Cell division, cell diversity and cellular organisation

What is the product of interphase?

A chromosome that consists of two sister chromatids, connected by a centromere

What is the difference between meiosis 1 and meiosis 2?

• Meiosis 1- homologous chromosomes are separated

• Meiosis II - chromatids (each half of a duplicated chromosome) are separated.

Describe the process of crossing over

1. Homologous chromosomes come together to form bivalents.

2. Non-sister chromatids wrap around each other and join up at the chiasmata

3. The chromosomes break up at the chiasmata, and sections of chromosomes swap over between non-sister chromatids.

4. The swapping over is only between non-sister chromatids.

5. The final chromatids still have the same genes, but they may have different alleles.

State what the cell cycle is and outline its stages.

Regulated cycle of division with intermediate growth periods.

1. Interphase

2. Mitosis or meiosis (nuclear division)

3. Cytokinesis (cytoplasmic division)

What happens during interphase?

G1 , S and G2 phase

What happens during G1 phase?

• Organelles replicate

• The cell increases in size.

• Protein synthesis

What happens during S phase?

DNA replicates- ready for mitosis

What happens during G2 phase?

• Cell keeps growing

• Proteins needed for cell division are made

What is the purpose of mitosis?

Produces 2 genetically identical daughter cells for:

* growth
* cell replacement/ tissue repair
* asexual reproduction

Name the stages of mitosis.

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Outline what happens during prophase in mitosis

• Chromosomes condense, becoming visible.

• The chromosomes consist of two identical chromatids called sister chromatids which are joined together at the centromere

• Centrioles move to opposite poles of the cell & mitotic spindle fibres form.

• The nuclear envelope & nucleolus break down = chromosomes free in cytoplasm.

Outline what happens during metaphase in mitosis

• Mitotic spindle fibres attach at the centromeres of each chromosome.

• The spindle fibres pull the chromosomes, making them line up at the metaphase plate

Outline what happens during anaphase in mitosis

• The sister chromatids separate at the centromere (the centromere divides in two)

• Spindle fibre begin to shorten

• The separated sister chromatids are pulled to opposite poles by the spindle fibres

Outline what happens during telophase in mitosis

• The chromatids decondense - can be called chromosomes again.

• Mitotic spindle breaks down

• Nuclear envelopes begin to reform around each set of chromosomes

What happens during cytokinesis in animals?

1. .Contractile division of cytoplasm.

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2. Cell membrane cleavage furrow forms.

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3. forms 2 daughter cells which are genetically identical to the original cell and each other

What happens during cytokinesis in plants?

• Cellulose starts to build up at the equator (the end plate).

• Plasma membrane forms in the middle of the end plate resulting in two fully separated plant cells

How is the cell cycle regulated?

• Checkpoints regulated by cell-signalling proteins

• These ensure damaged cells do not progress to next stage of cycle.

Describe what happens during the G1 phase checkpoint

• The cell checks that the chemicals needed for replication are present

• Chromosomes are checked for damage.

• If damage is detected then the cell does not advance into the S phase until repairs have been made

Describe what happens during the S phase checkpoint

• Chromosomes are checked to ensure they have been replicated.

• If all the chromosomes haven't been successfully replicated then the cell cycle stops

Describe what happens during the G2 phase checkpoint

• An additional check for DNA damage occurs after the DNA has been replicated.

• The cell cycle will be delayed until any necessary repairs are made

Describe what happens during the M phase checkpoint

• The final check- occurs during metaphase

• It determines whether the chromosomes are correctly attached to the spindle fibres

What is meiosis?

• A form of cell division where the nucleus divides twice

• Produces 4 genetically different haploid cells known as gametes from 1 diploid cell

Describe what happens during meiosis Ⅰ

• Its the first division, the reduction division.

• The pairs of homologous chromosomes are separated into 2 cells.

• Each cell only contain one full set of genes instead of 2 = haploid

• Crossing over occurs

What are homologous chromosomes?

• Pair of chromosomes with genes at the same locus.

• 1 maternal & 1 paternal.

• Some alleles may be the same while others are different

What happens during meiosis Ⅱ?

1. Independent segregation of sister chromatids.

2. Each cell divides again, producing 4 haploid cells.

How does meiosis produce genetic variation?

* Crossing over during meiosis Ⅰ.

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* Independent assortment (random segregation) of homologous chromosomes & sister chromatids.

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Result in new combinations of alleles.

How do cells become specialised?

* Some genes are expressed while others are silenced due to cell differentiation mediated by transcription factors.

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* Cells produce proteins that determine their structure & function.

What is a stem cell?

Undifferentiated cells that can divide indefinitely and turn into other specific cell types.

• Its a renewing source of undifferentiated cells

Name and define the 4 types of stem cell

* Totipotent: can develop into any cell type including the placenta and embryo.

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* Pluripotent: can develop into any cell type excluding the placenta and embryo.

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* Multipotent: can only develop into a few different types of cell.

