High school biology
|Gamete||A sex cell (in humans: sperm for males, and eggs for females)|
|Meiosis||A two-step process of cell division that is used to make gametes (sex cells)|
|Crossing over||Process in which homologous chromosomes trade parts|
|Interphase||Phase of the cell cycle where the cell grows and makes a copy of its DNA|
|Homologous chromosomes||Set of chromosomes (one from each parent), that are very similar to one another and have the same size/shape|
|Sister chromatids||Two halves of a duplicated chromosome|
|Diploid (2n)||Cell that contains two sets of homologous chromosomes|
|Haploid (n)||Cell that contains only a single set of genes|
The purpose of meiosis is to produce gametes, or sex cells. During meiosis, four daughter cells are produced, each of which are haploid (containing half as many chromosomes as the parent cell).
Stages of meiosis
Meiosis contains two separate cell divisions, meaning that one parent cell can produce four gametes (eggs in females, sperm in males). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase.
Before entering meiosis I, a cell must first go through interphase. This is the same interphase that occurs before mitosis. The cell grows, copies its chromosomes and prepares for division during the G phase, S phase, and G phase of interphase.
Meiosis I is the first round of cell division, in which the goal is to separate homologous pairs.
The phases of meiosis I.
Prophase I: The starting cell is diploid, 2n = 4. Homologous chromosomes pair up and exchange fragments in the process of crossing over.
Metaphase I: Homologue pairs line up at the metaphase plate.
Anaphase I: Homologues separate to opposite ends of the cell. Sister chromatids stay together.
Telophase I: Newly forming cells are haploid, n = 2. Each chromosome still has two sister chromatids, but the chromatids of each chromosome are no longer identical to each other.
The second round of cell division is meiosis II, in which the goal is to separate sister chromatids.
Phases of meiosis II
Prophase II: Starting cells are the haploid cells made in meiosis I. Chromosomes condense.
Metaphase II: Chromosomes line up at the metaphase plate.
Anaphase II: Sister chromatids separate to opposite ends of the cell.
Telophase II: Newly forming gametes are haploid, and each chromosome now has just one chromatid.
Common mistakes and misconceptions
- Interphase is not part of meiosis. Although a cell needs to undergo interphase before entering meiosis, interphase is technically not part of meiosis.
- Crossing over occurs only during prophase I. The complex that temporarily forms between homologous chromosomes is only present in prophase I, making this the only opportunity the cell has to move DNA segments between the homologous pair.
- Meiosis does not occur in all cells. Meiosis only occurs in reproductive cells, as the goal is to create haploid gametes that will be used in fertilization.
- Meiosis is important to, but not the same as, sexual reproduction. Meiosis is necessary for sexual reproduction to occur, as it results in the formation of gametes (sperm and eggs). However, sexual reproduction includes fertilization (the fusion between gametes), which is not part of the meiotic process.
Want to join the conversation?
- In telophase 1 and telophase 2, I am confused about the use of the word "haploid." They are both labeled "haploid," but they do not look the same.(20 votes)
- That is because in Telophase 1, the sister chromatids are still attached. Telophase 2 is basically splitting the 2 sisters apart, but, as each contains the same genetic info, it is still haploid. Hope that answers your query!(24 votes)
- I am confused about the number of sister chromatids between mitosis and meiosis. For example, human. After replication, there are 46 chromosomes (92 sister chromatids in total) in a cell, right?
During mitosis, they are split into 2 cells, each of which has 46 chromosomes (46 sister chromatids in total).
On the other side, during meiosis I, same as mitosis, they are split into 2 cells, each of which has 46 chromosomes (46 sister chromatids in total).
So, during meiosis II, these 2 cells was split again into 4 cells, each of which has 23 chromosomes (23 sister chromatids in total).
