Cellular Reproduction and Chromosomes Cell Cycle, Mitosis, Meiosis and Gene Expression

Cell Cycle Cells reproduce through a continuous sequence of growth and division known as the cell cycle Interphase – cells are making new molecules and DNA is copied in a process called DNA replication. Interphase broken into 3 steps: G1 – Gap 1 involves metabolic activities S – Synthesis involves DNA replication G2 – Gap 2 is preparation for cell division

Cell Cycle Timing of each phase can vary among different types of cells. Organ cell vs. skin cell Parent cell divides to create two new daughter cells (somatic cells have 46 chromosomes) Purpose of Mitosis Growth Regeneration of Damaged Tissue Maintenance of the Body

Chromosomes Hold the genetic information needed to maintain the cell and make new copies of the cell Made up of two sister chromatids which are held together with a centromere. Chromatids are exactly identical to one another

Phases of Mitosis Several events must happen to ensure that as a cell divides, the genetic material inside is properly shared between each new daughter cell. Phase order: PMAT & C Prophase Metaphase Anaphase Telophase Cytokinesis p. 144 figure 4 https://www.youtube.com/watch?v=mXVoTj06zwg

Prestep INTERPHASE Preparation for division The cell duplicates it’s genetic material (called chromatin) and creates two identical sister chromatids, joined by an object known as a centromere.

Mitosis Prophase chromatin, which is DNA and proteins, condenses and becomes visible Nuclear membrane and nucleolus disappear Centrioles made up of microtubules migrate to opposite poles of the cell Spindle fibres start to form between the two centrioles

Mitosis Metaphase Anaphase Spindle fibres attach to centromere Chromosomes line up on the cell’s equator (equatorial plate) Anaphase Centromere splits and chromatids are pulled to opposite poles of the cell

Mitosis Telophase Cytokinesis Chromatids reach the two opposite poles At this time each chromatid is considered a single non-replicated chromosome Chromosomes unwind and become less visible Cytokinesis Actual cell division Spindle fibres disappear, nucleolus reappears, nuclear membrane and in plant cells a new cell wall is formed

Meiosis, Chromosomes and Heredity

Meiosis and Chromosomes A zygote contains chromosomes from both parents Meiosis only occurs in reproductive organs and produces cells known as gametes (eggs or sperm) which are haploid (n). All other cells (somatic) are diploid (2n) and contain two copies of each type of chromosome.

Chromosomes The first part of meiosis reduces the chromosome number from diploid (2n) to haploid (n) known as reduction division. Each sperm or egg cell contains 22 autosomes and one sex chromosome (X or Y) The autosomes control almost all of the functions of the individual and the sex chromosomes determine the sex of the individual.

Reduction Division

Meiosis Almost the same as mitosis however there are two sequences of each of the phases. Interphase – chromosomes replicate Meiosis I - reduction division Prophase I, Metaphase I, Anaphase I, and Telophase I are all part of. Meiosis II - identical to mitosis. Prophase II, Metaphase II, Anaphase II and Telophase II

Meiosis Prophase I – homologous chromosomes pair which make up four chromatids called a tetrad. 23 pairs of homologous chromosomes, one copy of each pair came from each parent Crossing over - recombination of genes Page 154 figure 6

Meiosis Metaphase I – spindle fibres attach to the centromere of each chromosome Anaphase I - Homologous chromosomes are separated independently The centromere does not split

Meiosis Telophase I Needs to occur however can be a lengthy or short process Short  cell division goes directly to meiosis II Lengthy  chromosomes uncoil and nuclear membrane is formed (replication does not need to occur)

Meiosis Each cell beginning meiosis II is haploid Each cell at the end of meiosis II is also haploid although they are called gametes or spores. Meiosis II is exactly the same as Mitosis however there are only 23 chromosomes to split at the centromere in each cell instead of 46 as in the somatic cells Gametogenesis – process of creating sperm and eggs.

Gamete Formation - Male Spermatogenesis – male gamete formation A diploid germ cell (spermatogonium) undergoes the meiosis process to create 4 haploid cells Following meiosis II, cytoplasm lost and a tail develops

Gamete Formation Oogenesis – female gamete formation Diploid germ cell (oogonium) undergoes meiosis to create 1 haploid cell –primary oocyte is much larger than polar bodies After meiosis II, the cytoplasm is again unequally divided and only one cell is viable as a sex cell, the ovum (egg cell)

Genetic Variation Dependent on 2 factors: Crossing over – occurs during prophase I and the number of which is determined by the chromosome size (usually 2 or 3 cross overs per chromosome). Random Segregation – how each pair of homologous chromosomes line up during metaphase I is also extremely important as that determines which pole the chromosomes will go to. These two factors work together and are the basis behind genetic recombination

Genetic Variation

Diploid (2n) vs Haploid (1n)