BIO - - PROKARYOTIC CELLS Prokaryotic cells are cells that do not have a nucleus. Prokaryotic cells are also the oldest cells in the world. Prokaryotic cells have a more simple structure than eukaryotic cells. They do not have a nucleus or other organelles surrounded by a membrane. Prokaryotic organisms are always unicellular. (This is because their cell structure is too simple for the cells to become specialised, which is what occurs in multicellular organisms. However, they often group together to form colonies.) Organisms in the kingdoms Bacteria and Archaea are prokaryotic. Prokaryotes have a cell membrane. Prokaryotes also have a cell wall for support. (It contains a substance called peptidoglycan. Some cells have large amounts of peptidoglycan. They stain purple with a dye called crystal violet, and are known as Gram positive bacteria.) - Prokaryotic cells contain cytoplasm. (In a region of the cytoplasm called the nucleoid is a circular ring of DNA.) - Some prokaryotes have additional small loops of DNA known as plasmids. (Plasmids help bacteria survive in stressful situations, for example, they are involved in antibiotic resistance.) - - - - - BACTERIA Bacteria come in many different shapes including rods, spheres and spirals. Sometimes, bacteria cells can even join together to create long chains and other formations. A pathogenic bacterium is a type of bacteria that can cause disease. EUKARYOTIC CELLS basic structure. Inside a eukaryotic cell, there is much more organisation All the different specialised parts are separated into compartments. These compartments are separated from the rest of the cell by membranes. These are called membrane-bound organelles. (The outer cell membrane is made up of two layers of molecules called phospholipids.) Eukaryotic cells are complex enough to work together with other cells and form multicellular organisms. (some eukaryotic organisms can also be unicellular, such as the organisms in the kingdom Protista.) Organisms in the kingdoms Plantae, Animalia, Fungi and Protista are made up of eukaryotic cells. (They are divided into these kingdoms based on their cell structure) One of the main differences between prokaryotic and eukaryotic cells is how the DNA is stored. (In eukaryotic cells, the DNA is located inside the nucleus. The nucleus has a nuclear membrane separating it from the rest of the cell. This allows the DNA to be protected from damage.) - BOTH PROKARYOTIC AND EUKARYOTIC CELLS HAVE CYTOPLASM, CELL MEMBRANE, RIBOSOMES. - - CELL DIVISION Most eukaryotes reproduce by a type of cell division called mitosis. Prokaryotes and some unicellular eukaryotes reproduce using binary fission. In this process the cell grows and replicates its DNA, then when it is large enough it splits into two cells. - - - CELL ORGANELLES Organelles are specialised structures in a cell that perform specific functions. Plastids are double membrane bound organelles, mostly found in plants and algae. They are important for both manufacture and storage of chemicals in the cell. Chloroplasts are the organelles that exist in plants as the site of photosynthesis. Within the inner membrane is the stroma, a thick fluid containing lots of enzymes. Thylakoids are a part of the chloroplast containing the chlorophyll. MITOCHONDRIA This is a mitochondrion. Mitochondria (plural) are responsible for producing the energy needed to drive all of the cellular processes. Mitochondria are often called the 'powerhouses of the cell'. This is where most cellular respiration takes place, after glycolysis. Mitochondria come in many different sizes. They are generally described as rod or oval shaped. They are scattered throughout the cytoplasm. Mitochondria have a double membrane. Within the mitochondria, oxygen and glucose are combined, to make energy (in the form of ATP) for the cell to use. The enzymes needed for the reaction are held on the cristae (the folded inner membrane(to make work faster)). ENDOPLASMIC RETICULUM - The endoplasmic reticulum (ER) is a system of connected channels of membranes. - Parts of the membrane are able to pinch off into membranous sacs called vesicles. - These vesicles deliver substances throughout the cell. - Endo means internal; plasm is from cytoplasm; and reticulum means network. - The ER is a network of tubules within the cytoplasm that transports proteins around the cell. - Rough endoplasmic reticulum (RER) is studded with ribosomes. - These ribosomes produce proteins directly into the ER. - The ER then distributes the newly made proteins to their destinations. - RER is often found attached to the nuclear. If the proteins are distributed within the cell, then this process is called intracellular transport. If the proteins are distributed out of the cell, then this process is called intercellular transport. ● ● ● ● ● ● - When the ER has no ribosomes attached, it is referred to as smooth endoplasmic reticulum or SER. - Generally, the SER transports proteins and synthesises lipids (fats and oils). Because cell membranes are primarily made up of lipids, smooth ER is important for plasma membrane synthesis as well. - In liver cells, it breaks down drugs and produces steroid hormones. In some cells, the smooth ER also produces carbohydrates, and in others it stores