Am I right?(6 votes)
- Your thought is correct but plz notice, after splitting 2 sister chromosomes in mitosis, each single one is no longer a sister chromatid but a chromosome. So you cannot say 46 chromatids in total in your second graph, it is only 46 chromosomes or 23 pairs of homologous chromosomes:)
Meanwhile, in meiosis1, after splitting into two cells, each one contains 46 sister chromatids but only 23 chromosomes (notice that two sister chromatids is a chromosome).(7 votes)
- what is the difference between crossing over and synapsis?(4 votes)
- Synapsis is when the homologous chromosomes of the same size and length pair up. Crossing over is when bits of DNA are exchanged from each chromosome to produce genetically unique chromosomes. Though both happen in Prophase I, synapsis happens before the chromosomes can cross over. Hope that helps!(6 votes)
- What does Homologous mean?(4 votes)
- In biology it means similar in position, structure, and evolutionary origin but not necessarily in function for organs, but for chromosomes it means similar in position, structure, and evolutionary origin but not necessarily in function. hope this helps:)(7 votes)
- How can n divide to give 2n
Telophase 1 has 2 chromosomes each and they are n and Telophase 2 has 1 chromosomes in 1 cell and is also n how??(6 votes)
- it's because the germ cell goes through interphase another time before meiosis 2, so the chromosomes duplicate again.(1 vote)
- Is Meiosis a continues process or does it stop and begin whenever the body wants it to?(4 votes)
- Meiosis occurs in the testes in males and in the ovaries in females. In males, sperm production occurs in the testes almost continuously to replenish the supply. In females, egg cells are made from oocytes which are produce when the female is a fetus. Oocytes are cells arrested in meiosis I. Meiosis resumes when the oocyte is activated ready for ovulation and then arrests again in metaphase II. It is during fertilisation that the cell undergoes anaphase II so that the sister chromatids are finally separated(2 votes)
- so are the sister chromatids diploids?(3 votes)
- We say a cell is diploid or haploid or more,
Sister chromatids are just 'one-half' of the chromosome(4 votes)
- meiosis 2 is similar to mitosis because?(3 votes)
- Meiosis 2 is similar to mitosis because it separates the chromosomes to have sister chromatids in each cell. In both processes, you are separating the chromosome and dividing the cell to make 2 cells out of 1 (the only difference is that in meiosis, you're doing that for 2 cells to get 4). These are called daughter cells.(4 votes)
- what happens to cells ( what is their fate or role) that have completed the mitosis cycle and the meiosis process??(3 votes)
- after mitosis, cells go back to the interphase stage of the cell cycle and carry on the cell cycle. for human somatic cells, mitosis can only occur about 20-50 times before it undergoes apoptosis, a number known as the Hayflick limit. scientifically, this is beneficial because of the accumulation of mutations that may potentially be dangerous. an exception is cancerous cells such as with the TP53 mutation, where mitosis keeps going on continuously even if there is insufficient energy/organelles available, often resulting in tumours. note that some cells are stuck in a phase of the cell cycle known as G_0, where they do not go through mitosis till reverted back to the G_1 stage.
as for meiosis, cells are usually nourished so that they grow. in humans, the sperm is nourished by the sertoli cells in the testes to ensure that they grow and mature. the ova are developed inside the follicles of the ovary and typically the one ovum that matures the most is released from its follicle during ovulation. their fate is either fertilization, in which case the sperm and ovum form an embryo that develops in the fallopian tubes and then in the uterus. in case that this does not occur, the ovum is shed off with the uterine lining in a process known as menstruation. as for the sperm, they are usually just left in the testes, and if not released through a process such as ejaculation they may start to die in the testes.(4 votes)
- In the Starting Cell of Meiosis I, you say that it is the homologous chromosomes from mother and father that cross over... but how can this be if the cell has not been fertilized yet and Meiosis describes how a gamete cell is produced. Therefore, an egg would only have the chromosomes from the mother and a sperm would have only the chromosomes from the father.(3 votes)
- You received homologous chromosomes from your mother and some from your father, that is your genetic make up. I think the process of meiosis separates them so that the traits you pass on to your children can seem as if they are coming from grandparents or from you. I think it why I have blue like my dad and not brown ones like my mom. My maternal grandmother had blue eyes so she gave my mom the blue eyed chromosome. I was the one child of 4 with blue eyes. My eye color chromosomes from both my parents were different than for my siblings. I think meiosis has a role in that.(4 votes)