cell division and cellular organization

the cell cycle and mitosis

the cell cycle is the process of cell growth and divsion

starts when the cell has been produced by cell division and ends with the cell dividing to produce 2 identical cells

the cell cycle consists of a period of cell growth and dna replication (interphase) and a period of cell division (m phase)

m phase involves mitosis and cytokinesis

interphase= G1, G2 and s phase

the cell cycle is regulated by checkpoints to ensure there is no errors

phases

G1= cell grows and new organelles and proteins are made, rna transcription, biosynthesis

G2=cell keeps growing and proteins needed for cell division are made

s phase= cell replicates its dna, rapid to reduce risk of mutations

m phase= mitosis and cytokinesis, cell growth stops

G2 checkpoint= cell check whether dna has been replicated with any errors or damage

G1 checkpoint= the cell checks that chemicals needed for replication are present and for any damage to the dna before s phase

mitosis

it is needed for: growth, repairing damaged tissue, asexual reproduction

interphase comes before mitosis in cell cycle

cell carries out normal functions

cell's dna is unravelled and replicated

organelles are replicated

ATP content is increased

1)prophase

chromosomes get shorter and thicker- more distinct

centrioles divide and start moving to opposite poles of cell

nuclear envelope breaks down

nucleolus gets smaller and eventually disappears

2) metaphase

3)anaphase

chromosomes line up along the equator of the cell

spindle fibres (produced by centrioles) attach to centromeres of sister chromatids

at metaphase checkpoint cell checks that all the chromosomes are attached to spindle fibres before mitosis can continue

centromeres divide, separating each pair of sister chromatids

spindle fibres contract/ shorten pulling chromatids towards poles of cell (centromere first), motor proteins walk along tublin threads

4) telophase

chromatids reach opposite poles

they begin to uncoil and become longer- less distinct

a nuclear envelope forms around each group of chromosomes

5)cytokinesis

cytoplasm divides

forms to genetically identical daughter cells to the original

in animal cells a cleavage furrow forms to divide the cell membrane

staining chromosomes is used to see all the different phases

sexual reproduction and meiosis

meiosis produces gametes for sexual reproduction

in sexual reproduction two gametes (egg and sperm) join together at fertilization to form a zygote. the zygote then divides and develops into a new organism

meiosis involves reduction divison. cells that divide by meiosis have the full number of chromosomes to start with but the cells that are formed fro meiosis have half the number.

cells formed by meiosis are all genetically different because each new cell ends up with a different combination of chromosomes

meiosis involves 2 divisions

prophase 1= chromosomes condense, get shorter and fatter. chromosomes arrange themselves in homologous pairs and crossing over occurs. centrioles start to move to poles and nuclear envelope break down

metaphase 1= homologous pairs line up across equator and spindle fibres attach to centromeres, arrangement is random (independent assortment)

anaphase 1= spindle fibres contract, motor proteins walk along tublin threads separating homologous pairs - one chromosome goes to each end of the cell, crossed over area separates resulting in allele shuffling, centromeres don't divide

telophase 1= a nuclear envelope forms around each group of chromosomes and cytokinesis occurs to produce 2 haploid daughter cells

meiosis 2 = mitosis

in anaphase 2= pairs of sister chromatids are separated each new daughter cell inherits one chromatid from each chromosome to produce 4 haploid daughter cells

chromatids cross over in prophase 1

homologous pairs of chromosomes pair up

the chromatids twist around each other and bits of chromatids swap over

chromatids still contain the same genes but now have a combination of different alleles

meiosis produces cells that are genetically different

crossing over

each of the four daughter cells formed from meiosis contain chromatids with different alleles

independent assortment

each homologous pair of chromosomes in your cell is made up of one chromosomes from each parent

when the homologous pairs line up in metaphase 1 and are separated in anaphase 1, is completely random which chromosome from each pair ends up in the daughter cell

so the four daughter cells have different combinations of paternal and maternal chromosomes

leads to genetic variation in offspring

stem cells and differentiation

stem cells are unspecialised cells

multicellular organisms are made up of many different cell types that are specialised for their function

all specialised cells originate from stem cells

stem cells are found in embryos which can develop into any type of cell, adult stem cells are limited

stem cells differentiate into specialised cells

stem cells divide to become new cells

stem cells become specialised by differentiation

growth and repair

cells in bone marrow

bones are living organs containing nerves and blood vessels

adult stem cells differentiate to replace worn out eyrthrocytes and neutrophils

cells in meristem

differentiate into xylem and phloem

stem cells can develop into different specialised cell types so scientists could use them to replace damaged tissues in a range of diseases

stem cells are also used by scientists researching developmental biology e.g. how organisms grow and develop

cells are specialised for their particular function

animal cells

neutrophils (type of white blood cell)- flexible shape allows them to engulf foreign pathogens or particles. lysosomes in their cytoplasm contain digestive enzymes to break down engulfed particles

erythrocytes (red blood cells)- biconcave disc shape provides a large surface area for gas exchange. no nucleus, more room for haemoglobin (protein that carries oxygen), flexible, very few organelles

epithelial cells- cilia beat to move particles away, squamous epithelia are very thin and allow gas diffusion

sperm cells- have a flagellum to swim to egg, lots of mitochondria to provide energy to swim, digestive enzymes to enable the sperm to penetrate the surface of the egg

plant cells

palisade mesophyll cells- contain lots of chloroplasts so they can absorb a lot of sunlight, thin walls so carbon dioxide easily diffuses into cell, large vacuole so chloroplasts are closer to edge of cell, cytoskeleton to move chloroplasts

root hair cells- large surface area to maximise absorption, thin walls for entry of water and ions, lots of mitochondria to provide energy for active transport, carrier proteins for ion active transport

guard cells- tiny pores for gas exchange, thin outer walls and thickened inner walls force stomata to open allowing gas exchange

tissues, organs and systems

tissue= a group of cells specialised to perform a specific function

animal tissues

squamous epithelium- a single layer of flat cells lining a surface, found in lungs ,cells joined form continuous sheets by tight junctions and desmosomes

ciliated epithelium- a layer of cells covered in cilia, found in trachea

muscle tissue- made up of bundles of elongated muscle fibres. 3 types: smooth, cardiac, skeletal

cartilage- a type of connective tissue found in the joints. formed when chondroblasts secrete an extracellular matrix which they become trapped inside

plant tissues

xylem- contains hollow xylem vessels which are dead and strengthened with lignin

phloem- made up of sieve and companion cells , hols in sieve plates so sap can move through

G0 phase= cells may under go apoptosis (cell death)or differentiation

in plant cells cell wall is laid

occurs in anaphase 1 and 2