Vaia - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
Americas
Europe
Meiosis is defined as a form of cellular division by which sex cells, called gametes, are produced. This occurs in male tests and female ovaries in the human body to produce sperm cells and ovum, both needed for sexual reproduction. Gametes are haploid cells, and this means they contain only one set of chromosomes; in humans, this is 23 chromosomes (this value may differ between organisms). Conversely,…
Explore our app and discover over 50 million learning materials for free.
Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken
Jetzt kostenlos anmeldenNie wieder prokastinieren mit unseren Lernerinnerungen.
Jetzt kostenlos anmeldenMeiosis is defined as a form of cellular division by which sex cells, called gametes, are produced. This occurs in male tests and female ovaries in the human body to produce sperm cells and ovum, both needed for sexual reproduction.
Gametes are haploid cells, and this means they contain only one set of chromosomes; in humans, this is 23 chromosomes (this value may differ between organisms). Conversely, body cells, also called somatic cells, are diploid cells as they contain 46 chromosomes or 23 pairs of chromosomes. Upon sexual fertilization, when two haploid gametes use, the resulting zygote will contain 46 chromosomes. Meiosis is an important process because it ensures that zygotes have the correct number of chromosomes.
Haploid: one set of chromosomes.
Meiosis is also referred to as a reduction division. This means that the gametes contain only half the number of chromosomes compared to body (somatic) cells.
Meiosis begins with a diploid somatic cell which contains 46 chromosomes, or 23 pairs of homologous chromosomes. One pair of homologous chromosomes is composed of a maternally- and paternally-derived chromosome, each of which has the same genes at the same loci but differing alleles, which are different versions of the same gene.
Diploid: two sets of chromosomes
The end product of meiosis is four genetically different daughter cells, all of which are haploid. The steps taken to arrive at this end-stage requires two nuclear divisions, meiosis I and meiosis II. Below, we will discuss these steps in detail. Note that there are many similarities between meiosis and mitosis, another form of cellular division. Later on in this article, we will compare the differences between the two.
Meiosis I is composed of the stages:
Prophase I
Metaphase I
Anaphase I
Telophase I
However, we cannot forget about the stage preceding cell division, interphase. Interphase is divided into the G1 phase, S phase and G2 phase. To understand the changes in chromosome numbers during meiosis, we must first know what happens during interphase.
Interphase before mitosis is identical to interphase before meiosis.
In prophase I, the chromosomes condense, and the nucleus breaks down. The chromosomes arrange themselves in their homologous pairs, unlike mitosis, where each chromosome acts independently. A phenomenon called crossing over occurs at this stage, which involves the exchange of corresponding DNA between the maternal and paternal chromosomes. This introduces genetic variation!
During metaphase I, the homologous chromosomes will align on the metaphase plate, driven by spindle fibres, in a process called independent assortment. Independent assortment describes the array of the different chromosomal orientations. This also increases genetic variation! This is different to mitosis where individual chromosomes line up on the metaphase plate, not pairs.
Anaphase I involves the separation of the homologous pairs, meaning each individual from a pair is pulled to opposite poles of the cell through the shortening of spindle fibres. Although the homologous pair is broken, the sister chromatids are still attached together at the centromere.
In telophase I, the sister chromatids decondense and the nucleus reforms (note that two sister chromatids are still referred to as a chromosome). Cytokinesis is initiated to produce two haploid daughter cells. Meiosis I is usually referred to as the reduction division stage as the diploid number has halved to the haploid number.
Much like the previous stage, meiosis II is composed of
Interphase does not occur prior to meiosis II so the two haploid daughter cells enter prophase II immediately. The chromosomes condense and the nucleus breaks down once again. No crossing over occurs, unlike in prophase I.
During metaphase II, spindle fibres will align individual chromosomes on the metaphase plate, much like in mitosis. Independent assortment occurs during this stage as the sister chromatids are genetically different due to the crossing over events in prophase I. This introduces more genetic variation!
In anaphase II, the sister chromatids are pulled apart to opposite poles due to the shortening of the spindle fibres.
