2.S: Chromosomes, Mitosis, and Meiosis (Summary) - Biology

2.S: Chromosomes, Mitosis, and Meiosis (Summary) - Biology

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  • Chromosomes are complex and dynamic structures consisting of DNA and proteins (chromatin).
  • The degree of chromatin compaction involves proteins and varies between heterochromatic and euchromatic regions and among stages of the cell cycle.
  • Chromosomes can be distinguished cytologicaly based on their length, centromere position, and banding patterns when stained dyes or labeled with sequence-specific probes.
  • Homologous chromosomes contain the same series of genes along their length, but not necessarily the same alleles.Sister chromatids initially contain the same alleles.
  • Chromosomes are replicated by DNA polymerases and begin at an origin. Replication is bi-directional. Eukaryotes have multiple origins along each chromosome and have telomerase to replicate the ends.
  • Mitosis reduces the c-number, but not the n-number.Meiosis reduces both c and n.
  • Homologous chromosomes pair (sysnapse) with each other during meiosis, but not mitosis.
  • Several types of structural defects in chromosomes occur naturally, and can affect cellular function and even evolution.
  • Aneuploidy results from the addition or subtraction of one or more chromosomes from a group of homologs, and is usually deleterious to the cell.
  • Polyploidy is the presence of more than two complete sets of chromosomes in a genome.Even-numbered multiple sets of chromosomes can be stably inherited in some species, especially plants.
  • Endopolyploidy is tissue-specific type of polyploidy observed in some species, including diploids.
  • Both aneuploidy and structural defects such as duplications can affect gene balance.
  • Organelles also contain chromosomes, but these are much more like prokaryotic chromosomes than the nuclear chromosomes of eukaryotes.

Key Terms


core histones


30nm fiber

histone H1

scaffold proteins



satellite DNA











sister chromatid

non-sister chromatid







DNA polymerase

origin of replication



Hayflick limit

HeLa cells




prophase (I, II)

metaphase (I, II)

anaphase (I, II)

telophase (I, II)




syanapse, pair up

synaptonemal complex

reductional division

equational division






crossing over

chiasma (chiasmata)

polar bodies









replicated chromosome









Down syndrome







chromosome breakage











salivary gland chromosome


gene balance

cellular network




endosymbiont theory

organellar chromosome


2.S: Chromosomes, Mitosis, and Meiosis (Summary) - Biology

Importance of mitosis
Mitosis is the process that a somatic cell divides into two daughter cells. It is an important process in normal organism development. When mitosis is out of control, diseases such as cancer may occur.

Cell structures for mitosis
Mitosis requires a set of specialized cell structures. Chromosomes are the most important part for mitosis because they are duplicated and then separated evenly into two daughter cells. On a chromosome there is a special structure called centromere, where the spindle, a structure pulling the chromosomes to two poles, attach. The spindle is formed around a cytosolic structure called centrosome, which is main driving force for chromosome separation.

Mitosis is composed of prophase, prometaphase, metaphase, anaphase and telophase. The cell cycle phase between two mitoses is called interphase, where chromosomes are loose and stretched. During prophase and prometaphase, chromosome start to condense, in the metaphase, they are visible under microscope. Chromosomes are also lined up in the middle of cell and ready to be pulled to the two poles by spindle. This is done in anaphase and telophase, upon which nuclear separation is finished. Cytokinesis follow and two cells are generated.

Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material. It is divided into two steps: meiosis I and meiosis II. Meiosis I includes Prophase I, Metaphase I, Anaphase I and Telophase I. Meiosis II (Second division) includes Prophase II, Metaphase II, anaphase II and telophase II. Each of these phases are similar but not identical to the corresponding mitosis phases.

Difference between mitosis and meiosis
Meiosis is different than mitosis in terms of the cell division number. During mitosis, chromosomes are duplicated once, and cell divides once, therefore each daughter cell has equal chromosome number which is also equal to the mother cell’s. During meiosis, chromosomes are also duplicated, cell division occurs twice consecutively, leading the half of the chromosome number in 4 daughter cells. This process is used for generating germ line cells, the gametes. When gametes from male and female parents meet, they form normal diploid chromosome number again. Click to watch the movie tutorial on Mitosis and Meiosis" and see our genetics professor explain their difference in cell division.

What is the difference between mitosis and meiosis? The main concept to understand is in how the cells divide - as illustrated below:

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles (Figure 1). Non-kinetochore microtubules elongate the cell.

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell. During meiosis II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

Figure 1 In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

Summary on mitosis and meiosis

The cell division produces profound changes in the cells. The two existing types, mitosis and meiosis, occur in different ways. Check here a summary of what happens in both processes.