calcium ions. GOLGI APPARATUS ● Golgi body is one of the largest organelles in a cell. It contains enzymes which modify substances, generally by adding proteins or carbohydrates. They are then packaged in membrane 'pockets' called vesicles. Vesicles are tiny sacs made of phospholipids (like the cell membrane) cytosis(cell action) Vesicles transport substances into and out of the cell by joining with the cell wall (out of the cell, called exocytosis)(into the cell, called endocytosis). The Golgi body also produces lysosomes, another organelle. This organelle contains digestive enzymes that break parts of the cell into smaller subunits that can be reused in the cell. When a cell is old and worn out, one of these lysosomes will release its enzymes into the cell, digesting the parts of the cell and killing it. This is called 'programmed cell death' or apoptosis. - - CELL MEMBRANE - The cell membrane encloses the cytoplasm (the gooey stuff inside) of the cell and controls what enters and exits the cell. - The cell membrane is made of a phospholipid bilayer. - The bilayer is made up of hydrophobic (water hating) tails made of fatty acids (lipids) and hydrophilic (water loving) heads made of phosphate. Hence the name phospho+lipid. - The phosphate heads are charged and so interact with the water molecules which are polar. - The cell membrane is semi-permeable. (These allow small particles (like sugars and oxygen) to pass through, but not big ones. Things that cells need includes gases (oxygen and carbon dioxide), nutrients (sugars, fatty acids, and amino acids), and water, the main solvent in all cells. Things that the cells get rid of are wastes (mostly urea and excess carbon dioxide), and products that the cell produces for use elsewhere outside of the cell (e.g. hormones, mucus, etc). Proteins form an important part of the cell membrane. Some are attached to the outside of the membrane, and are known as extrinsic proteins. Others are inside the phospholipid bilayer, and are known as intrinsic proteins. Some proteins stretch from one side of the membrane to the other, and are known as transmembrane proteins. In some cases the transmembrane proteins form a protein channel. These are very important in allowing certain molecules to pass across the cell membrane. Another important component of cell membranes in animal cells is cholesterol. The membrane is fluid, which is stabilised by cholesterol. This is known as the fluid mosaic model of membrane structure. - - (in humans to much cholesterol may lead to heart disease) Plants do not have cholesterol. Some of the proteins have sugar groups attached to them. These are known as glycoproteins and are very important in cell signalling. Glycoproteins are protein molecules with sugar groups attached. (secreted in the golgi bodies) Function of glycoproteins in the cell membrane: - cell adhesion - cell receptors - antigen for cell recognition 2 TYPES OF ELECTRON MICROSCOPE - Transmission electron microscopes (TEM) - Scanning electron microscopes (SEM). - The TEM works by beaming electrons through a very thin slice of sample (2D image) - The SEM bounces electrons off the surface, giving a 3D image. ~ DIFFERENCES ~ PASSIVE TRANSPORT- DIFFUSION Diffusion is a form of passive transport, meaning that it does not require energy to - - - occur. - It is defined as the net movement of particles from an area of high concentration to an area of low concentration. - When the concentration gradient is high, this is an efficient technique of transporting tiny molecules, such as oxygen, through a semi-permeable barrier. Other nutrients, on the other hand, are unable to pass through the membrane and are required by the cell. Diffusion is increased by increasing temperature, moisture, surface area and concentration gradient. Multicellular organisms have specialised surfaces and transport systems to increase diffusion. (why multicellular organisms must not solely rely on diffusion for their nutrient requirements.) - Cells in the middle cannot get the nutrients they require. - The surface area:volume ratio is too small. - Large size makes it difficult materials with the surroundings Cells use diffusion to gain useful molecules and remove waste molecules. The smaller an object, the larger its surface area relative to its volume. - Therefore the smallest cube has the largest surface area:volume ratio, - While the largest cube has the smallest ratio. FOR AN EFFECTIVE DIFFUSION A LARGER SURFACE AREA:VOLUME RATIO IS THE BEST. The rate of diffusion depends on: PASSIVE TRANSPORT -OSMOSIS - Osmosis is the spontaneous nett movement of water molecules across a semi-permeable barrier from areas of lower to higher solute concentration. - Osmosis acts to equalise the concentration of the solutes on the two sides of a semi-permeable membrane. TONICITY Tonicity refers to the concentration of solutions: hypotonic = dilute, isotonic = equal, hypertonic = concentrated. - - - When plant cells lose water they become plasmolysed, whilst if they gain water they become turgid. (due to cell wall) When animal cells lose water they shrink called crenation, whilst if they gain water they swell and may burst. (no cell wall) Regulating water levels, or osmoregulation is important in living things. PASSIVE