Meiosis

• What is the cell cycle?

  Two main phases: Interphase (the growth period) and Cell division (Mitosis or meiosis- division of the nucleus) and (cytokinesis - division of the cytoplasm and organelles)
• What is interphase?


  Interphase has three stages:G1 (synthesis of cell components) S phase (DNA replication)G2 (preparation for meiosis/mitosis)DNA replication, to form two identical DNA strands, occurs in the S phase of interphase. After replication, the two genetically identical strands of DNA are known as sister chromatids. Sister chromatids are held together at the centromere, which adheres sister chromatids to each other vias cohesin (a protein complex) and is the site of kinetochore and microtubule attachment for movement of chromosomes. The cohesin forms a ring-like structure that wraps around strands of DNA, connecting the sister chromatids until anaphase. During interphase, nuclei are rounded or oval, DNA is granular and in chromatin form. Chromosomes are not present. Microtubules are distributed around the nucleus.
• What is the centromere?

  The section where sister chromatids are held together, via cohesin. It is also where kinetochores allow microtubule attachment
• What is cohesin?


  A protein complex. The cohesin forms a ring-like structure that wraps around strands of DNA, connecting the sister chromatids until anaphase
• What are kinetochores?


  Protein structures located at the centromere and they allow microtubule attachment
• How is DNA packaged?

  DNA is a double helix and is made of two antiparallel strands of nucleotides with two strands linked by hydrogen bonding between complimentary base pairs. Eukaryotic DNA is wrapped around histone proteins, forming nucleosomes. These nucleosomes coil and stack together to form chromatin fibres, and it is the chromatin fibres that form chromosomes.
• Why is DNA in chromatin form during interphase?

  Because DNA needs to be accessible to enzymes, during interphase most of the DNA is in chromatin form and not tightly coiled.
• What is a chromosome?

  Chromosome- molecule of DNA supercoiled around nucleosome- packaging histone proteins.
• What is a gene?

  Genes- sections of chromosomes that store genetic information that codes for a trait through transcription and translation of the genetic code into a protein.
• What is a homologous pair?

  In most eukaryotic cells, chromosomes are found in pairs, one inherited from the mother and one from the father. The pair is referred to as a homologous pair. The two chromosomes in a homologous pair have the same size, the same centromere location and the same genes in the same order. The two chromosomes of a homologous pair may have the same allele of a gene (homozygous) or different alleles of a gene (heterozygous)
• What are alleles?


  If there are variations in the sequence of nucleotides in a single gene, caused by mutation, they are known as alleles.
• What is a homozygous pair?

  Two chromosomes of a homologous pair have the same allele of a gene
• What is a heterozygous pair?

  The two chromosomes of a homologous pair have different alleles of a gene
• What is a diploid?

  Organisms and cells that contain a homologous pair of each chromosome are known as diploids. Nearly all mammals are diploids- with two complete sets of chromosomes in body cells (one copy paternal, one copy maternal)

  Diploid cells may be known as 2n.
• What is a karyotype?

  A karyotype arranges chromosomes from a person's cells in order from largest to smallest.

  It shows:

  The number of chromosomes.

  The size and shape of each chromosome.

  The pairs of chromosomes: Humans have 23 pairs of chromosomes (46 chromosomes total).

  22 pairs are autosomes (non-sex chromosomes).

  1 pair are sex chromosomes (XX for females and XY for males).
• What is a haploid?

  A haploid is a cell or organism with a single copy of each chromosome, therefore not containing homologous pairs. Eggs and sperm are examples of haploid cells. Haploid cells are represented by n.
• What is gametogenesis?


  Meiosis occurs in the process of gametogenesis (formation of sperm and egg cells), and occurs in two stages.
• What is reduction division?

  Meiosis is known as reduction division because it reduces the number of chromosomes in the daughter cells by half compared to the starting cell.
• What occurs during prophase I?

  The germ cell is diploid, 2n.

  Nuclear membrane begins to break down

  Kinetochore attaches to the centromere- The kinetochore attaches to the centromere, which is ready to attach to the spindle fibres.

  Spindle fibres form, the microtubules grow out from the centrosomes, which move to opposite ends of the cell, organising themselves into long, strong fibres stretching across the cell.

