OCR BIOLOGY - cell division

what is the cell cycle

is an ordered set of events resulting in cell growth and division into 2 genetically identical daughter cells

sequence of evetns that occurs between one cell division to the next

stages in the cell cycle

• G1 - growth phase, proteins are made

• checkpoint cell - checks that the chemicals needed for replication are present and for any damages to the DNA before enter S phase

• S - synthesis, cell replicates its DNA, ready to divide by mitosis

• G2 - growth stage 2, cell keeps growing and proteins needed for cell division are made —— all

• checkpoint cell - checks if all DNA has been replicated without any damage. cell can then enter mitosis

• INTERPHASE (growth stage)

• M - mitosis and cytokinesis

what is the significance of mitosis in life cycles

growth, tissue repair and asexual reproduction in plants, animals and fungi.

how is the cell cycle is regulated

use of checkpoints to control the cycle

• G1 checkpoint cell - checks that the chemicals needed for replication are present and for any damages to the DNA before enter S phase

• G2 checkpoint cell - checks if all DNA has been replicated without any damage. cell can then enter mitosis

the main stages in mitosis

1. Interphase

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase

mitosis: 1- Interphase

• cell carries out normal functions but prepares to divide

• DNA is unravelled and replicated which doubles its genetic content

• organelles are also replicated

• ATP content is increased - provides energy needed for cells.

mitosis: 2- Prophase

• chromosomes condenses so goes shorter and fatter

• 2 sister chromatid joined togather by centromere

• centrioles (tiny bundles of proteins) starts moving to opposite poles of the cell forming spindle fibres (network of protein fibres)

• the nuclear envelope breaks down and the chromosomes lies free

mitosis: 3- metaphase

the chromosomes becomes attached to the spindle by their centromere

the chromosomes are all lined up in the middle of the cell

at the M checkpoint the cell checks all chromosomes are attached to the spindle fibres before mitosis

mitosis: 4- Anaphase

• the centromere divides seperating sister chromatids

• spindles contract pulling chromatids to the opposite end

mitosis: 4- Telophase

chromatids reach the opposite poles

they uncoil and becoming long and thin

now chromosomes again

nuclear envelope forms around each chromosomes forming 2 nuclei

mitosis: 5- Cytokinesis

cytoplasm divides

cleavage furrow forms to divide the cell membrane

2 daughter cells genetically identical to each other made

cytokinesis begins at anaphase and finishes at telophase

seperate process from mitosis

mitosis in plant cell

mitosis steps same until seperation

plant cells have cell walls so wont form cleavage furrow

vesicles from the golgi apparatus begins to assemble in the same place where metaphase plate was

vesicles fuse with each other dividing cell surface membrane into 2

define meiosis

form of cell division where the nucleus divides twice resulting in halving of chromosomes and producing 4 haploid cells from one diploid.

the significance of meiosis

produces haploid cells and genetic variation by independent assortment and crossing over.

used in gametes and is a reduction division

stages of meiosis

1. interphase

Meiosis I

1. prophase 1

2. metaphase 1

3. anaphase 1

4. telophase 1

Meiosis II

1. prophase 2

2. metaphase 2

3. anaphase 2

4. telophase 2

meiosis I: prophase I

1. Prophase I -

   • chromosomes condenses

   • nuclear envelope disintegrates homologous chromosomes pairs up forming a bivalent

   • these chromosomes when moved through liquid cytoplasm the chromatids become entangled aka CROSSING OVER

   • due to this crossing over = chromatids have the same genes but different combinations of allele

   • spindle fibres form

meiosis I: Metaphase I

2. Metaphase I -

   • the homologous pairs of chromosomes assemble along the metaphase plate by spindle fibres from centromere instead of single chromosomes

• orientation of chromosomes on metaphase plate is random

• the maternal and paternal chromosomes can end up facing either pole aka INDEPENDANT ASSORTMENT results in different combinations of alleles facing towards the poles, this leads to genetic variation

meiosis I: Anaphase I

3. Anaphase I -

   • homologous chromosomes are pulled to opposite poles and chromatids stay joined together

   • sections of DNA on sister chromatid becomes entangled during crossing over now breaks off and rejoins resulting in exchange of DNA. point where chromatids breaks and rejoins is called CHIASMATA

   • when genes get exchanged between chromatids - RECOMBINANT. genes being exchanged may be different alleles of the same gene meaning the the combo of alleles being exchanged will be recombinant chromatids will be a different allele combo from original chromatid.

