Cell structure and division

cell

the smallest, basic membrane bound unit of life, responsible for all of life’s processes

cell theory- in order to continue living a cell requires

1. metabolic machinery capable of obtaining energy from the environment

2. the ability to use this energy to support essential life processes

3. a set of genes to control the synthesis of all the cell components and these are passed onto the next generation through cell division

4. a physical boundary between itself and the environment (cell membrane)

stem cell

a cell that can produce other cells which can develop into any kind of cell in the body

differences between prokaryotes and eukaryotes

3 shapes of bacteria

• coccus (round)

• bacillus (rods)

• spirilla (coils)

3 parts of bacteria

• external appendages and coverings

• cell envelope (cell membrane and cell wall)

• internal organelles

what is a cell wall made up of

peptidoglycan

what is peptidoglycan made up of

2 types of glycan chains (NAG and NAM)

what are the 2 types of glycan chain

N-acetyl glucosamine (NAG)

N-acteyl muramic acid (NAM)

Gram positive cell wall

• thick (20-80nm)

• porous

• contains teichoic acid

• stains purple

Gram negative cell wall

• thin (1-3nm)

• requires an extra layer of protection- outer membrane (porins)

• stains pink

4 ways antibiotics can affect bacterial cells

1. Disrupts cell wall synthese

2. Inhibits DNA replication- e.g. quinolones (ciprofloxacins)

3. Inhibits RNA synthesis

4. Inhibit protein synthesis- e.g. macrolides (erythomycin)

disrupts cell wall synthesis

binds to transpeptidases and inhibits the cross-linking of peptidoglycan subunits

inhibits RNA synthesis

binds to RNA polymerase e.g. rifampicin

porins

block the entrance of harmful chemicals and antibiotics making gram-negative bacteria more difficult to treat than gram-positive cells

what is attached to the outer membrane

lipopolysaccharide

lipopolysaccharide

acts as an endotoxin because it induces fever and shock in the human host

pilus

hair like appendage required for bacterial conjugation (transfer of genetic material)

capsule

polysaccharide layer, contains water to prevent the cell from dying out, protects the cell from phagocytosis helps adhere to surfaces

cell wall

rigid structure responsible for the characteristic shape and is made of cellulose

plasma membrane

consists of protein and phospholipid- transport, biosynthesis and energy transduction

plasmid

circular DNA, bacterial virulence

flagellum

enables movement and chemotaxis (process where cells move directionally in response to specific chemicals in the environment)

ribosomes

translate the genetic code from DNA to amino acids to produce proteins

nucleoid (DNA)

regulates growth, reproduction and function of the cell

cytoplasm

fluid that fills the whole cell, house and maintain an optimal environment for the organelles. It’s where chemical reactions take place

eukaryotic cells

basic units of animals and plants

mitochondria

turns glucose and oxygen into energy

vacuole

contains cell sap, which is a weak solution of sugar and salts

green chloroplasts

contains chlorophyll for photosynthesis

what does a cell membrane consist of

a double layer of phospholipid each about 2nm thick separated by a space approximately 3.5nm

what is embedded in the fluid mosaic in the cell membrane

proteins and other lipids

what is the cell surrounded by

a plasma membrane that controls the passage of materials into and out of the cell as well as receiving messages that regulate the behaviour of the cell

nucleus

• surrounded by a double membrane where pores of 40-100nm can be seen

• inside nucleic acids and proteins are found

• more densely packed chromatin granules appear as nucleoli (ribosomal RNA is made there)

cell organelles- mitochondria

• small elongated bodies

• consists of a double membrane, the outer membrane and much folded inner membrane

• contains enzymes and some DNA

• produces most of the cell’s adenosine triphosphate (ATP)

cristae

the folds in the inner membrane of the mitochondria

chloroplasts

• have a double membrane surrounding a storm where stacks of lamellae which contain chlorophyll

• performs photosynthesis

cell wall

a non-living layer found in plant cells and is derived from cellulose (a glucose polymer)

ribosome

carries out protein synthesis and is found in the rough endoplasmic reticulum

endoplasmic reticulum

can be rough- engages with protein synthesis

or smooth- involved in lipid metabolism

surface area is very large

lysosome

vesicles containing enzymes that break down proteins, lipids and nucleic acids

centriole

2 centrioles = a centrosome- attaches the cell’s cytoskeleton of microtubules

cytoskeleton

framework of tubular proteins gives cell its shape provides basis for movement

nucleus

DNA is held in the nucleus in chromatin fibres

nucleolus assembles ribosomes and communicates with cytoplasm through pores in the nuclear envelope

Golgi apparatus

flattened sacs that receive vesicles from the endoplasmic reticulum, modify, sort and package their contents and secrete it to the other organelles

plasma membrane

a bilayer of phospholipid molecules acts as a semi-permeable barrier in which proteins are embedded and acts as receptors, pumps and channels

viruses

they are tiny particles only visible in an electron microscope

2/3 parts of viruses

1. the genetic instructions in the form of DNA or RNA

2. A protein coat (capsid) that protects the DNA or RNA

3. in some cases an envelope of lipids that surround the protein coat when the virus is outside cells

capsid

a protein coat that surrounds and protects the DNA or RNA

how do viruses work

they replicate by taking over the machinery of a host cell and directing it to synthesis new virus particles

the newly synthesised virus particles are released from the cell and sometimes destroys the cell in the process

viruses bind to host cells in order to gain entry to the cell and incorporate their genome into the host cell DNA to achieve replication of the virus `

what do many virus cells target

glycans for cell attachment and entry

cell surfaces are especially enriched with complex carbohydrates, polysaccharide, glycoproteins and glycans

influenza

1. influenza virus binds to cell-surface slalic acid receptors through its surface glycoprotein hemagglutinin

