1. Mistakes in meiosis
1.1. 2 types of mistakes that can be made in meiosis: 1.Error during chromosome division 2.Errors during recombination. Errors can be analyzed by karyotypes
1.1.1. Nondisjunction in meiosis I, homologous chromosome fail to separate from each other this results in 2 gametes either with an extra chromosome or missing chromosome
1.1.2. 1. Error during chromosome division, nundisjunction is when chromosomes do not divide properly then there might be a missing or extra chromosome.The condition is called aneuploidy.
1.1.2.1. Nondisjunction in meiosis II sister chromatids fail to separate, results in 2 gametes one gamete with extra chromosome and one with a missing chromosome.
1.1.2.2. 3 types of aneuploidy: 1. Monosomy (1 missing chromosome) 2. Trisomy (1 extra chromosome) 3. Polysomy (1+ extra chromosome)
1.1.2.2.1. Example of monosomy (Turner's syndrome XO) under developed breasts and ovaries, characteristic facial hair, web of skin and constriction of aorta
1.1.2.2.2. Example of trisomy (Klinefelter syndrome XXY) poor beard growth, breast development and underdeveloped testicles. Usually this is fatal to the embryo, other syndromes include: Patau syndrome, Edwards syndrome and down syndrome.
1.1.3. 2. Errors during recombination this occurs when DNA is not exchanged properly and leads to abnormal chromosome shape (duplication, deletion, inversion and translocation.
1.1.3.1. Duplication, chromosome fragment attaches to a chromatid resulting in an extra copy of gene and gene is expressed more often.
1.1.3.2. Deletion, this where a chromosome fragment is lost resulting in the loss of genetic information. Could be serious if lots gene information is lost.
1.1.3.3. Inversion, chromosome fragment has a gene segment attached in the reverse order results is that the gene might not be expressed properly.
1.1.3.4. Translocation, chromosome fragment attaches to non-homologous chromosome and the gene may not be expressed properly.
2. Homologous chromosomes are the same size , same shape and the genes that code for the same trait. Gene codes for a specific change for example eye colour, flower colour etc.
3. G1 (1st growth phase) cells grow and prepare for division. Synthesis of components required for DNA synthesis this takes 40% of the cell cycle process
4. Meiosis
4.1. Special type of cell division in sexually reproducing organisms, produces gametes such as sperm and egg cells and goes through 2 rounds of division which produces 4 cells. The final gamete has 23 chromosomes and is haploid which is half a diploid.
4.1.1. Egg with 23 chromosomes + sperm with 23 chromosome = zygote with 46 chromosomes
4.1.1.1. Human life cycle: zygote > embryo > fetus > human. when the sperm reaches the egg they combine and form they zygote (product of fertilization)
4.2. There are 2 rounds of cell division meiosis I and meiosis II
4.2.1. Meiosis I
4.2.1.1. Prophase I, Chromatin condenses and turns into chromosomes, similar to mitosis centrioles move towards opposite poles, spindle fibers begin to form and nuclear membrane breaks down. Synapsis begins to take place and a tetrad forms; a pair of duplicated homologous chromosomes in total forms 4 chromatids.
4.2.1.1.1. Crossing over, it allows for genetic variation. Chromatids often break off and swap pieces .Crossing over happens in the chiasma
4.2.1.2. Metaphase I, tetrads line up along the equator randomly this allows for genetic variation. Spindle fibers attach to the pair of sister chromatids
4.2.1.3. Anaphase I, the pairs of chromosomes being to separate by being pulled towards opposite ends.The sister chromatids do not separate they stay together online mitosis.
4.2.1.4. Telophase I, the nuclear membrane reforms and spindle fibers disappear.
4.2.1.4.1. Cytokinesis I, forms 2 genetically different daughter cells and each of the daughter cell has half the number of chromosomes the parent cell has this is called haploid
4.2.2. Meiosis II
4.2.2.1. Prophase II, nuclear membrane begins to break down, spindle fibers start to form, centrioles being to migrate towards poles and there is no replication of DNA
4.2.2.2. Metaphase II, pairs of chromosomes start to align along the equator of the cell, spindle fibers attach to the chromatids and due to crossing over the chromatids are not identical anymore.
4.2.2.3. Anaphase II, the spindle fibers begin to pull chromatids apart.
4.2.2.4. Telophase II, nuclear membrane builds back together and reassembles itself, the chromosomes decondense and spindle fibers disappear
4.2.2.4.1. Cytokinesis II, forms 4 different daughter cell with different genetic material in each and each of the daughter cell has half the number of chromosomes the parent cells has, therefore they are haploid.