Mitosis: what it is, function and importance

Mitosis is a process of cell division where a cell originates two cells identical to the mother cell, that is, with the same number of chromosomes. The term mitosis comes from the Greek word Myths , which means to weave threads.

The function of mitosis is to ensure the growth and replacement of cells. The importance of this cell multiplication is to maintain the reproduction of single-celled beings, to effect healing and tissue renewal processes.

This type of cell division occurs in diploid cells and in some animal and plant cells. In a human cell, for example, there are 46 chromosomes. Mitosis promotes the appearance of two cells also with 46 chromosomes.

Phases of mitosis

Phases of mitosis


  • Each chromosome has a centromere that joins two filaments called chromatids.
  • The membrane that surrounds the nucleus, the library, is fragmented and the nucleolus disappears.
  • Chromosomes become shorter and thicker with the spiraling process.
  • The formation of spindle fibers facilitates displacement in the cytoplasm.


  • Nuclear material is dispersed in the cytoplasm due to the disappearance of the library.
  • The chromosomes are at the maximum degree of spiraling and are joined to the polar fibers of the mitotic spindle by the centromere region.
  • Chromosomes move to the median region of the cell, forming an equatorial plate.


  • The two sister chromatids are separated by dividing the centromere, becoming independent child chromosomes.
  • Each child chromosome goes to a pole of the cell by shortening the spindle fibers.
  • The genetic material that arrives at each pole is identical to that of the mother cell.


  • Nuclear division ends and the chromosomes de-spiralize, becoming long and thin filaments again.
  • There is disintegration of the spindle, reorganization of the nucleolus and reconstitution of the library.
  • The new nuclei acquire the same aspect as the interphase nucleus.
  • Cytokinesis causes the cytoplasm to divide and the strangulation to produce two cells.

In the interphase period , cells are not dividing. This phase is divided into three periods: G 1 (RNA synthesis), S (DNA synthesis) and G 2 (before duplication).

Differences between animal and plant mitosis

Mitosis in animal cellsMitosis in plant cells
Centric mitosis due to the presence of centrioles.Acentric mitosis due to the absence of centrioles.
Astral mitosis due to the presence of aster fibers.Anastral mitosis due to the absence of aster fibers.
Centripetal cytokinesis, that is, it occurs from the outside to the inside.Centrifugal cytokinesis, which occurs from the inside out.

When a pre-existing cell gives rise to a new cell, a cell cycle begins , which ends when duplication occurs and, consequently, the formation of daughter cells. Therefore, the cycle is the time it takes to complete all the changes.

Meiosis: what it is, function and importance

Meiosis is a process of two nuclear divisions, in which a diploid cell is transformed into four haploid cells by means of meiosis 1 and meiosis 2.

The function of meiosis is to reduce the number of chromosomes in diploid cells by transforming them into haploid cells and, finally, to ensure that there is a complete set of chromosomes in the generated haploid products.

The importance of meiosis lies in the development of genetic diversity, as it produces new gene combinations. Sexed life cycles are influenced by this process, with diversity being the raw material for natural selection and evolution.

Phases of meiosis 1

Phases of meiosis 1

It corresponds to the reductive step , which consists in reducing the number of chromosomes in half.

Prophase 1

  • The centrioles move to the poles of the cell.
  • Condensation of chromosomes occurs.
  • Formation of chromomers, which correspond to small and dense condensations on chromosomes.
  • There is an exchange of fragments between chromatids-homologues during crossing-over .

Metaphase 1

  • The cell membrane disappears.
  • Chromosomes are at the maximum level of condensation.
  • The kinetochore binds the pair of homologous chromosomes to the spindle fibers.
  • The homologous chromosomes are aligned in pairs in the equatorial region of the cell.

Anaphase 1

  • The homologous chromosomes are separated due to the shortening of the aster fibers.
  • The duplicated chromosome of each pair migrates to one of the poles of the cell.
  • Deconsensation begins.

Telophase 1

  • The library and nucleolus reorganize at each pole of the cell.
  • Cell division and formation of two haploids with half the number of chromosomes in the mother cell.
  • Cytokinesis occurs, that is, the division of the cytoplasm.

Phases of meiosis 2

Phases of meiosis 2

Stands for the equational stage , which consists of the division of cells and the number of chromosomes is the same as those that started the process.

Prophase 2

  • The library is broken and the nucleoli disappear.
  • Chromosomes condense.
  • Aster fibers are formed.
  • The cells are haploid, as they have a chromosome of each type.

Metaphase 2

  • The chromosomes are guided by the aster fibers and line up in the equatorial region of the cell.
  • Chromosomes are at the maximum degree of condensation.

Anaphase 2

  • The sister chromatids are directed by the aster fibers to opposite sides.
  • A chromatid becomes a simple chromosome.
  • Deconsensation begins.