TRANSPORT- FACILITATED DIFFUSION - Facilitated diffusion is a form of passive transport (meaning that it does not require energy to occur). - ________________________________________________________________________________________________________ HIERARCHY ORGANISATION LEVELS OF ORGANISATION - The smallest level of organisation is the cell. - Many cells combine to form a tissue. - The cells within a tissue are always of the same type. - For instance, muscle cells group together to form muscle tissue. - Humans have four types of tissue: connective, epithelial, muscle and nerve. - An organ is a structure that contains more than one type of tissue working together. The organ can then perform a specific function. An organ system is a group of related structures that are grouped together to perform a specific function. Organ systems allow you to digest food, move your body, transport essential nutrients to every cell in your body and excrete wastes. DESCRIPTION OF A TISSUE - Similar cells grouped together to perform the SAME function. DESCRIPTION OF AN ORGAN - Two or more tissues grouped together. DESCRIPTION OF AN ORGAN SYSTEM - Functionally related organs working together DESCRIPTION OF AN ORGANISM - Many organ systems working together - All multicellular organisms are made up of cells. ● Similar cells group together to form tissues. ● Tissues group together to form organs. ● Organs group together to form organ systems. ● All the organ systems together form the organism. - - - - - THE EXCRETORY SYSTEM The excretory system is incredibly important because our cells produce many waste products through metabolism, for example cellular respiration (process where cells gain energy). Without the excretory system, the build up in wastes would eventually poison our cells, resulting in death! Excretion is the removal of wastes that our body has produced (metabolic waste). Defecation or egestion is the removal of faeces (undigested food from our gastrointestinal tract). Our excretory organs are the lungs, liver, skin and kidneys. TYPES OF NITROGENOUS WASTES - Nitrogenous waste is any metabolic waste product that contains nitrogen. - Urea and uric acid are the most common nitrogenous waste products in: - terrestrial animals - freshwater fish excrete ammonia - marine fish excrete both urea and trimethylamine oxide. - Nitrogenous waste is produced when there is excess protein. - Proteins are made up of chains of amino acids. Freshwater and marine animals such as crabs and starfish remove their nitrogenous waste as ammonia. Many animals do not have enough water to excrete ammonia, so may convert ammonia into a less toxic substance called urea. This is formed by reacting ammonia with carbon dioxide. AMMONIA FORMULA NH3 THE KIDNEYS WHAT IS THE URINE? Urine is the term given to the end product created by the kidneys. After the kidneys filter out the excess water, salts and urea from our blood, they combine to form urine. The kidneys have three important internal regions. These are the renal medulla, the renal cortex and the renal pelvis. FUNCTIONS OF THE RENAL MEDULLA - The function of the medulla is to HOLD MASSES OF TISSUES called renal pyramids in each kidney. - The renal medulla is the inner region of the kidneys. FUNCTIONS OF THE RENAL CORTEX - The function of the renal cortex is to filter blood and remove unwanted substances out of the body. - The renal cortex is the outer portion of the kidney between the renal capsule and the medulla. FUNCTIONS OF THE RENAL PELVIS - The major function of the renal pelvis is to act as a funnel for urine flowing to the ureter. - The renal pelvis is the tube through which urine flows from the kidney to the bladder. NEPHRONS Each of your kidneys contain more than a million tiny filtering structures that help clean your blood. These are called nephrons. They filter blood to produce urine. Their functions include the removal of metabolic waste (excretion) and regulating water levels (osmoregulation). Blood enters the kidneys through the renal artery and leaves via the renal vein. - The nephron is divided 4 different regions, each with an important function: Bowman’s capsule (FILTRATION) First part of the nephron where blood is initially filtered (to form filtrate). Proximal convoluted tubule (REABSORPTION) A folded structure connected to the Bowman’s capsule where selective reabsorption occurs. Loop of Henle (WATER REABSORPTION) A selectively permeable loop that descends into the medulla and establishes a salt gradient. Distal convoluted tubule (REABSORPTION) A folded structure connected to the loop of Henle where further selective reabsorption occurs. FIRST PART OF THE NEPHRON IS THE BOWMAN’S CAPSULE - Each Bowman's capsule contains a network of capillaries, called a glomerulus. SECOND PART OF THE NEPHRON IS THE PROXIMAL CONVOLUTED TUBULE - reabsorption of filtrate in accordance with the needs of homeostasis (equilibrium) The filtrate now passes into the Loop of Henle which is found in the medulla region of the kidney. - The filtrate now passes into the Loop of Henle which is found in the medulla region of the kidney. - Overall this makes the urine much more concentrated and selectively reabsorbs some salts from the filtrate back into the blood. The next region of the