5. Karyotypes
5.1. Somatic cells (body cells) undergo mitosis to divide. The human somatic cells have 46 chromosomes diploid number (2 sets) 23 from father and 23 from mother, organized into 23 pairs of homologous chromosomes (contain one chromosome from each parent)
5.2. Alleles, 2 or more alternate forms of gene and there could be different versions of gene at the same location.
5.2.1. Dominant allele, use to code for a protein that will determine the characteristic of an organism for example the colour of someones hair (e.g., brown hair)
5.2.2. Recessive allele, only expressed when both alleles are recessive (e.g., blond hair)
5.3. Sex chromosomes, 23rd pair of homologs. In humans these are the X and Y chromosomes, females will receive an X chromosome from each parent = XX and males will receive X chromosome from their mother and Y chromosome from their father = XY. Homologs 1-22 are autosomes which determines the rest of the physical characteristics.
6. Dihybrid Crosses
6.1. Studying 2 characteristics at the same time, with 2 pairs of contrasting traits at the same time
6.2. The law of independent assortment, during gamete formation, segregating pairs of unit factors assort independently of each other, the 2 traits inherited totally independently of each other (colour is inherited independently of seed shape)
7. S (synthesis phase) replication of chromosome occurs this is the DNA synthesis and takes 39% of the cell cycle process
8. Human genome, full DNA sequence of humans, 1.5-2% of human genome contains genes that code for protein. 98% regions that repeat and regulate genes. Ordered by automated technology and analyzed by a computer.
8.1. Gene coding, sequence of nucleotides (A, G, C, T ) determines which protein building blocks are used
9. The cell alternates between periods of growth, DNA replication and active reproduction. Growth, duplication of genetic information, preparation for mitosis, carrying out normal cell functions and organelle duplication. Many processes occur during the cell cycle
9.1. G2 (2nd growth phase) final stage in preparation for cell division and prepares for mitosis this takes about 19% of the cell cycle
9.2. M (mitosis) all 4 phases of mitosis takes place and cell division happens and is completed, this takes about 2% of the cell cycle process.
10. Cell Division
10.1. One cell separating into 2 cells with identical genetic material
10.2. Cell division serves 3 purposes: 1. Healing and tissue repair 2. growth of an organism 3. reproduction of an organism
10.3. Transmission of genetic material through cell division happens 2 ways sexually and asexually
10.3.1. Asexually does not involve the blending of gametes or change of number of chromosomes. There is no need for mating, exact copy of parent cell, no genetic variation and is a quick process. Takes less than 24 hours to see millions of bacteria cells, one bacteria cell forms and then 1/2 hour later 2 cells show, 1/2 there are 4 cells, 1/2 hour later 8 cells shown within 15 hours a billion bacteria cells show.
10.3.2. Sexually by mating, some animals have a mating period and others such as humans don't. This produces an offspring which is a mix of both parents and there is lots of genetic variation.
10.4. There are 2 process that need to happen for a cell to divide, Mitosis and Cytokinesis.
10.4.1. Cytokinesis - the cytoplasm, cell membrane and organelles divide into 2 daughter cells
11. Mitosis
11.1. The chromosomes are replicated and divides into 2 halves, each half contains identical genetic material.
11.1.1. There are 4 phases that happen in the mitosis process (prophase, metaphase, anaphase and telophase.
11.1.1.1. Prophase, 2 pairs of centrioles move towards opposite poles (only animals have centrioles). Astral rays form around centrioles in a star shape, astral rays form from protein strands.Chromosomes start to become visible as pairs and join at the centromere.Nucleus disappears and chromosomes start to move to the centre of the cell.
11.1.1.2. Metaphase, chromosomes start to line up along the metaphase plate (equator of cell). Spindle fibers are attached at the centromere of each chromosomes, are held perpendicular to each other and the chromosomes are still condensed and thick.
11.1.1.3. Anaphase, 2 identical single stranded chromosomes form as the chromatids separate at the centromere. The spindle fibers shorten as each chromosome is pulled towards poles.
11.1.2. Telophase, in the phase the cell returns to interphase like conditions, the nuclear envelope reforms and the nucleus reappears and chromosomes uncoil and form into chromatin. Spindle fibers disappear, for a short time there is 2 nuclei in one cell and the cleavage furrow forms.
11.1.2.1. Cytokinesis, starts during telophase, the 2 daughter cells formed and the cytoplasm and organelles are distributed between the 2 cells. Cytoplasm creates a furrow by pinching the equator.
11.1.3. Interphase happens before mitosis begins, chromosomes spread throughout the nucleus and forms chromatin.
11.2. Plant cell mitosis
11.2.1. There is no centrioles, no astra rays and cytokinesis happens during late in anaphase.