  DNA condenses/supercoils into chromosomes

  Homologous chromosomes pair up, known as synapsis. during synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. Each homologous chromosome consists of two chromatids, so now there are four chromatids in total in a structure called a tetrad.

  Chromosomes cross over. Crossing over is the process where parts of the chromatids from one homologous chromosome exchange with parts of the chromatids from the other homologous chromosome. The crossing over creates new combinations of alleles (the different versions of genes), which is a big source of genetic variation. During crossing over, a section of the chromatid from the maternal chromosome might swap places with the chromatid from the paternal chromosome. these sections contain alleles. This means that after crossing over, the chromatids are no longer identical. The chromosomes that end up in the gametes (egg or sperm) will have a mix of maternal and paternal genes. After the chromatids physically exchange parts, they are still held together at the points of crossover, known as chiasmata. These chiasmata can be seen under a microscope and are an indicator that crossing over has happened.

  Crossing over produces recombinant chromosomes, which are chromosomes with new combinations of alleles not present in either parent.
• What is synapsis?


  Homologous chromosomes pair up, known as synapsis. during synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. Each homologous chromosome consists of two chromatids, so now there are four chromatids in total in a structure called a tetrad.
• What is a tetrad?


  Homologous chromosomes consist of two chromatids, so when they pair up there are four chromatids in a tetrad structure.
• What is crossing over?

  Crossing over is the process where parts of the chromatids from one homologous chromosome exchange with parts of the chromatids from the other homologous chromosome. The crossing over creates new combinations of alleles (the different versions of genes), which is a big source of genetic variation. During crossing over, a section of the chromatid from the maternal chromosome might swap places with the chromatid from the paternal chromosome. these sections contain alleles. This means that after crossing over, the chromatids are no longer identical. The chromosomes that end up in the gametes (egg or sperm) will have a mix of maternal and paternal genes. After the chromatids physically exchange parts, they are still held together at the points of crossover, known as chiasmata. These chiasmata can be seen under a microscope and are an indicator that crossing over has happened.

  Crossing over produces recombinant chromosomes, which are chromosomes with new combinations of alleles not present in either parent.
• What is chiasmata?


  After the chromatids physically exchange parts, they are still held together at the points of crossover, known as chiasmata. These chiasmata can be seen under a microscope and are an indicator that crossing over has happened.
• What are recombinant chromosomes?

  Crossing over produces recombinant chromosomes, which are chromosomes with new combinations of alleles not present in either parent.
• What occurs during metaphase I?

  The cell is still a diploid, 2n, meaning that it has two copies of each chromosome, one paternal and one maternal.

  Paired homologous chromosomes line up side-by-side in the centre of the cell, known as bivalents. This is called the metaphase plate. This is different from mitosis, where individual chromosomes line up (not pairs). Each chromosome of the bivalent will move to a different cell pole in Anaphase I, and this movement is random, creating variation.

  Spindle fibres attach to the kinetochores centromeres of each chromosome. One spindle fibre attaches to the centromere of the parental chromosome, and the other to the maternal chromosome.

  The way the chromosomes line up is random. This means that when the chromosomes are separated during the next stage (Anaphase I), each new cell could end up with a different mix of maternal and paternal chromosomes. This random alignment and separation of chromosomes is called independent assortment, and it is a big reason why genetic diversity happens. there are in fact 2^n possible ways chromosomes can independently assort to gametes
• What is independent assortment?

  The way the chromosomes line up is random. This means that when the chromosomes are separated during the next stage (Anaphase I), each new cell could end up with a different mix of maternal and paternal chromosomes. This random alignment and separation of chromosomes is called independent assortment, and it is a big reason why genetic diversity happens. There are in fact 2^n possible ways chromosomes can independently assort to gametes
• How many possible ways may chromosomes independently sort into gametes?


  There are in fact 2^n possible ways chromosomes can independently assort to gametes
• What occurs during anaphase I?

  The cell is still a diploid, 2n, meaning that it has two copies of each chromosome, one paternal and one maternal.

  The homologous pairs that were randomly lined up at the equator of the cell are pulled apart by the microtubules and spindle fibres and they shorten.

