Topic 4.2: Meiosis

Assessment Statement 4.2.1: State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei 4.2.2: Define homologous chromosomes 4.2.3: Outline the process of meiosis, including pairing homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells 4.2.4: Explain that non-disjunction can lead to changes in the chromosome number, illustrated by reference to Down’s syndrome 4.2.5: State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure 4.2.6: State that karyotyping is performed using cells colleced by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities 4.2.7: Analyze a human karyotype to determine gender and whether non-disjunction has occurred

Meiosis Meiosis is a form of cell division which results in gametes Although mitosis is similar to meiosis, there are some fundamental differences

Meiosis One 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 cell Cells which contain half the chromosome number are called haploid cells Cells with the full chromosome number are called diploid cells

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

Homologous chromosome In 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 genes The reason there are two of each is that one came from the father and the other from the mother

Homologous chromosome Although a pair of homologous chromosomes carry the same genes, they are not identical because the alleles for the genes from each parent could be different We use the letter n to denote the number of unique chromosomes in an organism In eukaryotes, there are n pairs of chromosome With two of each, that makes a total of 2n per cell Haploid- n Diploid-2n

Phases of meiosis Meiosis is a step-by-step process by which a diploid parent cell produces four haploid daughter cells Before the steps begins, DNA replication allows the cell to make a complete copy of its genetic information during interphase This results in each chromatid having an identical copy, or sister chromatid, attached to it at the centromere

Phases of meiosis In 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 cells

Phases of meiosis Another 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 over This 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 other’s initial position

Phases of meiosis Crossing over allows DNA from a person’s maternal chromosomes to mix with DNA from the paternal chromosomes In this way, the recominant chromatids which end up in the sperm or the egg cells are a mosaic of the parent cell’s original chromatids

Phases of meiosis Prophase I Chromosomes become visible as the DNA becomes more compact Homologous chromosomes, also called homologues, are attracted to each other and pair up – one is from the individual’s father, the other from the mother Crossing over occurs Spindle fibers made from microtubules form

Phases of meiosis Metaphase I The bivalents (another name for the pairs of homologous chromosomes) line up across the cell’s equator The nuclear membrane disintegrates

Phases of meiosis Anaphase I Spindle fibers from the poles attach to chromosomes and pull them to opposite poles of the cell

Phases of meiosis Telophase I Spindle and spindle fibers disintegrate Usually, the chromosomes uncoil and new nuclear membrane form Many plants do not have a telophase I state

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

Phases of meiosis Prophase II Metaphase II DNA condenses into visible chromosomes again New meiotic spindle fibers are produced Metaphase II Nuclear membranes disintegrate The individual chromosomes line up along the equator of each cell in no special order; this is called random orientiation

Phases of meiosis Anaphase II Centromeres of each chromosome split, releasing each sister chromatid as an individual chromosome The spindle fiber pull individual chromatids to opposite end of the cell Because of random orientation, the chromotids could be pulled towards either of the newly forming daughter cells In animal cells, cell membranes pinch off in the middle, whereas in plant cells, new cell plates form to demarcate the four cells

Phases of meiosis Telophase II Chromosomes unwind their strands of DNA Nuclear envelopes form around each for the four haploid cells, preparing them for cytokinesis

Down Syndrome Sometimes chromosomes do not separate the way they are expected to during the first or second meiotic division This results in an unequal distribution of chromosomes In humans this means that an egg cell or a sperm cell might have 24 instead of 23 chromosomes This 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 Syndrome In the case of Down’s 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 Down’s syndrome is referred to as trisomy 21 Having an additional chromosome brings about malformation of the digestive system and causes differing degrees of learning difficulties

Down Syndrome Down Syndrome is the most common chromosomal anomaly and affects approximately 1 birth in 800 The risk of Down’s syndrome increases as the age of the mother increases, particularly over the age of 35 Non-disjunction can happen with other chromosomes, and all of them can have a major impact on a child’s development Some developmental consequences are so severe that the fetus may not survive beyond a few weeks or months

Karyotypes A 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 shape The shape depends mainly on the position of the centromere

Karyotypes A 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 microscope 2) Photomicrograph images are obtained of the chromosomes during mitotic metaphase 3) The images are cut and separated, a process which can be done using scissors or using a computer 4) The images of each pair of chromosomes are placed in order by size and the position of their centromere

Karyotypes Obtaining cells for karyotyping An unborn baby’s cells can be extracted in one or two ways: either by a process called amniocentesis or by removing them from the chorionic villus Amniocentesis: Involves using a hypodermic needle to extract some of the amniotic fluid around the developing baby Inside the liquid, some of the baby’s cells can be found and used for the preparation of a karyotype Chorionic villus sampling: involves obtaining a tissue sample from the placenta’s finger-like projections into the uterus wall