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* Unipotent: can only develop into once type of cell.

Suggest some uses of stem cells

* Repair of damaged tissue e.g. cardiomyocytes after myocardial infarction.

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* Drug testing on artificially grown tissues.

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* Treating neurological diseases e.g. Alzheimer's & Parkinson's.

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* Researching developmental biology e.g. formation of organs, embryos.

Describe the structure of Neutrophils (white blood cells)

* flexible shape- allows them to engulf foreign particles or pathogens

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* many lysosomes in their cytoplasm- contains digestive enzymes to break down engulfed particles

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* multilobed nucleus allowing them to move and engulf material more easily

Describe the structure of Erythrocytes (red blood cells)

* biconcave- large SA

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* no nucleus- more room for hemoglobin

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* lots of haemoglobin in cytoplasm to carry oxygen.

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How do the specialised cells in blood form?

Multipotent stem cells in the bone marrow differentiate into:

* Erythrocytes- have a short lifespan & cannot undergo mitosis since they have no nucleus.

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* Leucocytes, including neutrophils.

State the relationship between a system and specialised cells.

specialised cells →
tissues that perform specific function →
organs made of several tissue types →
organ systems

Describe the structure of ciliated epithelia

* Have cilia (hair-like structures), which beat in a coordinated way to shift material along the surface of the epithelium tissue

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* Goblet cells secrete mucus- helps to trap dust, dirt and microorganisms - preventing them from entering vital organs where they may cause infection

Describe the structure of Simple squamous epithelium

* has a single layer of flattened cells on a basement membrane

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* The layer of cells forms a thin cross-section- shortens the diffusion pathway

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* It is permeable- allowing for the easy diffusion of gases

Describe the specialised structure of a spermatozoon.

* Have a flagellum- rotates so they can swim to the egg

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* Lots of mitochondria to provide energy to swim

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* The acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg

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* haploid nucleus

Describe the structure and function of palisade cells

• Large number of chloroplasts are present in the cytoplasm - maximise the absorption of light for photosynthesis

• The tall and thin shape - allows light to penetrate deeper and for many cells to be densely packed together

• Thin walls so CO2 can diffuse easily into the cell

Describe the structure and function of root hair cells.

* Specialised to absorb water and low-concentration minerals from soil.

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* Hair-like projections increase surface area for osmosis / carrier proteins for active transport.

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* Many mitochondria produce ATP for active transport.

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* Thin permeable cell wall-shorter diffusion distance for entry of water and ions

Describe guard cells

control the opening of the stomata to regulate water loss and gas exchange

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* Inner cell walls are thicker while the outer cell walls are thinner- The difference in the thickness of the cell walls allows the cell to bend when turgid

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* The cytoplasm has a high density of chloroplasts and mitochondria.

What are meristems?

• Totipotent undifferentiated plant cells

• Can develop into various types of plant cell, including xylem vessels & phloem sieve tubes.

Describe the structure of a vascular bundle.

Cambium (meristematic tissue)

Phloem tissue

Xylem tissue

Describe the xylem vessels

transport tissue for water and dissolved ions

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* No top and bottom walls between cells- forms continuous hollow tubes- water is drawn upwards towards the leaves by transpiration

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* Cells are essentially dead, without organelles or cytoplasm, to allow free movement of water

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* Outer walls are thickened with a substance called lignin- strengthens the tubes- helps support the plant

Describe the structure of phloem tissue.

* Sieve tube elements:
form a tube to transport sucrose in the dissolved form of sap.

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* Companion cells:
involved in ATP production for active loading of sucrose into sieve tubes.

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About cartilage

• Type of connective tissue found in joints

• Shapes and supports the ears, nose and windpipe

• Formed when chondroblasts secrete an extracellular matrix which they become trapped inside

What is erythropoiesis?

• Its when new erythrocytes are constantly being formed from bone marrow stem cells

• maintains the red blood cell count in the blood

What changes occur to adapt the original stem cell to become an erythrocyte

• The cell changes into a biconcave shape: this shape has a larger surface area- more oxygen can be absorbed through the cell surface

• Building up of haemoglobin in the cytoplasm: haemoglobin is the pigment that binds with oxygen

• The ejection of the nucleus: creates more room in the cytoplasm for haemoglobin, increasing the oxygen-carrying capacity

• An elastic membrane: this allows erythrocytes to change shape and therefore squeeze through narrow capillaries

What changes occur to adapt the original stem cell to become an neutrophil?

* Indentations form in the nucleus, giving it a lobed structure

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* Granules accumulate (these are lysosomes that contain hydrolytic enzymes)

What cells differentiate to form xylem and phloem?

Cambium- is a meristem which undifferentiated tissue in a plant that has the ability to give rise to new cells

What changes occur to adapt the cambium to become a xylem?

* **l**ose their cytoplasm

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* deposit lignin in their cell walls

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* lose their end cell walls

What changes occur to adapt the cambium to become a phloem?

