Cellular division and organisation.

• Chromosomes are made from DNA tightly wrapped around histones and some RNA.
• There are 46 chromosomes in the human body, 22 autosomes and 1 sex chromosome.
• Homologous chromosomes are identical chromosomes that carry the same genes.
• Sister chromatids are two copies of one chromosome joined together at the centromere.
• In interphase, there are 3 phases. Gap phase 1, synthesis, and gap phase 2.
• In gap phase 1, the cell grow and new organelles and proteins are made.
• Gap phase 2 is when the nucleolus starts making ribosomes again.
• During synthesis, DNA is replicated to make sister chromatids, so there are a total of 92 chromatids, and 46 chromosomes.
• Checkpoints are a series of events that occur in the cell cycle to ensure that the cell is ready to divide.
• The first checkpoint occurs after gap phase 1 which commits the cell to the cycle. The 2nd is after gap phase 2 which checks that DNA has replicated properly. The 3rd checkpoint occurs during metaphase. It checks that the chromosomes have attached to the spindle fibres correctly.
• Mitosis has 4 phases, PMAT.
• Prophase- chromosomes condense and become visible. Centrioles begin move to the poles of the cell, forming spindle fibres. The nuclear envelope/membrane breaks down, so chromosomes lie free in the cytoplasm.
• Metaphase - Chromosomes line up along the equator of the cell. Spindle fibres attach to centromeres on the chromosome.
• Anaphase - Sister chromatids separate from one another at the centromere. Each set of chromatids moves towards opposite ends of the cell pulled by the spindle fibre.
• Telophase - Nuclear envelopes form around the sets of chromatids at either end of the cell. Chromatin begins to uncoil into chromosomes. Cytokinesis occurs (cell wall forms).
• Mitosis allows the growth of organisms, repair of tissues, replace of cells and asexual reproduction.
• Mitosis occurs in plants meristematic tissue and all over an animals body.
• Mitosis produces 2 diploid, genetically identical daughter cells.
• Cancer occurs due to uncontrolled cell division, which creates a rapid replication forming tumours that can damage surrounding tissue and spread through the blood to other areas of the body.
• Meiosis is used for sexual reproduction as it reduces the number of chromosome pairs to haploid so they can fuse together during fertilisation to create a new diploid zygote with genetic variation.
• Prophase 1 - Homologous chromosomes pair up (bivalents) and exchange DNA segments via crossing over to create recombinant chromosomes. The nuclear envelope breaks down and spindle fibres begin to form.
• Metaphase 1 - Bivalent chromosomes line up along the metaphase plate at the equator of the cell.
• Anaphase 1 - Sister chromatids separate from one another and move towards opposite poles of the cell pulled by microtubules attached to centromeres.
• Telophase 1 - Nuclear envelopes reform around sister chromatids at either pole of the cell. Chromosomes decondense into chromatin. In animal cells, the DNA uncoils whilst in plants they go straight on to the second stage.
• Interkinesis - A short gap between two mitotic divisions where the cytoplasm divides but not the nucleus.
• Prophase 2 - Spindle fibres attach to kinetochores of chromosomes and condensed chromosomes are visible under a light microscope. Centrioles are now at 90 degrees prior to the first stage.
• Metaphase 2 - Chromosomes align themselves along the metaphase plate at the centre of the cell. Spindle fibres attach to centromeres.
• Anaphase 2 - Sister chromatids separate by separating the centromere and move towards opposite poles of the cell. The spindle fibre pulls them apart.
• Telophase 2 - Nuclear envelope forms around daughter chromosomes which then decondense into chromatin. Cytokinesis occurs as the plasma membrane pinches inwards forming two new cells with their own nuclei.
• Tissues are groups of cells that work together to carry out a specific function. Some examples are xylem, phloem and muscle tissues.
• Organs are made up of a collection of tissues working together to perform a particular function. Examples include the stomach (muscular, nervous and glandular) and the leaf (spongy + palisade mesophyll, xylem and phloem)
• Organ systems are organs working together as a single unit. Some examples are the respiratory system (lungs, trachea and bronchi) and the circulatory system (heart, arteries and veins)
• Stem cells are undifferentiated cells that can self renew to produce more undifferentiated cells, or differentiate into specialised cells due to chemical signals.
• Embryonic stem cells come from embryos and have the potential to develop into any type of tissue
• Differentiation is when an unspecialised cell becomes specialised through changes in its structure and function
• Adult stem cells only form one type of tissue but they can divide rapidly to replace damaged cells
• Adult stem cells only exist in certain parts of the body and cannot form all types of cell
• Hematopoietic stem cells differentiate into blood cells, such as white, red and platelets. They are found in the bone marrow of people at all ages and in the umbilical cord in new-borns.
• Plant stem cell are found in the meristematic tissue. Plant cells rely on stem cells for growth and repair.
• Apical meristems in plants consist of roots and shoot tips.