  Sister chromatids are NOT separated, the chromosome remain as an x with sister chromatids attactched at the centromere.
• What occurs during telophase I?

  Telophase I occurs at the same time as cytokinesis.

  New nuclei begin to form around the chromosomes at each pole

  DNA uncoils from chromosomes to form chromatin

  The spindle fibres break apart

  Two haploid daughter cells are formed, and each daughter cell has one copy of each chromosome type, either paternal or maternal, at random.
• What occurs during prophase II?

  Cells are now haploid, n, as each cell has one copy of each chromosome type. This may either be maternal or paternal, and is randomised from the independent assortment during metaphase I.

  DNA condenses/supercoils into chromosomes

  Nuclear membrane begins to break down

  Spindle fibres form, the microtubules grow out from the centrosomes, which move to opposite ends of the cell to prepare to pull the chromosomes apart, organising themselves into long, strong fibres stretching across the cell known as spindle fibres. The spindle fibres will later attach to the chromosomes to help pull them apart, and are known as the mitotic spindle.

  Kinetochore attaches to the centromere- The kinetochore attaches to the centromere, which is ready to attach to the spindle fibres, which will then allow the chromosomes to move around.
• What occurs during metaphase II?

  Cells are still haploid, n, as each cell has one copy of each chromosome type. This may either be maternal or paternal, and is randomised from the independent assortment during metaphase I.

  Chromosomes now align at the cell equator

  Sister Chromatids are still attactched to one another via cohesin in the centromere

  The mitotic spindle attaches to the kinetochores on each chromosome
• What occurs during anaphase II?

  Cells are still haploid, n, as each cell has one copy of each chromosome type. This may either be maternal or paternal, and is randomised from the independent assortment during metaphase I.

  The cohesin holding the sister chromatids together is cut via the enzyme separase, which is kept inactive by a protein known as securin before anaphase.

  Motor proteins of the microtubules pull the sister chromatids along the spindle fibres which begin to shorten and pull the sister chromatids towards the opposite poles of the cell.
• What occurs during telophase II?

  Telophase II occurs at the same time as cytokinesis.

  Cells are still haploid, n, as each cell has one copy of each chromosome type. This may either be maternal or paternal, and is randomised from the independent assortment during metaphase I.

  Spindle fibres break apart and the DNA uncoils to form chromatin.

  The DNA is no longer X shaped, because the sister chromosomes have moved to opposite poles of the cell.

  At the end of cytokinesis, there are four haploid daughter cells
• What is nondisjunction?


  Nondisjunction is the failure of chromosomes to separate during either anaphase I or anaphase II, therefore resulting in gametes that have an incorrect number of chromosomes.

  In anaphase I, nondisjunction occurs when homologous chromosomes fail to separate properly. One gamete receives two of the same chromosome and the other receives no copy of that chromosome. All of the resulting gametes have an incorrect number of chromosomes. In anaphase II, a nondisjunction may occur when sister chromatids fail to separate properly. Half of the gametes that result have an incorrect number of chromosomes, but the other half are not affected. If a gamete with an extra chromosome fuses with a normal gamete, the zygote will have three copies of the chromosome. Usually, the zygote ill not survive and will be flushed out of the female body.
• What is down syndrome and why is is more common with a higher maternal age?

  Down syndrome occurs when humans have an extra chromosome #21.

  The risk of down syndrome affected offspring increases with maternal age, as the proteins that help keep chromosomes together such as cohesin, and older women have lower amounts of these proteins in their eggs. This leads to instability of the homologous chromosome pairs and a higher likelihoods of nondisjunction occurring in anaphase I.
• What is genetic variation?

  Variation is a defining feature of life. Variation within a species is called intraspecies variation. It is inheritable, and is transmitted from parents to offspring.

  Genetic variation allows some organisms to survive better than others in their environment. Therefore, those individuals are more likely to reproduce and pass on their beneficial variation. This is known as natural selection, and is the main driving force of evolution.

  Genetic variation may result from:

  genetic mutation

  gene flow (movement of genes between different organism groups)

  meiosis (through prophase I and metaphase I)

  sexual reproduction