MEIOSIS HL
Nature of science:
Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. (1.8)
Understandings:
• Chromosomes replicate in interphase before meiosis.
• Crossing over is the exchange of DNA material between non-sister homologous chromatids.
• Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
• Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
• Homologous chromosomes separate in meiosis I.
• Sister chromatids separate in meiosis II.
• Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
• Chromosomes replicate in interphase before meiosis.
• Crossing over is the exchange of DNA material between non-sister homologous chromatids.
• Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
• Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
• Homologous chromosomes separate in meiosis I.
• Sister chromatids separate in meiosis II.
• Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
chromosome replication
before meiosis can occur, the cell must prepare for cell division during interphase. An important step in preparation is DNA replication, enabling the cell to have a copy of each chromosome
The exchange of dna material
During prophase 1 of meiosis the process of synapsis brings together 2 homologous chromososmes in a pair called a bivalent. Recall that the term homologous means that the chromososme are the same length, they have hteier centrometers I the same position and generally they contain the same genes stored at the same loci. The major difference betweent ehm is that one chromosome in the bivalent came from the person’s mother and the other from the father. As each parent has different alleles for each of the genes along the chromatids, the two homologous chromosomes are by no emans identical.
Mixing genetic material between non-sister chromatids in other word between paternal and maternal crhomosomes can occur when the chromatids intertwine and break. In order for crossing over to function correctly identical breaks must occur at exactly the same position in each non sister chromatids.
During prophase 1 of meiosis the process of synapsis brings together 2 homologous chromososmes in a pair called a bivalent. Recall that the term homologous means that the chromososme are the same length, they have hteier centrometers I the same position and generally they contain the same genes stored at the same loci. The major difference betweent ehm is that one chromosome in the bivalent came from the person’s mother and the other from the father. As each parent has different alleles for each of the genes along the chromatids, the two homologous chromosomes are by no emans identical.
Mixing genetic material between non-sister chromatids in other word between paternal and maternal crhomosomes can occur when the chromatids intertwine and break. In order for crossing over to function correctly identical breaks must occur at exactly the same position in each non sister chromatids.
New combinations of alleles
for crossing over to happen, each chromatid involved has to have a separated tip. The two segment each connect ot the corresponding position on the other chromatid. The two tips ar thus swiftched and each resulting chromatid has a segment of the other’s genetic material. The place where the 2 connect to each other is called chiasma or chiasmata.
Once they are attached at their chiasmata the chromatids repel to each other and twist around to make interesting shapes.
Once they are attached at their chiasmata the chromatids repel to each other and twist around to make interesting shapes.
chiasmata
AS we have seen the process of crossing over happens when two non-sister chromatids swap segments of their DNA. This emans that a maternal chromosmme can end up with a asegment of genetc chrosmosome of the father and vice versa. Thus a chromosome originally carrying a recessive allele could end up with a dominant allele that was traded during crossing over. This creates more diversity in humans.
AS we have seen the process of crossing over happens when two non-sister chromatids swap segments of their DNA. This emans that a maternal chromosmme can end up with a asegment of genetc chrosmosome of the father and vice versa. Thus a chromosome originally carrying a recessive allele could end up with a dominant allele that was traded during crossing over. This creates more diversity in humans.
Homologous chromosome separation
The homologous chromosomes are pulled to opposite ends of the cell during meiosis 1, notably visible during anaphase 1.
The difference of anaphase 1 in mitosis and meiosis is that instead o the sister chromatids of a single replicated chromosome being pulled apart the way they are in mitosis, the bivalent in meiosis are separated so that each homologous chromosome is pulled to opposite ends of the cell.
The difference of anaphase 1 in mitosis and meiosis is that instead o the sister chromatids of a single replicated chromosome being pulled apart the way they are in mitosis, the bivalent in meiosis are separated so that each homologous chromosome is pulled to opposite ends of the cell.
Sister chromatid separation
It is not until meiosis 2 that the sister chromatids of each chromosome are separated. In anaphase 2 the centromeres of the chrosmomes split, releasing each sister chromatid to become an individual chromosome. The spindle microtubes pull individuals chromatid to opposite end of the cell, because of random orientation the chromatid can be pulled towards eiahter of the newly forming daughter cells. Which then during telophase 2 a new nuclear membrane can form to have 4 new cells.
Independent assortment of genes
Gregor Mendel law of independent assortment states that when gametes are formed the separation of one pair of allels between the daughter cells is independent of the separation of another pair of alleles. What this means is that as a general rule one alleles does not fllow another when it is passed on to the gamete. The law of independent assortment implies that alleles that determineidfferent characteristics will eb transmitted independently to the next generation.
Understandings:
• Gene loci are said to be linked if on the same chromosome.
• Unlinked genes segregate independently as a result of meiosis.
• Variation can be discrete or continuous.
• The phenotypes of polygenic characteristics tend to show continuous variation.
• Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.
• Gene loci are said to be linked if on the same chromosome.
• Unlinked genes segregate independently as a result of meiosis.
• Variation can be discrete or continuous.
• The phenotypes of polygenic characteristics tend to show continuous variation.
• Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.
Applications and skills:
• Skill: Drawing diagrams to show chiasmata formed by crossing over.
• Skill: Drawing diagrams to show chiasmata formed by crossing over.