nephron is the distal convoluted tubule, which is found in the cortex region of the kidney. - 'Distal' means away from, as this part of the nephron is away from the Bowman's capsule. - In this region there is more selective reabsorption of salts by active transport. The filtrate moves from the distal convoluted tubule into the collecting duct. - These tubes connect the nephron to the renal pelvis and carry the urine towards the ureter. - The collecting ducts are regulated by hormones such as ADH (anti-diuretic hormone) which controls the amount of water reabsorbed by this region. The remaining filtrate travels down the collecting ducts through the renal pelvis to the ureter. - This tube carries urine down from the kidneys to the bladder where it is stored. Eventually it is released from the body via the urethra. The blood in the capillaries within the kidney have now reabsorbed all the glucose and amino acids back from the filtrate, as well as some salts and water. - They now join together to form the renal vein, which carries the filtered blood back towards the heart. - - - - - - - IN CONCLUSION Overall the kidneys filter the blood in the Bowman's capsule (ultrafiltration), then selectively reabsorb needed substances back from the filtrate as it passes along the nephron. This ends up with urine, containing urea, some salts and some water, which is carried to the bladder, and clean blood being returned to the body via the renal vein. HOMEOSTASIS The ability of an organism to adapt for changes in its environment by maintaining internal stability. NEGATIVE FEEDBACK A negative feedback loop occurs in biology when the product of a reaction leads to a decrease in that reaction. A negative feedback loop brings an organism closer to a target of stability or homeostasis. OSMOREGULATION Osmoregulation is the maintenance of constant osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content. PHOTOSYNTHESIS Photosynthesis uses water, the sun, and the air to produce glucose (C6 H12 O6). Glucose is food for plants. It gives them the energy they need to grow. We can write photosynthesis as an equation: Photosynthesis is the process by which green plants and other organisms use sunlight to synthesis nutrients from carbon dioxide and water. Photosynthesis is the number one source of oxygen in the atmosphere. Without photosynthesis, carbon cycle could not occur oxygen surviving life would not occur/survive. Without photosynthesis there would be little to no oxygen on the planet. How plants gather the reactants needed for photosynthesis: Sunlight shines into the plant through the transparent top layer. Water enters the plant through the roots. Carbon dioxide diffuses into the plant through the stomata. These products meet in the chloroplasts, where chlorophyll converts them to glucose and oxygen. - Chlorophyll takes in sunlight and converts it into chemical energy, this helps the reaction to happen. - This is because photosynthesis is endothermic and needs energy to occur. ENDOTHERMIC: a procedure that requires the absorption of heat. - Chlorophyll is also the reason why leaves are green. This diagram below clearly labels the different structures of a leaf and the ways that reactants enter and leave. STEM CELLS - Another difference is that stem cells may divide endlessly to produce additional stem cells, but other cells in the bo longer divide after they have specialised. - - - - Stem cells have been found in the bone marrow, brain, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, other places (but not all tissues). Stem cells are cells that are unspecialised cells that can divide to become any kind of cell. Stem cells can divide into two by the process of mitosis. (Remember, mitosis is the type of cell division that produc identical cells.) There are two types of stem cells. Pluripotent or embryonic stem cells are those that can become any type of cell in the body. They are found in embryos that are just a few days old. Embryonic stem cells are created by leftover embryos donated by patients. Unlike tissue specific stem cells, these pluripotent; this means that they grow onto any kind of tissue in the body. Tissue specific stem cells replace the existing cell in your organs as they were out and die Tissue-specific or adult stem cells are cells that could become any type of cell in the specific tissue they a in. Tissue-specific stem cells can be found in various parts of a healthy body. Specialised cells are cells with special structure and function Difference between adult (tissue-specific) and embryonic (pluripotent) stem cells. - Tissue specific stem cells in our bodies are to replace damaged cells in the tissues they are found in. - Chemicals or other factors released from damaged cells trigger the differentiation of the stem cells. - Stem cells do not heal all tissues in the human body. Summary - Pluripotent stem cells are essential during development, because all the specialised tissues that make up a person from these unspecialised cells - After development, tissue specific stem cells are found in specific tissues - The differentiation of stem cells are triggered by chemicals from damaged cells. - However stem cells can not repair severe damages to body