Cell Division and Cellular Organisation

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  • Created by: katie
  • Created on: 07-02-16 17:14

Cell Cycle

Starts when cell produced by cell division and ends with cell dividing to produce  2 identical cells.

Period of cell growth and DNA replication called Interphase. Interphase is subdivided into three seperate growth stages called G1, S and G2.

G1 - Cell grows, new organelles and proteins made.

S - Synthesis - Cell replicates DNA ready to divide by mitosis.

G2 - Cell keeps growing, proteins for division made.

Period of cell division called M Phase. M Phase involves mitosis and cytokenesis.

Cell cycle regulated by checkpoints. Checkpoints occur at checkpoints during cycle to make sure it's ok for process to continue.

G1 CP - Cell checks chemicals needed for replication present and any damage to DNA before S

G2 CP -  Cell checks if ALL DNA replicated without damage. If damaged cell can't enter mitosis

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Cell carries out normal functions but prepares to divide.

DNA unravelled and replicated.

Organelles replicated too so there's spare ones.

ATP content increases (ATP provides energy for cell division)

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Needed for growth in mullticellular organisms and for repair of damaged tissues.

Used by some animals of plants to reproduce asexually.

Continuous process describes in several stages:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase

Chromosome Structure

Mitosis begins, chromosomes made of  two strands joined together in the middle by centromere. Seperate strands called chromatids. Two strands on same chromosome are called sister chromatids. There's 2 strands because each chromosome already has identical copy of itself made during interphase. When mitosis is over new chromatids end up as one strand chromosomes in new daughter cells.

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Mitosis 1 -- Prophase

Chromosomes condenses, getting shorter and fatter.

Tiny bundles of proteins called Centrioles start moving to opposite ends of cell forming network of protein fibres called the spindle. Nuclear envelope breaks down and chromosomes free in cytoplasm.


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Mitosis 2 -- Metaphase

The chromosomes line up along middle of cell (at spindle equator)

Become attatched to spindle by centromere.

At metaphase checkpoint cell checks that all chromosomes attachtech to spindle before mitosis continues.


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Mitosis 3 -- Anaphase

Centromeres divide seperating each pair of sister chromatids.

Spindles contract  which pulls chromatids to opposite ends of cell, centromere first.


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Mitosis 4 -- Telophase

Chromatids reach opposite poles on spindle.

Uncoil, become long and thin 

Now called chromosomes again.

Nuclear envelope forms around each group of chromosomes so now 2 nuclei.


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Cytoplasm divides

In animal cells a cleavage furrow forms to divide cell membrane.

Now 2 daughter cells that are genetically identical to original cells and to each other.

Cytokinesis usually begins during anaphase and ends in telophase. It's seperate to the process of mitosis.

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Mitosis Overview


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Investigating Mitosis -- Practical

Can stain chromosomes to see under microscope so you can watch them during mitosis.

To do this, stain specimen on slide and examine under microscope.

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Sexual Reproduction and Mitosis

Normal body cells have diploid number of chromosomes, each cell has 2 of each chromosome (a pair - one from mom and one from dad).

Chromosomes that make up each pair are same size and have same genes although could have different versions of those genes - Alleles. These pairs of maching chromosomes are called homologous chromosomes.

Gametes have a haploid number of chromosomes (one cope of each chromosome). At fertilisation a haploid sperm fuses with haploid egg making a cell with a normal diploid number of chromsomes. Half chromsomes from father and half from mother. Diploid cell produced by fertilisation is called a zygote.


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Cell division that happens in reproductive organs to produce gametes.

Involves a reduction in division.

Cells that divide by meiosis are originall diploid but the cells formed are haploid, the chromosome number halves. 

The cells formed are genetically different because each new cells have a different combination of chromosomes

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Interphase for Meiosis

Whole of meiosis begins with interphase. During interphase, cells DNA unravels and replicates. This produces double armed chromsomes called sister chromatids.

After interphase, cells enter meiosis 1. Meiosis 1 is reduction division (halves chromosome No.). Ithas four stages:

  • Prophase 1
  • Metaphase 1
  • Anaphase 1
  • Telophase 1
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Meiosis 1 -- Prophase 1

Chromosomes condense, fatter and shorter.

Homologous chromosomes pair up. 

Crossing over occurs.

Like in mitosis, centrioles begin to move to opposite ends of cell forming spindle fibres.

Nuclear envelope breaks down.

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Meiosis 1 -- Metaphase 1

Homologous pairs line up across centre of cells.

Attatch to spindle fibres by centrioles.

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Meiosis 1 -- Anaphase 1

Spindles contract pulling pairs apart.

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Meiosis 1 -- Telophase 1

Nuclear envelope forms around each group of chromsomes.

Cytokinesis occurs.

2 haploid daughter cells produces.

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Meiosis 2

2 daughter cells undergo:

  • Prophase 2
  • Metaphase 2
  • Anaphase 2
  • Telophase 2
  • Cytokiesis

These are the same as in meiosis 1 except with half the number of chromsomes. 

In anaphase 2 the sister chromatids are seperated - each new daughter cell inherits one chromatid from each chromosome.

4 haploid daughter cells produced.

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Meiosis 1 & 2 Overview


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Genetic Variation

Differences between an individual's genetic material. 

Meiosis is important so that it creates genetic variation - it makes gametes which are all gentically different.

During fertilation any egg can fuse with any sperm which also creates variation.

This means new individuals have a new mixture of alles making them genetically unique.