Telophase 2

  • The cells formed are haploid.
  • The library is reorganized and the nucleolus reappears.
  • Cytokinesis causes cell separation to occur.

The whole process can be summarized as follows:


Differences between animal and plant meiosis

Meiosis in animal cellsMeiosis in plant cells
Gametic meiosis due to gamete formation:sperm (male gamete) and egg (female gamete).Sporic meiosis due to spore formation.

Mitosis Stages

Mitosis is the process of cell division that forms two genetically identical nuclei from on parent cell nucleus. It is used for:

  • Asexual reproduction (e.g. Paramecium)
  • Growth (increasing cell number)
  • Repair and Maintenance (replace damaged cells with identical replacements)

Although we traditionally break down mitosis into a series of stages and sub-stages, it is actually a continuous process. In the micrographs opposite, you can see that mitosis is not necessarily synchronised and looks much messier than the clean, idealised textbook diagrams!

Interphase - Not strictly a stage of mitosis, this is where the cell prepares to divide by growing, storing energy, replicating organelles and replicating DNA.

Prophase - The chromosomes supercoil and become visible under a light microscope. The chromosomes assume their classic &aposX&apos shape - two sister chromatids joined in the middle at the centromere. Other key events are:

  • Nuclear Envelope breaks down
  • Centriole divides in two, travels to opposite poles of the cell to form the spindle.

Metaphase - An easy stage to identify, Metaphase is characterised by the chromosomes lining up, single file, along the middle (the equator) of the cell. At this point, each chromosome becomes attached to the spindle at its&apos centromere.

Anaphase - Another easily recognisable stage! Anaphase sees the chromosomes split at the centromere, separating the sister chromatids:

  • The sister chromatids are pulled apart to opposite poles of the cell
  • At this point, each chromatid becomes an individual chromosome - identical to the original parent chromosome
  • Spindle fibres shorten, pulling each chromatid by the centromere - this causes the chromatids to look like Vs

Telophase - a simple stage to recognise - you will see two nuclei starting to form in early telophase in late telophase you will no longer be able to see the chromosomes, just two complete nuclei at opposite ends of the cell.

2.S: Chromosomes, Mitosis, and Meiosis (Summary) - Biology

Living organisms are constantly making new cells. They make new cells in order to grow and also to replace old dead cells. The process by which new cells are made is called cell division. Cell division is occurring all the time. Around two trillion cell divisions occur in the average human body every day!

Types of Cell Division

There are three main types of cell division: binary fission, mitosis, and meiosis. Binary fission is used by simple organisms like bacteria. More complex organisms gain new cells by either mitosis or meiosis.

Mitosis is used when a cell needs to be replicated into exact copies of itself. Everything in the cell is duplicated. The two new cells have the same DNA, functions, and genetic code. The original cell is called the mother cell and the two new cells are called daughter cells. The full process, or cycle, of mitosis is described in more detail below.

Examples of cells that are produced through mitosis include cells in the human body for the skin, blood, and muscles.

Cell Cycle for Mitosis

    Prophase - During this phase the chromatin condenses into chromosomes and the nuclear membrane and nucleolus break down.

Meiosis is used when it is time for the entire organism to reproduce. There are two main differences between mitosis and meiosis. First, the meiosis process has two divisions. When meiosis is complete, a single cell produces four new cells instead of just two. The second difference is that the new cells only have half the DNA of the original cell. This is important for life on Earth as it allows for new genetic combinations to occur which produces variety in life.

Examples of cells that undergo meiosis include cells used in sexual reproduction called gametes.

Diploids and Haploids

The cells produced from mitosis are called diploids because they have two complete sets of chromosomes.

The cells produced from meiosis are called haploids because they only have half the number of chromosomes as the original cell.

Simple organisms such as bacteria undergo a type of cell division called binary fission. First the DNA replicates and the cell grows to twice its normal size. Then the duplicate strands of DNA move to opposite sides of the cell. Next, the cell wall "pinches" off in the middle forming two separate cells.


Mitosis creates two diploid somatic daughter cells that are clones of the parent cell.

• A somatic cell spends most of its time in interphase, growing and replicating DNA in preparation for mitosis.

• The four phases of actual mitotic division are prophase, metaphase, anaphase and telophase. These names will not need to be memorized for the AP® Biology exam.

Meiosis creates four haploid gamete daughter cells, each containing half of the original cell’s genetic material.

• The phases of meiosis vary in a few key ways from those of mitosis, but follow the same general phase order twice. Again, the names of the phases will not need to be memorized.

That’s all there is to it! Can you describe each of the stages and key structures of mitosis and meiosis for the AP® Bio exam?

Need help preparing for your AP® Biology exam?

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