Meiosis Flashcards
Fertilization
combining of sex cells, or gametes, 2 haploid gametes fuse to form a diploid (2n) zygote
Zygotes
cell created by the fusion of two
gametes; fertilized egg
Diploid vs haploid
Haploid cells are those that have only a single set of chromosomes (n), while diploid cells have two sets of chromosomes (2n)
What type of cells undergo meiosis
germ cells
What types of cells are produced
gametes (sex cells)
Meiosis I
Preceded by DNA replication in S phase
- Homologous chromosome pairs separate, but the individual chromosomes (sister chromatids) stay together
Prophase 1
Chromatin begins to condense
- Synapsis: specific parallel alignment (pairing) of homologous chromosomes
- Tetrad: four chromatids of each pair of homologous chromosomes
Prometaphase
Chiasmata: regions of attachment that form between non sister chromatids, crossing over and exchanging genetic material, resulting in recombinant chromatids and increases genetic variability of the products
Metaphase 1
Homologous pairs line up on
equatorial (metaphase) plate
Anaphase 1
Homologous chromosomes – each with two chromatids – move to opposite poles of the cell
Telophase 1
Chromosomes gather into nuclei
- Original cell divides
Independent assortment
chromosome pairs align randomly during metaphase I, each haploid cell receives an entire set of genes, but only one from each pair (again, the pair comes from each parent)
Meiosis II
Not preceded by DNA replication
- Sister chromatids are separated
- Chance assortment of the chromatids contributes to genetic diversity
- Final products are four haploid daughter cells (n)
Prophase II
Chromosomes condense again after brief interphase (but chromosomes do not replicate)
Metaphase II
Centromeres of paired chromatids line up along equatorial plate of each cell
Anaphase II
Chromatids separate, becoming chromosomes
- Move to opposite poles
- each new cell has different genetic makeup
Telophase II
Chromosomes gather into nuclei and cells divide
Products
Each of the four cells has a nucleus with a haploid number of chromosomes, DNA replicates once, nucleus divides twice
Nondisjunction
- Homologous pairs fail to separate at anaphase I
- Sister chromatids fail to separate in meiosis II
- Result: aneuploidy – chromosomes lacking or present in excess, lack of cohesins cause homologs to go to the same pole
Human example of nondisjunction
Down syndrome results from a gamete with two copies of chromosome 21
- After fertilization, there are three copies (trisomy) of chromosome 21
Main differences between mitosis and meiosis
mitosis produces two identical daughter cells with the same number of chromosomes as the parent cell, while meiosis produces four genetically different daughter cells with half the number of chromosomes as the parent cell