Finally, telophase II involves the decondensing of chromosomes and the reforming of the nucleus. Cytokinesis creates a total of four daughter cells, all of which are genetically unique due to the genetic variation that was introduced during both cellular divisions.
Some of the differences between the two cellular divisions were explained in the previous section, and here, we will clarify these comparisons.
Mutations describe random changes in the DNA base sequence of chromosomes. These changes usually occur during DNA replication, where there is the potential for nucleotides to be incorrectly added, removed or substituted. As the DNA base sequence corresponds with an amino acid sequence for a polypeptide, any changes may affect the polypeptide product. There are four main types of mutations:
Although mutations arise spontaneously, the presence of mutagenic agents can increase the rate of mutations. This includes ionizing radiation, deaminating agents and alkylating agents.
Ionizing radiation can break DNA strands, altering their structure and increasing the chances of mutations arising. Deaminating agents and alkylating agents alter the nucleotide structure and thereby cause the incorrect pairing of complementary base pairs.
These mutations result in a codon becoming a stop codon, which terminates the polypeptide synthesis prematurely. Stop codons do not code for an amino acid during protein synthesis, preventing further elongation.
Missense mutations result in the addition of an incorrect amino acid in place of the original amino acid. This will harm the organism if the properties of the new amino acid are significantly different from the original amino acid. For example, the amino acid glycine is a nonpolar amino acid. If serine, which is a polar amino acid, is incorporated instead, this mutation may alter the polypeptide structure and function. Conversely, if alanine, another nonpolar amino acid, is incorporated, the resulting polypeptide may remain the same because alanine and glycine have very similar properties.
Silent mutations occur when a nucleotide is substituted, but the resulting codon still codes for the same amino acid. The genetic code is described as 'degenerate' as multiple codons correspond with the same amino acid—for example, AAG codes for lysine. However, if a mutation occurs and this codon becomes AAA, there will be no change as this also corresponds with lysine.
Frameshift mutations occur when the 'reading frame' is altered. This is caused by the addition or deletion of nucleotides, causing every successive codon after this mutation to change. This perhaps is the most lethal kind of mutation as every amino acid may be altered, and therefore, the polypeptide function will be dramatically affected. Below are examples of the different types of mutations that we have discussed.
Meiosis forms four genetically unique haploid gametes by undergoing two nuclear divisions, meiosis I and meiosis II.
Genetic variation is introduced during meiosis through crossing over, independent segregation and random fertilization.
Mutations involve changes to the DNA base sequence of genes, increasing genetic variation.
The different types of mutations include nonsense, missense, silent and frameshift mutations.
Meiosis describes the process of producing four haploid gametes, all of which are genetically different. Two rounds of nuclear division must take place.
Meiosis occurs in our reproductive organs. In males, meiosis occurs in the testes and females, in the ovaries.
Four daughter cells are produced in meiosis, all of which are genetically unique and haploid.
Meiosis involves two cell divisions and these are considered meiosis I and meiosis II.
The first division of meiosis differs from mitosis due to crossing over and independent assortment. Crossing over involves the exchange of DNA between homologous chromosomes while independent assortment describes the lining up of homologous chromosomes on the metaphase plate. Both of these events do not occur during mitosis as they are exclusive to meiosis.
How would you like to learn this content?
94% of StudySmarter users achieve better grades.
Sign up for free!94% of StudySmarter users achieve better grades.
Sign up for free!How would you like to learn this content?
Free biology cheat sheet!
Everything you need to know on . A perfect summary so you can easily remember everything.
Be perfectly prepared on time with an individual plan.
Test your knowledge with gamified quizzes.
Create and find flashcards in record time.
Create beautiful notes faster than ever before.
Have all your study materials in one place.
Upload unlimited documents and save them online.
Identify your study strength and weaknesses.
Set individual study goals and earn points reaching them.
Stop procrastinating with our study reminders.
Earn points, unlock badges and level up while studying.
Create flashcards in notes completely automatically.
Create the most beautiful study materials using our templates.
Sign up to highlight and take notes. It’s 100% free.