Cell Structure

How are cells organised?

• Groups of cells are organised into tissues

• Tissues are organised into organs

• Organs may be organised into organ systems

Cell definition

• Smallest, basic membrane bound unit of life

• Responsible for all of life’s processes

4 aspects of cell theory

1. All living organisms are composed of one or more units called cells

2. Each cell is capable of maintaining its vitality independently

3. Cells can only arise from other cells

4. Viruses are not cells

What things does a cell require in order to continue living?

1. Metabolic machinery capable of obtaining energy from the environment e.g. light energy, degradation of foodstuffs

2. The ability to use this energy to support essential life processes e.g. movement of materials from one part of the cell to another, transport of molecules in and out of the cell

3. A set of genes to control the synthesis of all the cell components » these are passed onto the next gen through cell division

4. A physical boundary between itself and the environment » the cell membrane

Stem cell definition

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

Types of cells

1. Prokaryotic » bacteria

2. Eukaryotic » animal, plant, fungi

How many cells are present in a human body?

From 1 to 10¹²

What is every living cell surrounded by?

• A plasma membrane

• Inside is a viscous fluid, the cytoplasm

Differences between prokaryotes and eukaryotes

Bacteria (prokaryotes)

3 major shapes of bacteria cells

1. Coccus (round)

2. Bacillus (rods)

3. Spirilla (coils)

Bacteria (prokaryotes)

Label the structural features of a bacteria cell

Bacteria (prokaryotes)

State the function of each feature of a bacterial cell:

1. Pilus

2. Capsule

3. Cell wall

4. Plasma membrane

5. Plasmid

6. Flagellum

7. Ribosomes

8. Nucleoid (DNA)

9. Cytoplasm

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

Capsule: polysaccharide layer, contains water to prevent cell from drying out, protects cell from phagocytosis, helps cell adhere to surfaces

Cell wall: rigid structure responsible for shape of cell

Plasma membrane: consists of proteins and phospholipids, involved in transport, biosynthesis and energy transduction

Plasmid: circular DNA

Flagellum: enables movement and chemotaxis (movement away from harm and towards beneficial environments e.g. nutrients)

Ribosomes: translate genetic code from DNA to amino acids to proteins

Nucleoid (DNA): regulates growth, reproduction and function of the cell

Cytoplasm: fluid that fills the whole cell, maintains optimal environment for organelles

Bacteria (prokaryotes)

What 3 parts can the structure of a bacterial cell be divided into?

1. External » appendages and coverings

2. Cell envelope » cell wall and membranes

3. Internal organs

Bacteria (prokaryotes)

What are the appendages on bacteria for?

• Movement (flagella)

• Adhesion to surfaces (pilli, fimbriae, glycocalyx)

Bacteria (prokaryotes)

What does the cell envelope contain

• Cell membrane

• Cell wall (of varying thickness)

Bacteria (prokaryotes)

What does the cell wall contain?

• Peptidoglycan

  » made from long glycan chains cross-linked by peptides

Bacteria (prokaryotes)

What are the 2 types of glycan chains that make up peptidoglycan?

1. N-acetyl glucosamine (NAG)

2. N-acetyl muramic acid (NAM)

Bacteria (prokaryotes)

Why are cell walls strong and rigid?

• Cell wall is made up of peptidoglycan

• Cross-linking between amino acids in different glycan chains occurs with the help of DD-transpeptidase

• This results in a 3 dimensional structure that is strong and rigid

Bacteria (prokaryotes)

What is an example of an antibiotic that interferes with bacterial cell wall synthesis?

• Penicillin

• Acts by binding to transpeptidases and inhibiting the cross-linking of peptidoglycan subunits

• A bacterial cell with a damaged cell wall cannot undergo cell division and so will die

Bacteria (prokaryotes)

How are bacteria classified?

Bacteria are classified as Gram-positive or Gram-negative based on differences in their cell wall

» done through differences in the staining of their cell walls with crystal violet dye (Gram stain)

Bacteria (prokaryotes)

Gram-positive cell wall

• Thick (20-80nm) layer of peptidoglycan

• This layer is porous and contains teichoic acid

  » teichoic acid maintains the cell wall and gives the cell surface an acidic (-) charge

• The cell wall stains purple with Gram stain

Bacteria (prokaryotes)

Gram-negative cell wall

• Thin (1-3nm) layer of peptidoglycan

• Because cell is thin, gram-negative cells require an extra layer of protection called the outer membrane

  » the outer membrane is a phospholipid membrane with tiny holes called porins

• The cell wall stains pink with Gram stain

Bacteria (prokaryotes)

Why are Gram-negative bacteria more difficult to treat than Gram-positive bacteria?

Porins block the entrance of harmful chemicals and antibiotics

Bacteria (prokaryotes)

Why can Gram-negative bacteria induce fever and shock?

• Attached to the outer membrane is a highly-branched fatty sugar called lipopolysaccharide (LPS)

• LPS acts as an endotoxin causing shock and fever

Bacteria (prokaryotes)

Why do Gram-positive cell walls stain purple but Gram-negative cell walls stain pink?