tissues. ___________________________________________________________________________________ - When scientists discovered stem cells' potential, they wanted to see if they could utilize them to treat diseases or r damaged tissues. - This is called stem cell therapy or regenerative medicine. - The first step is to collect stem cells and multiply them in a laboratory setting. - Adult stem cells are only found in very small numbers in specific tissues. - They are also tissue-specific, meaning that they can only differentiate into specialised cells of the tissue from whic were extracted. - In order to use them to replace damaged cells, there must be a large number of them. - This means they must be cultured in the laboratory until there are enough cells. - Embryonic stem cells are easier to isolate and grow in the laboratory. - They have the advantage of being able to differentiate into any cell type in the body. - However, there are moral and ethical issues about using embryonic stem cells in research. This is due to the fact t cells are obtained by destroying an embryo that would otherwise have developed into a human person. Because o some people are opposed to the usage of embryonic stem cells. - After the stem cells have been isolated and grown in large enough numbers in the laboratory, they must be tested humans. - Stem therapy= using stem cells to treat diseases or injuries Advantages of using embryonic stem cells over adult stem cells for stem cell therapy: - Embryonic stem cells are easier to isolate and grow in the lab - Embryonic stem cells can become any type of cell in the body. - Disadvantages of using embryonic stem cells over adult stem cells for stem cell therapy: - The destruction of the embryo which could have become a human being. - embryonic stem cell research in Australia is legal. (donated with consent) - Stem cell therapy is currently used to treat leukemia and serious burns. Summary - Stem cell therapy is when stem cells are used to treat diseases and repair damaged tissues. - The stem cells must first be isolated and then grown in the lab to increase the number of cells. They are then impla damaged tissues where they differentiate into specialised cells to restore function to the tissue. - Stem cell therapy is currently used to treat leukemia. - Leukaemia is when there are cancerous white blood cells in the bone marrow. - These cells take up all the space and resources in the bone marrow so that healthy red blood cells cannot form. - Chemotherapy can be used to eliminate cancerous cells, after which healthy haematopoietic stem cells from a don implanted into the patient's bone marrow. These stem cells have the ability to develop into new blood cells. - what induced pluripotent stem cells are. (advantages and disadvantages that they have compared to using adult stem cells or embryonic stem cells.) VCE Biology: Unit 2 Revision - Centrioles: organelles made up of microtubules that produce spindle fibres during cell division. - Chromosome: a threadlike structure made up of DNA and proteins found in the nucleus of eukaryotic cells - Chromatid: one half of a duplicated chromosome. - Apoptosis: cell suicide - Binary fission: an asexual reproduction in which one cell divides to form two identical cells. (occurs in prokaryotes) - Cell cycle: the phases in the life of a cell from one cell replication to another. G1 (gap 1): growth of a cell occurs with replication of many organelles. (interphase) - G2 (gap 2): more growth occurs and the cells get ready for replication. (interphase) - Cell cycle checkpoint: a point in the cell cycle where the cell is checked for any errors. (shown on diagram) - Interphase: cell grows, and prepares for cell division. (consists of G1, S, G1) - Cytokinesis: the division of the cytoplasm creating two new daughter cells. (shown on diagram) - Daughter cells: The name given to the newly created cells during cell replication. PMAT - Mitosis: The process of nuclear division in eukaryotic cells; divided into 4 stages: PROPHASE, METAPHASE, ANAPHASE, and TELOPHASE. - Anaphase: A Phase of MITOSIS: chromatids of each chromosome are pulled apart in opposite poles. - Metaphase: A phase of MITOSIS: chromosomes line up along the metaphase plate in single Asexual and Sexual reproduction - In asexual reproduction, an individual makes offspring without a partner. - The offspring produced are genetically identical to their parents! This means they have the same alleles. - We call these offspring clones, or clonals. - Asexual reproduction does not use gametes or include the process of meiosis or fertilization. - mitosis is the main process used in asexual reproduction. Mitosis is a type of cell division. (Mitosis has many uses. It is usually used for growth and recovery from injury. When you grow taller each year, or when you heal a broken bone, you are using mitosis.) - sexual reproduction uses meiosis and asexual reproduction uses mitosis. - These are both types of cell division, meaning they make new cells by splitting up old cells - However, mitosis makes cells that are identical to the original cell. It is used to do lots of different things. - Meiosis makes haploid cells that have only half the DNA of the original cell. It is only used to make gametes. budding and vegetative propagation. - In budding, mitosis