploid stages that are multicellular
Sexual life cycles - most fungi and some protists
- Fertilization produces a unicellular diploid zygotę that undergoes mitosis to produce haploid cells
- Haploid cells then undergo mitosis to produce unicellular descendants or a multi-cellular haploid organism
- Gametes are produced from haploid cells via mitosis
Meiosis Overview
- Producing haploid cells from diploid cells, involves 2 rounds of cell divisions but one round of DNA replication
- 4 haploid cells are produced from 1 diploid cell
Meiosis I - Prophase I
- Chromosomes condense
- Spindle forms
- Nuclear envelope breaks down
- Crossing over occurs
- Kinetochore forms
- Microtubules attach
- Non-sister chromatid break at points, and homologues held together by the synaptonemal complex in a process called synapsis
What is crossing over?
- exchange of DNA between non-sister chromatid during prophase I
- increases genetic variability
What is synapsis?
The fusion of chromosomes pairs at the start of meiosis
What is chiasma
- point of contact between non sister chromatids
Meiosis I - Metaphase I
homologous chromosome pairs align at metaphase plate
Meiosis I - Anaphase I
- Cohesions breakdown along chromosome arms
- Homologous chromosomes move to opposite poles
Meiosis I - Telophase I and cytokinesis
- Forms two haploid cells
- Each chromosome still consists of two sister chromatid
Meiosis II Overview
- Equivalent to mitosis, sister chromatids split but they are not genetically identical
- Each haploid cell from meiosis I produces two haploid cells
Meiosis II - Prophase II
- mitotic spindle starts to form
- nuclear envelope fragments
- kinetochore forms
- microtubules attach
Meiosis II - Metaphase II
chromosomes align at metaphase plate
Meiosis II - Anaphase II
- cohesion sprung centromere cleaved
- sister chromatid separate
- cell elongates
Meiosis II - Telophase II and cytokinesis
- nuclei form
- cells split
- product is 4 haploid cells
What events are unique to meiosis?
- Synapsis and crossing over
- Alignment and separation of homologous chromosomes
What is the main advantage of sexual reproduction?
The new combinations of alleles
What are the 3 factors that contribute to genetic variability?
- Independent assortment of chromosomes
- Random fertilization
- Crossing over
What is independent assortment of chromosomes?
- alignment of each pair of homologues is independent of other pairs of homologues 2^n possibilities
- where n = number of pairs of chromosomes
What is random fertilization?
- any sperm and egg can fuse to produce a zygote
- 2^n * 2^n possibilities