* lose some of their cytoplasm and organelles

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* develop sieve plates (located at ends of the cells)

Name the 3 types of muscle in the body and where they are located

Cardiac: exclusively found in heart.

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Smooth: walls of blood vessels and intestines.

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Skeletal: attached to incompressible skeleton by tendons.

When does cytokinesis usually begin?

• Begins in anaphase

• Ends in telophase

• Its a separate process to mitosis

What happens during prophase 1 ?

• DNA condenses and becomes visible as chromosomes

• Chromosomes arrange themselves into homologous pairs (bivalents)

• Crossing over of non sister chromatids occurs at chiasmata.

• Centrioles start moving to opposite ends of the cell forming spindle fibres

• Nuclear envelope breaks down and nucleolus disappears

What happens during Metaphase 1?

• The homologous pairs of chromosomes line up across the centre of the cell

• Chromosomes attach to the spindle fibres by their centromeres

• The maternal and paternal chromosomes in each pair position themselves independently of the others

What happens during Anaphase 1 ?

• Spindle fibres contract separating the homologous pairs

• Centromeres do not divide- sister chromatids of each chromosome remain attached to one another and don't come apart

What happens during Telophase 1 during ?

• Chromosomes assemble at each pole

• Nuclear envelope re forms around the two groups of chromosomes

• Chromosomes decondense.

• Animal cell undergoes cytokinesis

• Cleavage furrow forms

• The cytoplasm divides

• 2 haploid daughter cells are produced

What happen during Prophase 2?

• The chromosomes condense and coils and become visible again.

• Nuclear envelope breaks down

• Centrioles move to opposite poles - spindles formation begins.

What happens during Metaphase 2?

• Individual chromosomes assemble on the centre (metaphase plate)

• Attached to spindle fibre by the centromere

• Due to crossing over, the chromatids are not identical

• Independent assortment and more genetic variation is produced.

What happens in anaphase 2?

• Centromeres divide

• Sister chromatids of the individual chromosomes are pulled to opposite poles

Telophase 2

• The chromatids decondense- become chromosomes again

• Nuclear envelope reforms around each set of chromosomes and the nucleolus becomes visible.

• Cytokinesis results in the division of cells forming 4 daughter cells.

• Cells are haploid because of the reduction division.

• Cells will be genetically different from each other and the parent cell

What is the significance of meiosis in life cycles?

• Produces haploid cells

• Produces genetic variation by independent assortment and crossing over.

Describe the structure and function of guard cells in plants

* Form stoma.

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* When turgid, stoma opens;

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* when flaccid, stoma closes.

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* Walls are thickened by spirals of cellulose.

Name two types of cell that can be found in phloem tissue.

* sieve (tube) element
* companion (cell)

When does independent assortment occur?

metaphase I and metaphase II

When does the formation of the spindle apparatus occur?

Prophase I

When does the separation of sister chromatids occur?

anaphase II

When does the formation of nuclear membranes occur?

telophase II

When do the chromosomes get pulled to opposite poles?

Anaphase I

Explain why meiosis needs to have twice as many stages as mitosis.

• To halve chromosome number

• To separate homologous pairs (of chromosomes) and sister chromatids

Before the division of the nucleus of a cell, the genetic material must replicate. Explain why this is essential.

* so that cells are genetically identical / have same DNA

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* so both (daughter) cells receive a full copy

Explain what is meant by a homologous pair of chromosomes

• One maternal and one paternal

• Carry same genes

• Pair up in meiosis

• Similar length

State what is meant by the term tissue.

A group of specialised cells working together to perform a particular function

Function of squamous epithelial

* acts as a surface
* short (diffusion) pathway

Function of ciliated epithelial

* move mucus
* secrete mucus

Name two parts of a plant where meristematic tissue can be found.

* just behind tip of root

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* just behind tip of shoot

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* vascular bundle

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* bud

Name the type of nuclear division that produces two genetically identical nuclei

mitosis

G 0

The phase when the cell leaves the cycle, either temporarily or permanently.

Differentiation

* A cell that becomes specialised to carry out a particular function is no longer able to divide.

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* It will carry out the function indefinitely and not enter the cell cycle

Why might a cell leave a cycle (temporarily or permanently)

• Differentiation

• Damaged DNA = cell arrest

• Get older = more cells

Chromosomes

Structures of condensed and coiled DNA in the form of chromatin.

Chromatids

Two identical copies of DNA held together at the centromere

Centromere

A region where two chromatids are held together

Chromatin

Uncondensed DNA in a complex with histones

Crossing over

* Sections of DNA which become entangled, breaks and rejoins

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* During prophase 1 of meiosis = genetic variation

Independent assortment

• The production of different combinations of alleles in daughter cells

• Its caused due to the random alignment of homologous pairs along the equator

• Each chromosome is inherited randomly and independent of other chromosomes

• Results in genetic variation

What is a Zygote?

The initial diploid cell formed when 2 gametes are joined by the means of sexual reproduction