Meiosis (2) Flashcards
What is Meiosis?
Meiosis is cell division that takes place in the reproductive organs of both plants and animals to produce gametes (sex cells) in animals and spores in plants.
In meiosis, the number of chromosomes is ____ from two sets (__) in the parent cell to one set (__) in each of the daughter cells formed, i.e. the number of chromosomes is halved.
reduced
(2n)
(n)
What are the gametes/ spores formed called?
The gametes/spores formed are called haploid as they only have one set of chromosomes, i.e. one chromosome from each homologous pair.
In sexual reproduction, a male haploid gamete fuses with a female haploid gamete during fertilization to form a ____ zygote.
diploid
Where does meiosis take place in animals? (2)
In animals, meiosis takes place in the reproductive organs, the testis and ovaries.
- The formation of sperm cells in the testis is called spermatogenesis.
- The formation of egg cells or ova in the ovaries is called oogenesis.
Where does meiosis take place in plants? (2)
In plants, meiosis takes place in the formation of spores in sporangia. In seed-bearing plants:
- microsporangia are the pollen sacs in the male anthers - megasporangia are the ovules in the female ovaries.
The DNA of the parent cells is replicated in ____ preceding both mitosis and meiosis. However, in meiosis, replication is followed by two divisions.
interphase
What is meiosis 1?
Meiosis 1 is a reduction division that results in two cells being formed each with half the number of chromosomes of the parent cell, i.e. the haploid (n) number.
What is meiosis 2?
Meiosis 2 is a copying division that involves the two haploid cells formed, each dividing again by mitosis to form 4 haploid cells.
Meiosis 1- a reduction division (5)
- Early prophase 1
As in mitosis, the chromosomes become short and fat and are visible as two chromatids joined by a centromere. From here on the behaviour of the chromosomes in meiosis 1 and in mitosis differs. - Late prophase 1
The chromosomes of homologous pairs lie alongside one another, forming a bivalent. The centrioles move to opposite poles. A spindle, made up of protein threads, develops across the cell from the two centrioles. It is at this stage that crossing over takes place. - Metaphase 1
The bivalents (not the chromosomes) move to the middle of the cell and line up on the equator. The centromeres become attached to the spindle threads. - Anaphase 1
The centromeres do not split. The bivalents separate and chromatids) are pulled away from each other by the contracting spindle threads. The chromosomes move to opposite poles of the cell. the chromosomes (not the - Telophase 1
The cytoplasm then divides (cytokinesis) to form two haploid cells, i.e. both the new cells only have one of each homologous pair of chromosomes.
Meiosis 2 - a copying division
The two chromatids making up each chromosome need to separate. Each of the haploid cells will therefore divide again by mitosis.
- Late prophase 2
Each chromosome is made up of two chromatids joined by a centromere. A spindle, made up of protein fibres develops. The nuclear membrane disappears. - Metaphase 2
The chromosomes move to the middle of the cell where they line up at the equator. The centromeres become linked to the spindle threads. - Anaphase 2
The centromeres play, allowing each chromosome to separate into two chromatids. Spindle threads contract and pull the chromatids apart. The chromatids, which are now called daughter chromosomes, move to opposite ends (poles) of the cell. - Telophase 2
Daughter chromosomes group together at the poles. A / new nuclear membrane starts to form around each set of daughter chromosomes. - Cytokinesis
The cytoplasm starts to divide forming two new daughter cells, each with the haploid number of chromosomes. This process is called cytokinesis. A new nucleolus forms.
What is the result of meiosis 2?
At the end of meiosis four new, non-identical, haploid cells are formed from one parent cell, each with half the original number of chromosomes. The gametes are not identical to the parent cell.
