Cell division

Cell cycle stages

1. Interphase (G1, S, G2) - longest stage

2. Nuclear division

3. Cytokinesis

G0

• In this phase, cells may undergo apoptosis (programmed cell death), differentiation or

  senescence.

Interphase G1

• Cell grows

• Protein synthesis occurs, RNA, enzymes

• Organelles replicate

• Preparation for s phase

G1 Checkpoint

• Checks is cell has grown to correct size

• Checks if DNA is damaged

• Checks if cells have resources

Interphase S phase

• DNA replicated for mitosis

• 46 Chromosomes 46 chromatids ———> 46 chromosomes 92 chromatids

• When all chromosomes have been duplicated,

  each one consists of a pair of identical sister

  chromatids.

• This phase is rapid, and because the exposed

  DNA base pairs are more susceptible to

  mutagenic agents, this reduces the chances of

  spontaneous mutations happening.

Interphase G2

• Cell grows

G2 Checkpoint

• Special chemicals ensure that the cell is ready

  for mitosis by stimulating proteins that will be

  involved in making chromosomes condense

  and in formation of the spindle.

• Checks DNA is correclty replicated

Why are these checkpoints important?

• to prevent uncontrolled division that would lead to tumours (cancer)

• to detect and repair damage to DNA (for example, damage caused by UV light).

What does the specific sequence ensure?

• The cycle cannot be reversed

• The DNA is only duplicated once during each cell cycle.

Why is mitosis used?

• Growth

• Tissue repair

• Asexual reproduction of single celled protoctists e.g. amoeba and paramecium to produce new individuals

• Creates body cells

M phase

• Cell growth stops

• mitosis occurs

M phase checkpoint

• Checks in metaphase

• Makes sure chromosomes are lined up correctly in center

• Checks chromosomes are attached to spindles correctly

What happens if cell does not meet checkpoint requirement?

• Cell will pause in checkpoint until issue can be fixed

• Will undergo apoptosis - if cannot be fixed (self - destructs) - ensures this cell cannot divide

Prophase

• Replicated chromosomes consist of two pairs identical sister chromosomes

• Chromosomes condense shorten and thicken - DNA supercoils

• Nuclear envelope breaks down

• Centrioles create spindle fibres

Metapahse

• Chromosomes line up on equator spindle fibres attach to centromere

• Checkpoint at this stage ensures every chromosome is attached to spindle correctly

Anaphase

• Spindle fibres contract and pull chromatids by centromeres to opposite poles

• Chromosomes are separated by centromeres, and sister chromatids move to opposite ends

• Needs energy - ATP

Telophase

• Chromosomes become longer and thinner - no longer visible

• Spindle fibres disintegrate, nuclear membrane forms

• The cell now contains two nuclei each

  genetically identical to each other and to the

  parent cell from which they arose.

Cytokinesis

• Cytoplasm splits - forms cleavage furrow forms in middle of cell

• Cytoskeleton causes cell membrane to draw inwards until the cell splits in two

• Plant cells - cell membrane splits due to fusing of golgi apparatus vesicles. Cell wall forms new sections around membrane

Observing mitosis

• Visible under light microscope in onion garlic and root tips

• A thin slice of root tip is placed on a microscope slide and broken down with a needle

• An acetic orcein stain is added (which stains chromosomes) - makes visible

• Cover slip is pushed down - to squash tip to ensure a single layer of cells so light can pass through

Mitotic index

• The number of cells in mitosis/ total number of cells x 100

Meiosis

• Two nuclear divisions - resulting in 4 genetically different haploid daughter cells

• Haploid n = one copy of chromosomes 23

• Diploid 2n = two copies of chromosomes 46

• Creates - Gametes - haploid as they fuse together - as a fertilised egg they have 46

• Reduction division - starting cells has 46 chromosomes - sperm and egg cells have only 23

How are genetic differences introduced

• Crossing over

• Independent assortment

• Any sperm can fertilise egg

Significance of meiosis

• Sexual reproduction increases genetic variation

• This increases its chances of survival with environment changes and selctional pressures

• Gives some individuals characteristics that enable them to be better adapted to the

  change.
  

