Science: Meiosis

Meiosis

A process that converts diploid nuclei to haploid nuclei, occurs in sex organs, involves two divisions

What is the diploid number in a human somatic cell?

Any cell with two chromosome sets is called a diploid cell and has a diploid number of chromosomes, abbreviated 2n. For humans, the diploid number is 46 (2n = 46), the number of chromosomes in our somatic cells. (23 pairs)

Homologous chromosomes

Are matched in length, centromere position, gene locations

Diploid cells

Chromosomes are in homolgeous pairs (2n). 2 chromosomes from mom and two from dad

Meiosis- True or False Involves one division. Involves two divisions. Nuclear membrane breaks down. Cell membrane breaks down. Nucleoli disappear. Spindle forms. Resulting cells are somatic cells. Resulting cells are gametes. Resulting cells are genetically identical. Resulting cells are genetically different. Produces diploid cells. Produces haploid cells.

F, T, T, F, T, T, F, T, F, T, F, T

Meiosis II

Follows meiosis I without chromosomes duplication (interphase). The chromosomes are split into 2 cells, so there is half the amount of genetic information in the haploid cells.

Tetrad

Four chromatids that have come together (2 chromosomes) • A tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

X-chromosome inactivation

In each cell of a human female, one of the two X chromosomes become tightly coiled and inactive, can inactivated maternal or paternal chromosome, this promotes a balance between the number of X chromosomes in autosomes

In which human organs does meiosis occur?

It produces sex cells (gametes). Depending on the gender, it occurs in the testes in males and ovaries in females. For females, the sex cells are formed before their birth, and thus have a fixed number of gametes. For males, sperm can be produced from puberty onwards.

Syndromes caused by nondisjunction

Klinefelter syndrome (male) (XXY), Turner syndrome (Female) (XO)

Karyotype

Shows stained and magnified versions of chromosomes

Anaphase II

Sister chromatids separate and chromosomes move toward opposite poles. Centromeres of sister chromatids finally separate, and the sister chromatids of each pair are now individual daughter chromosomes. The move toward opposite poles of the cell.

Gametes

Sperm and eggs. They are haploid. In Males 4 sperm cells are produced that is functional and in females only one egg cell is produced that is functional.

Translocation

The attachment of a segment to a nonhomologous chromosome; can be a reciprocal

Random fertilization

The combination of each unique sperm with each unique egg increases genetic variability

Nondisjunction

The failure of chromosomes or chromatids to separate during meiosis , fertilization after this causes zygotes with altered number of chromosomes

Deletion

The loss of a chromosome segment

Locus

The position of a gene, different versions of a gene may be found at the same this on maternal and paternal chromosomes

Duplication

The repeat of a chromosome segement

Inversion

The reversal of a chromosome segment

Chiasma

The site of attachment and crossing over

Fertilization

The union of sperm and egg

Trisomy 21 (Down Syndrome)

Three copies of chromosome 21, have specific facial features, susceptibility to disease, shortened life span, mental retardation, variation in characteristics

What would the human chromosome number be in the resulting zygote if meiosis (reduction division) failed to occur?

When meiosis goes wrong, it can result in gametes that have more or fewer chromosomes than they should. Human gametes normally have 22 upaired nonsex chromosomes and one unpaired sex chromosome, and a normal human zygote has a total of 23 paired chromosomes. When an abnormal gamete fuses with another to form a zygote, the resulting individual can have many abnormalities, such as Down or Tuner Syndrome.

Crossing over

When nonsister chromatids exchange genetic information. It produces 4 genetically different chromatids.

What is the chromosome number in a zygote?

When the gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. The type of cell division that produces gametes with half the normal chromosome number is called meiosis. Zygote- one- celled organism formed by the union of a sperm and a fertilized egg

Females

X and X

Homologous Chromosomes

• The two versions of each chromosome are called homologous chromosomes. (chromosome 1 from the mother and chromosome 2 from the father) *Chromosomes that have the same sequence of genes and the same structure. One chromosome (homolg) is maternal and the other homolg is paternal

Metaphase I

Chromosome tetrads are aligned on the metaphase plate, (Cell equator) midway between the spindle poles. The chromosome is condensed and its sister chromatids still attached at the centromere. In each tetrad, homologous chromosomes are held together at cross over sites. The homologous chromosomes of each tetrad are positioned to move towards opposite poles of the cell. Independent assortment occurs- each pair of chromosome independently aligns at the cell equator, equal probability of maternal or paternal chromosome facing a pole- leads to varied offspring.

Prophase I

Chromosomes coil and become compact, homologous chromosomes come together as pairs by synapsis, each pair with four chromatids is called a tetrad, nonsister chromatids exchange genetic information by crossing over. The chromosomes condense further as the nucleoli disappear. The centrosomes move away from each other, and the spindle starts to form. The nuclear envelope breaks down and the chromosome tetrads are captured by the spindle.

