Cell Cycle: Mitosis

• Cell Cycle (1): Mitosis

Overview

• Multicellular organisms start as a single cell and become trillions of cells through cellular reproduction
• Reproduction continues to grow or replace worn/damaged tissues
• Cellular reproduction is necessary for creating new cells and organisms
• Asexual reproduction involves only one organism and doesn't need sperm or egg
• Sexual reproduction needs both a male and female as well as sperm and egg
• Cell division is the process where a cell replicates to create two identical daughter cells

Chromosomes

• The nucleus holds all genetic material to direct the body's functions
• Chromosomes are composed of DNA, which contains instructions within genes
• Each species has a specific number of chromosomes, eg humans have 46
• Chromosomes contain proteins that aid organization, DNA and proteins are called chromatin
• Humans have 46 chromosomes organized into 23 pairs
• Twenty-two chromosome pairs are autosomes (females and males both have them)
• Sex chromosomes consist of one pair that decide gender
• Males have X and Y and chromosomes, while females have two X chromosomes
• The Y chromosome consists of SRY genes that causes testes to develop
• Karyotype displays the chromosomes in a cell
• When a cell separates, chromatin condenses to create chromosomes
• Staining chromosomes can create light and dark bands, which can used by computers based on size and shape

Mitosis

• Mitosis is cell division that starts when a fertilized egg divides
• It ensures cells becomes diploid and have the correct number of chromosomes
• During cell separation, a cell divides into two parts called sister chromatids
• Replicated or duplicated chromosomes contain two sister chromatids that contain same genes
• Genes are the units of heredity that are responsible for eye colour and metabolism
• Collectively, genes contain the instructions for our characteristics
• The centromere holds chromatids until a cell division occurs
• Cell division splits sister chromatids, creating new independent chromosomes for new cells
• After replication, chromosomes contain sister chromatids joined at the centromere

Cell Cycle

• The cell cycle is split into interphase and cell division
• Most of the cell cycle is spent in interphase, where organelles perform functions and cells prepare to divide
• During interphase, the cell increase organelles, grows, and doubles the amount of chromatin, and synthesizes DNA
• Interphase has three stages: G1, S nd G2
• G₁ stage-the cell performs normally and accumulates materials for DNA synthesis.
• S stage-DNA replication and each chromosome consists of two identical sister chromatids
• G₂ stage-synthesizes proteins
• The amount of time in interphase can vary
• Nerve and muscle cells do not complete the cell cycle and are arrested in G1, becoming a Go stage
• Embryonic cells spend very little time in G1 and complete cell growth in few hours

Mitosis and Cytokinesis

• Cell division after interphase has two stages: M or mitosis stage and cytokinesis
• Mitosis is nuclear division, also called duplication division because new nucleus has the same number of chromosomes from the former cell
• Cytokinesis is the division of the cytoplasm
• The cell cycle is continuous in certain species
• Mitosis is balanced by apoptosis or programmed cell death, which occurs when cells are excessively damaged or not needed
• The cell cycle is controlled by checkpoints to delay until conditions are met
• G1, G2 and Metaphase are checkpoints
• External factors, hormones, and growth factors may also control cell cycle

Cell Cycle Controls

• G1 checkpoint-If DNA is harmed, the cell will commit apoptosis, if growth singles are present it will undergo growth and nutrients are available
• G2 checkpoint - Mitosis occurs when mitosis happens only if DNA has properly replicated. Apoptosis can occur if DNA is damaged
• M checkpoint- Mitosis does not continue without chromosomes aligned properly
• External signal, hormones, and growth factors can stimulate a cell to divide
• The external signal binds to plasma membrane and relays info to cells
• Proteins create a chain and perform a signal transaction pathway that is followed by cell stimulation
• Once the last signal is made in signal transduction, genes are activated
• Proto-oncogenes are genes that stimulate the cell cycle, and tumor suppressors are used to inhibits the cell cycle

Apoptosis

• Apoptosis programmed cell death where cell fragments are engulfed by blood cells
• Apoptosis is triggered by internal and external signals
• Apoptosis helps to keep the number of cells at appropriate level
• Apoptosis helps in normal growth and development, such as tadpole tail

Mitosis

• Creates cells in developing fetus, embryo's, and/or child
• It replaces cells in adults
• During mitosis, new cells are created and daughter cells generated which are both identical to the parent cell
• As mitosis begins, chromosomes condense until each daughter cell gets a complete set of chromosomes
• DNA replication occurs during the S phase of interphase
• At the end of the phase, sister chromatids contain two identical parts
• Mitosis is split into prophase, metaphase, anaphase, and telophase
• The stages are continuous

The Mitotic Spindle

• The centromere is the microtubule organizing of a call
• After duplicating, chromosomes and centrosomes separate and form the poles of the mitotic spindle, where they accumulate microtubules

Stages of Mitosis

• In prophase, the duplicated centrosomes move to opposite ends of the nucleus, spindle fibres appear, the envelope fragments, chromosomes condense (now visible)
• In metaphase, aligned chromosomes linked to spindle fibres align on metaphase plate and spindle poles
• In Anaphase, Sister chromatids are now called chromosomes and move to various poles of the spindle.
• In telophase,chromosomes become indistinct forming chromatin, the cell divides into daughter nuclei, the nuclear envelope and nucleolus reappear
• In plant cells during metaphase, microtubules go opposite ends and attach to centromeres
• In Cytokinesis animal cells, cleavage furrow forms and anaphase ends where contractile fibres are formed

Cytokinesis

• Cytokinesis occurs through division of the cytoplasm and organelles
• Plant cells build plasma membrane and cell walls between cells
• Cell plate plate-newly formed plasma membrane where molecules that create membrane are released