Interphase

• DNA replicates - 46 chromosomes and 46 chromatids - 46 chromosomes and 92 chromatids

• Cell growth

Prophase 1

• Replicated chromosomes consist of two identical sister chromosomes

• Chromosomes condense shorten and thicken - DNA supercoils

• Nuclear envelope breaks down

• Centrioles create spindle fibres

• Homologous pairs (bivalent) line up - same size contain same genes in same location

• Non - sister chromatids cross over - forming chiasma and transfer genetic information - new combination of alleles to form recombinant chromosomes

Metaphse 1

• Homologous pairs line up on equator spindle fibres attach to centromere

• Checkpoint at this stage ensures every chromosome is attached to spindle correctly

• Independent assortment - homologous pairs line up in random order of maternal and paternal chromosomes

• 2²³ combinations

Anapahse 1

• Spindle fibres contract and pull Chromosomes by centromeres to opposite poles

• Needs energy - ATP

Telophase 1

• Cytoplasm splits - forms cleavage furrow forms in middle of cell

• Chromosomes become longer and thinner - no longer visible

• Spindle fibres disintegrate, nuclear membrane forms

• There is then a short

  interphase when the chromosomes uncoil.

Cytokinesis

• Cytoskeleton causes cell membrane to draw inwards until the cell splits in two set of

  chromosomes

• Each new nucleus contains half the original number of chromosomes, but each chromosome consist of two chromatids

Prophase II

• If the nuclear envelopes have reformed, then they now break down.

• The chromosomes coil and condense, each one consisting of two chromatids.

• The chromatids of each chromosome are no longer identical, due to crossing over in prophase 1.

Metaphase II

• Chromosomes line up in the middle

Anphase II

• The centromeres divide

• The chromatids of each chromosome are pulled apart by motor protein that drag them along the tubulin threads of the spindle, towards opposite poles.

Telophase II

• Nuclear envelopes form around each of the four haploid nuclei.

• cells now divide to give four haploid cells

Entrhocytes

• Erythrocytes carry oxygen from the lungs to respiring cell

• Derive from stem cells in the bone marrow

• Large SA:V ratio - very small - oxygen to diffuse across their membranes and easily reach all regions inside the cell.

• Biconcave shape to increase surface area

• Well developed cytoskeleton to increase cell flexibility to fit through narrow capillaries

• No nucleus to create more space for haemoglobin in oxygen transport

Neutrophils

• Neutrophils make up about 50% of the white blood cells in your body

• Have a lobed nucleus to squeeze through capillary walls

• Flexible to surround and engulf pathogens

• They are attracted to and travel towards infection sites by chemotaxis.

• Have lysosomes which contain lyysozymes

Spermatoza

• The many mitochondria carry out aerobic respiration. The ATP provides energy for locomotion of the tail to propel the cell towards the ovum.

  Sperm cells are small but long and thin, they can move easily.

  • Once the sperm reaches an ovum, digestive enzymes are released from the acrosome

  The enzymes digest the outer protective covering of the ovum, allowing the sperm head to enter the ovum.

• The head of the sperm contains the haploid male gamete nucleus and very little cytoplasm.

Palisade cells

• Adapted for photosynthesis

• • they contain many chloroplasts – the organelles that carry out photosynthesis

• They have a large vacuole so that the chloroplasts are positioned nearer to the periphery of the cell, reducing the diffusion distance for carbon dioxide

• Thin cell walls with little space between for CO2 to diffuse in

Root hair cells

• The hair-like projection greatly increases the surface area for absorption of water and mineral ions, such as nitrates, from

• The root hair cells have special carrier proteins in the plasma membranes in order to actively transport the mineral ions in.

• These cells will also produce ATP, as this is needed for active transport

Guard cells

• Flexible cell wall - thicker inner wall cells bend when turgid to open stomata and close when flaccid to control water loss by transpiration

• In spongy mesophyll with air spaces for oxygen to diffuse out

Squamous epithelial cells

• Single layer of flat cells for a short diffusion distance

Ciliated epithelial cells

• Hair like projections sway away mucus from lungs

• Goblet cells release mucus to trap dust and pathogens

Cartilage

• Firm and flexible provides structural support

• Prevents bones rubbing together - causes damage

Muscle

Tissues can contract and relax for movement

Xylem

• Elongated tissue, hollow dead cells -

• Wide lumen higher volume of water

• lignin strengthens and makes walls waterproof

• Madoe of meristem tissue

Phloem

• Made of sieve tube elements, perforated end walls and lacking most organelles to make transport easier

• Companion cells provide resources - sucrose

• Made of meristem tissue

Totipotent

• Cells can divide into any type of body cell. Occur only for limited time in embryos

Pluripotent

• Cells can divide into any type of cells apart from placenta

• Used in research - to cure disorders

• If doesn’t work can continually divide into tumour

• Ethical issues destroying embryo after

Multipotent

• Found in mature mammals

• Differentiate into limited cells e.g. blood cells

Unipotent

• Can only divide into one type of cell