   • causes genetic variation from new combo of alleles so sister chromatids are no longer identical

meiosis I: Telophase I

4. Telophase I -

   • chromosomes assemble at each pole and nuclear membranes reforms

   • chromosomes uncoil

   • cell undergoes cytokinesis and divides into 2

   • the reduction of chromosomes number from diploid to haploid is complete

Meiosis II: Prophase II

5. Prophase II -

   • chromosomes consists of 2 chromatids, condenses and becomes visible again

   • the nuclear envelopes breaks down

   • spindle formation begins

Meiosis II: Metaphase II

6. Metaphase II -

   • individual chromosomes assemble on the metaphase plate

   • chromatids are no longer identical due to crossing over - there is independent assortment and more genetic variation

Meiosis II: Anaphase II

7. Anaphase II -

   • chromatids of individual chromosomes are pulled to opposite poles after division of centromeres

Meiosis II: Telophase II

8. Telophase II -

   • chromatids assemble at the poles

   • chromosomes uncoil and form chromatins again

   • nuclear envelopes reform

   • nucleolus becomes visible

   • cytokinesis results in the division of 4 daughter cells making cells haploid as its reduction reaction

   • will be genetically different from each other and from parent cell due to crossing over and independent resortment.

specialised cell: erythrocytes

• aka RBC

• flattened biconcave shape = increase SA:volume

• essential in transporting oxygen

• dont have nuclei = increases space available for haemoglobin

• flexible = squeezes through narrow capillaries

specialised cells: neutrophils

• aka WBC

• essential role in immune system

• multi lobed nucleus = easier to squeeze through small gaps to get to sites of infection

• granular cytoplasm = many lysosomes which contains enzymes used to attack pathogens

specialised tissue: squamous epithelialium

• made of squamous epithelial cells

• aka pavement epithelium = flat appearance

• thin = squats/ flat cells make up it so its one cell thick

• present when rapid diffusion across a surface is essential

• forms lining of the lungs = rapid diffusion of oxygen into the blood

specialised tissue: ciliated epithelial cells

• made up of ciliated epithelial cell

• hair like structure of structure is called cilia = beats in rhythmic manner

• lines up the trachea = mucus to be swept away from the lungs

• goblet cells also present = releasing mucus to trap unwanted particles present in the air = prevents bacteria reaching alveoli once in lungs

specialised cell: sperm cells

• male gametes

• delivers genetic information to female gamete (ovum)

• contains a flagella = capable of movement and has many mitochondria present so supplies energy needed to swim

• acrosome = digestive enzymes released to digest protective layer around ovum and lets sperm penetrate through it - leads to fertilisation

specialised plant cell: palisade cells

• present in mesophyll = contains chloroplasts to absorb large amount of light for photosynthesis

• cells = rectangular box shapes, closely packed for continuous layer

• thin cell walls = increases rate of diffusion of CO₂

• large vacuole = maintains turgor pressure

• chloroplasts cna move within cytoplasm = to absorb more sunlight

specialised plant cell: root hair cells

• present at surface of roots near growing tips

• long extensions called root hairs

• these increases SA of the cell

• maximises the uptake of water and minerals from the soil

specialised plant cell: guard cells

• forms small opening of leaves surface called stomata

• necessary for CO2 to enter plant for photosynthesis

• guard cell = loses water = less swollen due to osmotic forces = changes shape

• stoma closes = prevents further water loss from the plant

• cell wall of the guard = one side thicker so the cell dont change shape symmetrically as it changes volume

cartilage tissue

• connective tissue

• found on the outer ear, nose, and the end of bones

• contains fibres of proteins and elastin and collagen

• firm and flexible connective tissue

• made up of chondrocyte cells embedded in extracellular matrix

• prevents end of bones rubbing against each other

• fishes have bones made up of cartilidge

muscle tissue

• shortens in length in order for bones to move

• different types of muscle fibres:

• skeletal muscle fibres - attached to the bone

• myofibrils - contains contractile proteins

xylem tissue

• vascular tissue

• responsible for transport of water and minerals throughout plants

• composed of vessel elements - elongated dead cells

• walls strengthened by lignin = waterproof - provides structural support for plants.

phloem tissue