We get genetic variation in gametes because of:

  • Crossing over of chromatids
  • Independant assortment of chromsomes
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Crossing over of chromatids

During prophase 1 of meiosis 1. homologous pairs of chromosomes come together and pair up. 

Chromatids twist around each other and bits of chromatids swap over.  

Chromatids still contain the same enes but now have different combinations of alleles.

The crossing over of chromatids in meiosis 1 means that each of the 4 daughter cells formed from meiosis 2 contain different alleles.


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Independent assortmentof chromosomes

Each homologous pair of chromsomes in your cells is made up of one maternal and one paternal. 

When they line up in metaphase 1 and are seperated in anaphase 1, it's completley random which chromosome from each pair ends up in whcih daughter cell so the 4 daughter cells produced by meiosis have a completley different combination of the maternal and paternal chromsomes.


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Stem Cells

Multicellular organsims are made up of many different types of cells that are specialised. The specialised cells originally came from stem cells.

Stem cells are unspecialised cells which can develop into different types of cells.

Stem cells are found in:

  • early embryos
  • bone marrow
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Stem cells divide to become new cells  which then become specialised.This is called differentiation.

Stem cells are also able to divide to produce more undifferentiated stem cells (they can renew themselves).

In animals, stem cells are used to replaced damaged cells.

Human Example:

Bones are living organisms which contain nerves and blood vessels. Main bones in the body have bone marrow in the centre. This is where adult stem cells divide and diffrentiate to replace worn out blood cells - erythrocytes and neutrophils.

Plant Example:

Stem cells found in meristems. In stem and root stem cells of the vascular  cambium divide and diffrentiate to become xylem vessle and phloem sieve tubes.

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Stem cells and medicine

Heart disease: reasearchers trying to develop a way to use stem cells to make a replacemtn for the heart cels to repair the damages tissue.

Alzheimers: Hoping to use stem cells to regrow healthy nerve cells which are damged in people with alzheimers resulting in memory loss.

Parkinsons: Transplanted stem cells might help to regenerate the dopamine producing cells which are needed to to release dopamine needed to control movement.

Stem cells are also used un research to develop biological knowledge such as:

  • How organsims grow and develop
  • Understanding develpmental disorders and cancer
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Specialised cells -- Human Examples

Eryocytes - carry oxygen to blood. Bioconcave disc shape provides large SA for gas exchange. They have no nucleus so more room for haemoglobin, the protein that carrues oxygen.

Neutrophils - defend body against disease. Flexible shape means they can engulf foreign particles or pathogens. Many lyosomes in their cytoplasm contain digestive enzymes to break down engulfed particles.

Epithelial cells -  cover surfaces of organsisms. Cells joined by interlinking cell membranes and a membrane at their base. Ciliated epithelia have microvilli (folds in mebrane). This increases SA. Squamous epithelia are very thin and allow efficient gas diffusion.

Sperm cells - have flagellum so they can swim to egg. Lots of mitrochondria to provide energy to swim. Acrosome contains lots of digestive enzymes to enable sperm to penentrate surface of egg.

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Specialised cells -- Plant Examples

Palisade mesophyll cells - in leaves, do most of photsynthesis. Contain many chloroplasts so they can absorb lots of sunlight. Walls thin so carbon dioxide can diffuse easily into cell.

Root hair cells - absorb water and mineral ions frm soil. Have large SA for absorbtion and a thin, permeable cell wall for entry of water and ions. Cytoplasm contains extra mitochondria to provide energy needed for active transport..

Guard cells - found in pairs with gap between them to form a stoma. Tiny pores in surface of leaf for gas exchange. In light, guard cells take up water into their vacuoles and become turgid. Their thin outer walla and thickened inner walls force them to bend outwards, opening the stomata. This allows the leaf to exchange gases for photosynthesis.

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Group of cells that are specialised to work together to carry out a particular function. A tissue can contsin more than 1 type of cell.

Animal Examples:

Squamous epithelium tissue - single layer of flat cells lining surface. Found in many places of body including alveoli in lungs and provides thin exchange surface for substances to diffuse across quickly.

Ciliated epithelium - layer of cells covered in cilia. Found on surfaces where things need to be moved.

Muscle tissue - made up of bundles of elongated cells called muscle fibres. Different types:

  • Smooth
  • Cardiac
  • Skeletal

All sligthly different strucures.

Cartilage - type of connective tissue. Shapes and sipports nose, ears, windpipe. Formed when cells called chondroblasts secrete extracellular matrix which they become trapped inside.

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Tissues -- Plant Examples

Xylem - transports water around plant and supports plant. Contains hollow xylem vessel cells which are dead and living parenchyma cells.

Phloem - transports sugars around plant. Arranged in tubes and made up of sieve cells, companion cells and some ordinary plant cells. Each sieve has end walls with holes in them so that sap can move through them easily. End walls are called sieve plates.

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Group of different tissues that work together to perform a particular function.


Lungs - animal organ which carry out gas exchange. The contain squamous epithelium tissue (in alveoli) and ciliated epithelium tissue (in bronchi). Also have elastic connective tissue and vascular tissue.

Leaf - Plant organ which carries out gas exchage and photosynthesis. Contains palisade tissue and epidermal tissue and xylem and phloem tissue in the veins.

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Organ systems

Organs work together to form an organ system. Each system has a particular function.

Respiritory system - made up of all organs, tissues and cells involved in gas exchange. Lungs, trachea, larynx, nose , mouth and diaphram are part of the respiritory system.

Circulatory system - made up of organs involved blood supply. The heart, arteries, veins and capllaries are all part of this system

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