2. cell takes up the virus by endocytosis

3. this vesicle fuses with a lyosome

4. virus takes up the lysosome and uses the acidity to break down the coat. Hemagglutinin undergoes structural changes and inserts itself into the cell membrane

5. membrane fusion and releases RNA into the host cell

6. intracellular replication

7. a cell surface neuraminidase cleaves sialic acid from the cell membrane allowing viral escape

viral treatment- drugs

• antibiotics have no effect

• antiviral drugs- inhibit or disable viral proteins that are a part of the cell entry mechanism or the viral replication and release process

viral treatment

• viral infections provokes an immune response that usually eliminates the virus

• the immune response can be induced artificially with vaccines

vaccines

contains an agent that could be dead virus particles or one of the viral surface proteins

how do vaccines work

the agent stimulates the body’s immune system to recognise the agent as a threat, destroy it and to further recognise and destroy any similar viruses it may encounter in the future

mitosis

replication of cells where division results in 2 identical daughter cells

meiosis

division that results in 4 gametes (Sex cells) with half the chromosome number

chromosomes

threadlike structure of nucleic acids and protein found in the nucleus

chromatid

2 threadlike strands that make up the chromosome

centromere

region of a chromosome where the microtubules of the chromosome join

telomere

cap at the end of each chromosome arm which maintain stability

cell cycle

series of events that occur when a cell divides and grows

cell division structure

chromosome structure

stages of cell cycle

1. interphase

2. prophase

3. metaphase

4. anaphase

5. telophase

stages of mitosis

2. prophase

3. metaphase

4. anaphase

5. telophase

stages of interphase

1. G1 stage

2. S stage

3. G2 stage

G1 stage

the synthesis of RNA and protein occurs

• preparing to replicate DNA by synthesising the mRNAs and proteins required to execute the future steps

• cell usually grows larger and some organelles are copied

S stage

replication of DNA us complete and duplication of histone proteins occurs

• the process of replication generates sister chromatids, which are identical pairs of chromosomes

• these sister chromatids are attached to each other by a centromere

• a centromere is a specialised sequence of DNA that links the sister chromatids and is important throughout mitosis

G2 stage

synthesis of organelles like mitochondria occurs

• the cell undergoes additional growth, replenishes energy stores and prepares and reorganises the cytoplasmic components for division including duplicating some organelles ad dismantling the cytoskeleton

• G2 ends when mitosis begins

how are chromosomes formed

• DNA is packaged by special proteins called histones to form chromatin

• chromatin further condenses to form chromosomes

• chromatins is a lower order of DNA organisation whereas chromosomes are higher order of DNA organisation

prophase

• chromosomes condense and become visible inside the nuclear membrane as long thin entangled threads

• threads become shorter and thicker and the chromosomes care seen as separate structures

• the nucleous gradually disappears

• the centrioles migrate to opposite ends of the cell

• the spindle of microtubules begins to form

• proteins catalyse the condensation process (the chromosomes become compact) including cohesion and condensin

• cohesion forms rings that hold the sister chromatids together

• condensin forms rings that coil the chromosomes into highly compact forms

pro-metaphase

• phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles

• the spindle microtubules now have direct access to the genetic material of the cell

• each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome

• the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere

• the actual number of microtubules that attach to the kinetochore varies between species

at the end of pro-metaphase…

• kinetochore microtubules attach the chromosomes to the spindle pole

• interpolar microtubules extend from the spindle pole across the equator

• astral microtubules extend from the spindle pole to the cell membrane

metaphase

• nuclear membrane disappears and each chromosome is seen as 2 chromatids joined at the centromere

• the chromosomes become attached by their centromeres to the equator of the spindle

anaphase

• the centromere splits

• enzymatic breakdown of cohesion- which linked the sister chromatids together during prophase- causes this separation to occur

• the spindle microtubules to which centromeres are attached shorten and drag chromatids away from the equator to opposite ends of the cell

anaphase A

the kinetochore microtubules shorten and draw chromosomes toward the spindle poles

anaphase B

the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes

telophase

• the chromatids assemble at the poles and a nuclear membrane forms around each group

• inside the nuclear envelope chromatids become uncoiled, nucleoli reform and the nucleus takes on a granular appearance

• cell division is completed by cytoplasmic cleavage (cytokinesis)

cytokinesis

the cell membrane between the 2 nuclei separated to form 2 daughter cells each of which is a copy of the original cell

why do people get Down syndrome

they have an extra chromosome

prophase I

the chromosomes condense and the nuclear envelope breaks down- crossing over occurs

metaphase I

pairs of homologous chromosomes move to the equator of the cell

anaphase I

homologous chromosomes move to the opposite poles of the cell

telophase I & cytokinesis

chromosomes gather at the poles of the cells

the cytoplasm divides

synapsis

pairing of homologous chromosomes

how does crossing over work

spindle fibres will pull the homologous chromosomes together creating tetrads leading to crossing over

this creates genetic diversity

prophase II

a new spindle forms around the chromosomes

metaphase II

chromosomes line up at the equator

anaphase II

centromere divides and chromatids move to the opposite poles of the cells

telophase II and cytokinesis

a nuclear envelope forms around each set of chromosomes and the cytoplasm divides

what happens when fertilisation occurs

the 2 haploid cells fuse to form one cell zygote

because of recombination and reassortment the gametes have a varied combination of genes