is used to grow a new organism off the side of the parent. When the organism is mature it splits off from the parent, leaving behind a scar. - Vegetative propagation is when a mature plant grows special structures, from which new plants can grow. - In sexual reproduction, two individuals combine their DNA to produce offspring that have genetic information from both parents. - Gametes are specific haploid cells produced during meiosis, a type of cell division. You may be familiar with the terms sperm and egg. - Individuals have two alleles for each gene. Their gametes, however, have only one allele. So gametes contain half of an individual's DNA. - During sexual reproduction, the mother donates an egg and the father donates a sperm. - The sperm and egg fuse together in a process called fertilization to make a single cell called a zygote. - This combines the DNA found in the gametes. So the zygote ends up with two alleles for each gene: one from the mother's egg and one from the father's sperm. - Using a type of cell division called mitosis, the zygote grows into an embryo, then into a foetus and eventually into a baby. - In summary: - In sexual reproduction, two parents come together to make offspring. - Each parent donates one gamete. Gametes are special cells produced by a type of cell division called meiosis. - The gamete from the father and the gamete from the mother fuse, in a process called fertilization, to make a cell called a zygote. - The zygote grows into an individual.______________ - The zygote inherits all the DNA in the sperm and the egg. - This means the child gets half of their DNA from their father's sperm and half from their mother's egg! Advantages and Disadvantages - Asexual reproduction is often a lot easier to do, since it does not require a partner. Finding a partner can be difficult if individuals are far apart. - Asexual reproduction can also be cheaper, since it takes a lot of energy to find suitable partners. - We can see how expensive sexual reproduction is when we consider the elaborate courtship behaviours and display structures that many animals have. - With asexual reproduction seeming so much easier, cheaper and safer, you have to wonder why species reproduce sexually at all! - However It is risky to have all your offspring be identical to one another. If the environment changes, and the traits those offspring have are not favourable, they can all die! - Sexual reproduction produces offspring with different allele combinations and, as a result, slightly different traits. - If the environment changes, there is a good chance that at least one of the offspring will have traits that are suited to the new environment. - By creating variation in your offspring, you can increase the chances that some offspring survive! - Overall, asexual reproduction is easier and less energetically more expensive than sexual. However, it is also riskier. It puts all your eggs in one basket by making offspring that are identical to one another. - Sexual reproduction makes offspring with different traits, and so spreads the odds around. This makes sexual reproduction favourable in unpredictable and changing environments. - 1. 2. 3. 4. 5. 6. 7. Meiosis: - A 2N cell is divided into 4 unique 1N daughter cells which develop into sex cells. - In meiosis cells divide twice so meiosis has two rounds of cell division. - First two daughter cells are produced but The cells divide again to produce Four daughter cells.. The final four daughter cells are haploid meaning they have only one copy of each chromosome.(Cells produced in mitosis are diploid) EXTRA: The only haploid cells in your body are gametes:Sperm and Egg Meiosis makes gametes. Gametes are used in sexual reproduction, The sperm will fuse with the egg, fertilising it.This produces zygote and then mitosis makes it a baby The zygote inherits All of the chromosomes found in the sperm and eggs so gametes need to be haploid to make sure the zygote gets the correct number of chromosomes. Steps of Meiosis: Before meiosis starts the parent cell replicates its DNA. Then the chromosomes move to the centre of the cell and nuclear membrane breaks down Homologous chromosomes are separated (This step does not happen in mitosis).Nuclear membrane reform. Then the cell membrane splits the cell in half and the first part is done.(Haploid daughter cells) In the second part the chromosomes move to the centre of the cell again. Nuclear membrane breaks down again. But this time sister chromatids are separated by a spindle fibre network. Then the cell membrane pinches both cells in half and FOUR haploid daughter cells called gametes are produced ! - Because the gametes are haploid the genetic information is different from the diploid parent cell, The gametes are also different from one another. Summary of meiosis: - Before meiosis starts the parent cell replicates its DNA, meiosis then involves two rounds of cell division. - These rounds of cell division are very similar but the main difference is that in the first round the homologous chromosomes are separated while in the second round the sister chromatids have separated. - This produces Four Haploid Gametes. The genetic material is different from the parent cell and different from one another.