Prophase II

Chromosomes coil and become compact. Nuclear envelope breaks down. A spindle forms in each haploid cells and moves chromosomes toward the middle of the cell.

Telophase II

Chromosomes have reached the poles of the cell, a nuclear membrane forms around each set of chromosomes, with cytokinesis, four haploid cells are produced. The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes. Each haploid (4) cell results in one genetically different chromatid in each. The chromatids are called chromosomes.

In which phase does the tetrad form and crossing over occur?

Crossing over occurs during synapsis when the chromosomes are bundled in tetrads. This occurs in prophase of meiosis I. Tetrads by the synapsis of homologous chromosomes. This is one crossing over occurs which is the exchange of genetic material across the synapse between either homologous chromosome. Crossing over involves the switching of genes between nonsister chromatids of homologoues which allows the mixture of maternal and paternal genetic material with new, recombinant chromosomes

Metaphase II

Duplicated chromosomes align at the cell equator. Spindle fibers line up the sister chromatids of each chromosome along the equator (metaphase plate) of the cell. Sister chromatids of each chromosome pointing toward opposite poles.

Telophase I

Duplicated chromosomes have reached the poles, a nuclear envelope forms around chromosomes (spindle breaks down) in some species, each nucleus has the haploid number of chromosomes. The cytoplasm of the cell divides, and a cleavage furrow forms. Two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.

Describe the number of pairs of chromosomes in somatic cell vs. a gamete.

Each pair of chromosomes comprises one chromosome inherited from the father and one inherited from the mother. For example, in humans, somatic cells contain 46 chromosomes organized into 23 pairs. By contrast, gametes of diploid organisms contain only half as many chromosomes. There are 46 chromosomes in somatic cells and 23 chromosomes in gametes.

Independent orientation at metaphase I

Each pair of chromosomes independently aligns at the cell equator, there is an equal number of combinations of the maternal or paternal chromosome facing a given pole, the number of combinations for chromosomes packaged into gametes is 2(to the nth power) were N= haploid number of chromosomes *Independent orientation of chromosomes increases the possibilities of the offspring because every chromosome randomly forms during meiosis. Independent assortment is the random lining up of the homologoues during metaphase of metaphase one. Between different gametes, there are 2n different possibilities of how the homologoues could line up. Chromosomal and paternal chromosomes line up during this phase.

Zygote

Has a diploid chromosome number, one set from each parent. Product of fertilization.

Haploid cells

Has one chromosome from each pair (n)- one from mom and one from dad

Polyploid

Have more than two chromosome sets, observed in many plant species, seen less frequently in animal species

Somatic cells

Have pairs of homologous chromosomes, receiving one member of each parent

Autosomes

Have the same size genetic composition

Outcomes of meiosis vs mitosis

Meiosis: four genetically different cells with half the number of chromosomes then original cells Mitosis: Two genetically identical cells with the same chromosomes number as the original cell

Describe the behavior of the tetrad during meiosis.

Prophase I - homologous chromosomes join together to form a tetrad in a process called synapsis. Metaphase I - homologous pairs line up along the central axis of the cell. They move towards the equator. Anaphase I - homologous chromosomes are separated and pulled toward the nearest pole; sister chromatids move together. Tetrads are pulled apart by the spindle fibers. Telophase I - the chromosomes have formed two groups; one group at each pole. When the diploid parent cell divides, each daughter cell has only 1 of each kind of chromosome. Prophase II - the chromosomes condense and become visible in a light microscope. Metaphase II - the chromosomes line up along the central axis (equator) of the cell. Anaphase II - the centromeres are broken and the sister chromatids are pulled to opposite poles. Telophase II - the chromatids (now chromosomes) group at the poles. It produces four gametes.

Anaphase I

Sister chromatids making up each doubled chromosome remain attached at their centromeres. Spindle fibers shorten, and the homologous chromosomes start to separate from each other. They move towards opposite poles of the cell.

Describe the genes carried on homologous chromosomes.

The 22 pairs of homologous chromosomes contain the same genes but code for different traits in their allelic forms since one was inherited from the mother and one from the father. So humans have two homologous chromosome sets in each cell, meaning humans are diploid organisms. Genes on the homologous chromosomes are in the exact same position and they have the same centromere position.

What is the haploid number in a human gamete?

The term haploid can also refer to the number of chromosomes in egg or sperm cells, which are also called gametes. In humans, gametes are haploid cells that contain 23 chromosomes, each of which a one of a chromosome pair that exists in diploid cells. (23 chromosomes)

Males

X and Y

Sex chromosomes

X and Y differ in size and genetic composition