3.3 MEIOSIS
Nature of science:
Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8)
Understandings:
• One diploid nucleus divides by meiosis to produce four haploid nuclei.
• The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
• Orientation of pairs of homologous chromosomes prior to separation is random.
• Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
• Crossing over and random orientation promotes genetic variation.
• Fusion of gametes from different parents promotes genetic variation.
• One diploid nucleus divides by meiosis to produce four haploid nuclei.
• The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
• Orientation of pairs of homologous chromosomes prior to separation is random.
• Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
• Crossing over and random orientation promotes genetic variation.
• Fusion of gametes from different parents promotes genetic variation.
meiosis 1
- Prophase I:
chromosomes condense
spindle forms
pairing of homologous chromosomes
– Metaphase I:
homologous chromosomes line up at equator
– Anaphase I:
Spindle attach to chromosomes and pull homologous chromosomes to separate poles
– Telophase I:
Chromosomes arrive at opposite poles of cell
Spindle disappears
chromosomes condense
spindle forms
pairing of homologous chromosomes
– Metaphase I:
homologous chromosomes line up at equator
– Anaphase I:
Spindle attach to chromosomes and pull homologous chromosomes to separate poles
– Telophase I:
Chromosomes arrive at opposite poles of cell
Spindle disappears
meiosis 2
-Prophase II:
chromosomes condense
spindle forms
-Metaphase I:
individual chromosomes line up at equator
-Anaphase I:
Spindle attach to chromatids and pull sister chromatids to separate poles
- Telophase I:
Chromatids arrive at opposite poles of cell
Spindle disappears
chromosomes condense
spindle forms
-Metaphase I:
individual chromosomes line up at equator
-Anaphase I:
Spindle attach to chromatids and pull sister chromatids to separate poles
- Telophase I:
Chromatids arrive at opposite poles of cell
Spindle disappears
producing 4 haploid cells
The vast majority of cells in a person’s body each contain 46 chromosomes. Sperm and egg cells cannot contain 46 chromosomes for the simple reason that if they did, when they fused together during fertilization the baby would be formed would have a total of 92 chromosomes and each generation would double its chromosome number, making an impossible amount of DNA to deal with. To avoid this problem humans and other animals, produce egg and sperm cells in such way that the bumber of chromosomes in their nuclei is halved. Hence the sperm and egg cells only contain 23 chromosomes.
The halving of the chromosome number
Whereas mitosis produces diploid 2n nuclei containing 46 chromosomes, meiosis produces haploid cells that contain 32 chromosomes each representing half of one pair.
DNA is replicated before meiosis The reason why chromosomes are represented as having the shape of the letter X or H, is because at this stage in the chromosome as existence the DNA has been replicated so that a full copy of the original DNA has been produced. As a result, the single chromosome comprises 2 sister chromatids side by side and joined in the middle at the centromere |
pairing of homologous chromosomes and crossing over
Meiosis is the process by which a diploid parent cell produces 4 haploid daughter cells. Before the steps begin, DNA replication allows the cell to make a complete copy of its genetic information during interphase. This results in each chromatic having an identical copy attached to it at the centromere. In order to produce 4 cells, the parents cell must divide twice, the first meiotic division makes 2 cells and the second divisions makes a total of 4 cells.
One characteristic that distinguished meiosis from mitosis is that during the first step called prophase 1 there is an exchange of genetic material between the nonsister chromatids in a process called crossing over. This trding of segments of genes happens when sections of 2 homologous chromatids break at the same point , twist around each other and then each connects to the others initial position.
Crossing over allows DNA form a person’s maternal chromososme to mix with DNA form the paternal chromosomes. This helps increase the variety among offspring if one combination of alleles is more favorable for survival than other.
One characteristic that distinguished meiosis from mitosis is that during the first step called prophase 1 there is an exchange of genetic material between the nonsister chromatids in a process called crossing over. This trding of segments of genes happens when sections of 2 homologous chromatids break at the same point , twist around each other and then each connects to the others initial position.
Crossing over allows DNA form a person’s maternal chromososme to mix with DNA form the paternal chromosomes. This helps increase the variety among offspring if one combination of alleles is more favorable for survival than other.
- homologous chromosomes = chromosomes pairs that are of the same length and staining pattern and possess genes for the same characters at the same length
random orientation
In Metaphase 1 the homologous chromosomes line up along the centre of the cell. The way they happen to line up is by chance, this is another adaptation that increases variety in teh offspring. The result of random orientation is that a male will only very rarely produce 2 sperm cells that are identical.
fertilization and variation
Crossing over during prophase 1 and random orientation during metaphase 1 allow variation in the offspring. However there is one other way the genetic variation is also promoted: fertilization. When the egg and sperm cells meet, there is a great deal of chance involved. The probability of a woman having the same egg twice is 1 in 8 388 608.
extra or missing chromosome
Sometimes errors occurs during meiosis and a child receives an atypical number of chromosome such as 47 instead of 46. One such anomaly is called Down syndrome and it happens when there is an extra chromosome in the 21st pair, it results from a phenomenon called non-disjunction, where chromosomes fail to separate during anaphase 1.
Theory of knowledge:
• In 1922 the number of chromosomes counted in a human cell was 48. This remained the established number for 30 years, even though a review of photographic evidence from the time clearly showed that there were 46. For what reasons do existing beliefs carry a certain inertia?
Applications and skills:
• Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
• Application: Studies showing age of parents influences chances of non- disjunction
• Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
• Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Guidance:
• Preparation of microscope slides showing meiosis is challenging and permanent slides should be available in case no cells in meiosis are visible in temporary mounts.
• Drawings of the stages of meiosis do not need to include chiasmata.
• The process of chiasmata formation need not be explained.