Topic 4.2: Meiosis

Topic 4.2: MeiosisAssessment Statement4.2.1: State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei4.2.2: Define homologous chromosomes4.2.3: Outline the process of meiosis, including pairing homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells4.2.4: Explain that non-disjunction can lead to changes in the chromosome number, illustrated by reference to Downs syndrome4.2.5: State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure4.2.6: State that karyotyping is performed using cells colleced by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities4.2.7: Analyze a human karyotype to determine gender and whether non-disjunction has occurredMeiosisMeiosis is a form of cell division which results in gametesAlthough mitosis is similar to meiosis, there are some fundamental differences

MeiosisOne characteristic which makes meiosis unique is that each new cell which results from it has only half the number of chromosomes that a typical cell in that organism has.Humans have 46 chromosomes in their cells, but in the sperm and egg cells, there are only 23 chromosomes in each cellCells which contain half the chromosome number are called haploid cellsCells with the full chromosome number are called diploid cells

MeiosisThis type of cell division is called a reduction division because the number of chromosomes has been reducedThis reduction is necessary in gamete production because during sexual reproduction, each parent contributes 50% of the genetic informationThe cells formed from cell division are referred to as daughter cells

Homologous chromosomeIn a diploid cell, the 46 chromosomes can be grouped into 23 pairs of chromosomes called homologous chromosomes.Homologous means similar in shape and size and it means that the two chromosomes carry the same genesThe reason there are two of each is that one came from the father and the other from the mother

Homologous chromosomeAlthough a pair of homologous chromosomes carry the same genes, they are not identical because the alleles for the genes from each parent could be differentWe use the letter n to denote the number of unique chromosomes in an organismIn eukaryotes, there are n pairs of chromosomeWith two of each, that makes a total of 2n per cellHaploid- nDiploid-2nPhases of meiosisMeiosis is a step-by-step process by which a diploid parent cell produces four haploid daughter cellsBefore the steps begins, DNA replication allows the cell to make a complete copy of its genetic information during interphaseThis results in each chromatid having an identical copy, or sister chromatid, attached to it at the centromerePhases of meiosisIn order to produce a total of four cells, the parent cell must divide two times: the first meiotic division makes two cells and then each of these divides during the second meiotic division to make a total of four cellsPhases of meiosisAnother characteristic which distinguishes meiosis from mitosis:During the first step, called prophase I, there is an exchange of genetic material between non-sister chromatids in a process called crossing overThis trading of segments of genes happen when sections of two homologous chromatids break at the same point, twist around each other and each connects to the others initial position

Phases of meiosisCrossing over allows DNA from a persons maternal chromosomes to mix with DNA from the paternal chromosomesIn this way, the recominant chromatids which end up in the sperm or the egg cells are a mosaic of the parent cells original chromatidsPhases of meiosisProphase IChromosomes become visible as the DNA becomes more compactHomologous chromosomes, also called homologues, are attracted to each other and pair up one is from the individuals father, the other from the motherCrossing over occursSpindle fibers made from microtubules form

Phases of meiosisMetaphase IThe bivalents (another name for the pairs of homologous chromosomes) line up across the cells equatorThe nuclear membrane disintegrates

Phases of meiosisAnaphase ISpindle fibers from the poles attach to chromosomes and pull them to opposite poles of the cell

Phases of meiosisTelophase ISpindle and spindle fibers disintegrateUsually, the chromosomes uncoil and new nuclear membrane formMany plants do not have a telophase I state

Phases of meiosisAt the end of meiosis I, cytokinesis happens: the cell splits into two separate cellsThe cells at this point are haploid because they contain only one chromosome of each pairEach chromatid still has its sister chromatid attached to it, so not S phase is necessaryNow meiosis II takes place in order to separate the sister chromatids

Phases of meiosisProphase IIDNA condenses into visible chromosomes againNew meiotic spindle fibers are producedMetaphase IINuclear membranes disintegrateThe individual chromosomes line up along the equator of each cell in no special order; this is called random orientiation

Phases of meiosisAnaphase IICentromeres of each chromosome split, releasing each sister chromatid as an individual chromosomeThe spindle fiber pull individual chromatids to opposite end of the cellBecause of random orientation, the chromotids could be pulled towards either of the newly forming daughter cellsIn animal cells, cell membranes pinch off in the middle, whereas in plant cells, new cell plates form to demarcate the four cells

Phases of meiosisTelophase IIChromosomes unwind their strands of DNANuclear envelopes form around each for the four haploid cells, preparing them for cytokinesis

Down SyndromeSometimes chromosomes do not separate the way they are expected to during the first or second meiotic divisionThis results in an unequal distribution of chromosomesIn humans this means that an egg cell or a sperm cell might have 24 instead of 23 chromosomesThis unexpected distribution of chromosomes is due to a non-disjunction, a process by which two or more homologous chromosomes stick together instead of separating

Down SyndromeIn the case of Downs syndrome, non-disjunction happens in the 21st pair of chromosome: the child receives 3 instead of 2.Such an anomaly is called a trisomy and Downs syndrome is referred to as trisomy 21Having an additional chromosome brings about malformation of the digestive system and causes differing degrees of learning difficulties Down SyndromeDown Syndrome is the most common chromosomal anomaly and affects approximately 1 birth in 800The risk of Downs syndrome increases as the age of the mother increases, particularly over the age of 35Non-disjunction can happen with other chromosomes, and all of them can have a major impact on a childs developmentSome developmental consequences are so severe that the fetus may not survive beyond a few weeks or months

KaryotypesA karyotype is a photograph of the chromosomes found in a cell arranged according to a standard format.The chromosomes are placed in order according to their size and shapeThe shape depends mainly on the position of the centromere

KaryotypesA karyotype is made by the following steps:1) The cells are stained and prepared on a glass slide to see their chromosomes under a light microscope2) Photomicrograph images are obtained of the chromosomes during mitotic metaphase3) The images are cut and separated, a process which can be done using scissors or using a computer4) The images of each pair of chromosomes are placed in order by size and the position of their centromereKaryotypesObtaining cells for karyotypingAn unborn babys cells can be extracted in one or two ways: either by a process called amniocentesis or by removing them from the chorionic villusAmniocentesis: Involves using a hypodermic needle to extract some of the amniotic fluid around the developing babyInside the liquid, some of the babys cells can be found and used for the preparation of a karyotypeChorionic villus sampling: involves obtaining a tissue sample from the placentas finger-like projections into the uterus wall