Meiosis Flashcards
Why do humans reproduce sexually
Humans and most other animals reproduce sexually. This allows for ____genetic variety___________________.
How is a zygote formed
two cells that fuse together to make a new “baby” organism. After fertilization, the fused gametes form a __zygote____________
What are gametes and what are human gametes
specialized cells are called ____gametes____________ or sex cells.
Sperm and egg cells are human gametes
Where does meiosis occur
In males- in the _testes_____________
In females- in the __ovaries___________
What is the general process for meiosis
1) Chromosome # needs to be cut in half
2) Creates genetic diversity (can help evolution of species)
Describe the pairs of chromosomes for humans
Pairs 1-22 are said to be __homologous chromosomses_ (they go together because they have same traits but are not identical).
Homologous chromosomes are similar is size and shape and carry genetic information for the same genes.
What is meiosis
Meiosis is a special type of cell division that halves the number of chromosomes in gametes. This insures that the correct number of chromosomes are passed on.
Sex detemrination
The sex chromosomes consist of a matching pair of homologous chromosomes in females and a partially matching pair in males.
The larger chromosome is the _X______.
XX for females and XY for males.
Females produce eggs that only contain the X chromosome, males produce sperm that are either X or __Y____.
This system is known as the XX/XY sex-determination system.
Stages of Meiosis
Meiosis has two rounds of cell division:
_____meiosis I___________
_____meiosis II ____________
Each round is divided into 4 phases:
Prophase, Metaphase, Anaphase, Telophase
There is only one duplication of the chromosomes.
Prophase 1
Chromosomes condense & shorten, visible
Spindle fibres form
Centrioles moving towards poles of the cell
Homologous chromosomes form a tetrad made of 4 chromatids
Process of forming a tetrad is called ___synapsis___________
Crossing Over
Occurs during prophase 1
In the tetrad, chromatids often break at the end and swap places with the sister chromatid
This process is called recombination or crossing over and allows for genetic variation
The location of recombination is called the ___chlasma_____________
Metaphase 1
Tetrads line up along equator randomly
= ___Independent___________Assortment
Spindle fibres attach to the pair of sister chromatids
Anaphase 1
The sister chromatids do __not____ separate here (different than mitosis)
Instead, the ___pairs________ of chromosomes move apart to opposite poles
The chromosome number is reduced from 2n (diploid) to n (haploid)
Telophase 1
Chromosomes condense slightly, nuclear membrane may form
Cytokinesis occurs forming two genetically different daughter cells
Each daughter cell has half the number of chromosomes as the parent cell (____haploid - n______________)
Meiosis 2
Similar to mitosis, but no duplication of chromosomes during a very short interphase (no G1 or S phase)
Each chromosome (made of two “mixed” chromatids) lines up at equator, centromeres split, and each chromosome is pulled to opposite poles
End result – ____four haploid cells___________________________