Gram-positive cell walls have a thicker layer of peptidoglycan so can retain more dye

Bacteria (prokaryotes)

How can antibiotics affect bacterial cells?

• Disrupt cell wall synthesis e.g. Penicillin

  » bind to transpeptidases inhibiting the cross-linking of peptidoglycan subunits

  » damaged cell walls mean cells cannot undergo cell division causing cell death

• Inhibits DNA replication e.g. Quinolones such as Ciprofloxacin

• Inhibits RNA synthesis by binding to RNA polymerase e.g. Rifampicin

• Inhibits protein synthesis e.g. Macrolides such as Erythromycin

Eukaryotic cells

Label the structural features of eukaryotic cells

Eukaryotic cells

Ribosome

• Contains RNA

• Site of protein synthesis

• Attached to the RER

Eukaryotic cells

Endoplasmic reticulum

• 3 dimensional network of membranes which extends throughout the cell

• Large surface area

• RER is covered with ribosomes and is involved in protein synthesis, proteins are folded inside the RER

• SER is in involved in lipid metabolism

Eukaryotic cells

Lysosome

• Vesicles containing hydrolytic enzymes

• These enzymes break down proteins, lipids and nucleic acids

Eukaryotic cells

Peroxisome

• Vesicles that produce hydrogen peroxide

• They then destroy any excess hydrogen peroxide produced

Eukaryotic cells

Centriole

• 2 centrioles make up a centrosome

• The cell’s cytoskeleton of microtubules is attached to the centrosome

Eukaryotic cells

Cytoskeleton

• Makes up the framework of tubular proteins

• Gives an animal cell its shape and provides basis for movement

Eukaryotic cells

Nucleus

• Largest organelle in an animal cell

• Contains genetic material in the form of DNA

• DNA is held in chromatin fibres

• During cell division the chromatin granules form chromosomes

• Assembles ribosomes

• Pore in the double layered nuclear envelope allow things to move in and out of the cytoplasm

Eukaryotic cells

Mitochondria

• Has a double membrane

• Inner membrane is folded forming cristae with a matrix on the inside

• Matrix contains enzymes needed for respiration

• Site of aerobic respiration » produces ATP

Eukaryotic cells

Golgi apparatus

• Flattened sacs that receive vesicles from the Endoplasmic reticulum

• Modify, sort and package the contents of the vesicles and secrete it to other organelles

• In the Golgi carbohydrates are added to proteins coming from the ER to form glycoproteins

Eukaryotic cells

Plasma membrane

• Double layer of phospholipids (bilayer)

• Acts as a semi-permeable barrier

• Embedded in the fluid mosaic are proteins, lipids and cholesterol

• Acts as pumps and channels

Eukaryotic cells

Compare plant and animal cells

Similarities:

• Ribosomes

• Endoplasmic reticulum

• Lysosome

• Centriole

• Cytoskeleton

• Mitochondria

• Golgi apparatus

Differences:

• Plant cells have a cell wall, animal cells do not

• Plant cells have a large fluid filled vacuole, animal cells do not

• Plant cells contain chloroplasts, animal cells do not

Eukaryotic cells

Vacuole

• Temporary food store containing sugars and amino acids

• When vacuole is full, cell is turgid

• Membrane surrounding vacuole is called the tonoplast

Eukaryotic cells

Chloroplast

• Absorb light for photosynthesis to produce carbohydrates

• Surrounded by double membrane

• Thylakoid membranes inside which can form a stack called a granum

• Grana linked by lamellae

What is the cell wall made of in plants, algae, fungi and prokaryotes?

Plants: cellulose

Algae: cellulose

Fungi: chitin

Prokaryotes: murein e.g. bacteria

Compare eukaryotes and prokaryotes

Similarities

• Cell membrane

• Cytoplasm

• Ribosomes

• DNA

Differences

• P: no membrane bound organelles E: membrane bound organelles

• P: smaller ribosomes E: bigger ribosomes

• P: have a capsule E: no capsule

• P: smaller E: bigger

• P: circular DNA E: linear DNA

• P: DNA not bound to histones E: DNA bound to histones

• P: plasmids E: no plasmids

• P: cell division by binary fission E: cell division by mitosis

What are viruses?

• Tiny particles, not cells

• Only visible in an electron microscope

What do viral particles consist of?

1. Genetic material in the form of RNA or DNA

2. A protein coat (capsid) that surrounds RNA OR DNA

3. In some cases a lipid envelope that surrounded the protein coat when the virus is inside cells

What was the first virus to be discovered?

• Tobacco mosaic virus

  » Positive-sense single stranded RNA virus

  » Infects a wide range of plants especially tobacco

Why can viruses only replicate by infecting a host?

• Viruses do not contain their own biochemical machinery

• So they bind to host cells, enter the cell and incorporate their genomes into the host cell’s DNA to replicate the virus particles

What do viruses target on cell surfaces for cell attachment and entry?

Glycolipid molecules (glycans) on the cell surface

Summarise the process of viral replication

1. Virus attaches to the host cell by targeting glycans on the cell surface

2. Virus injects its genetic material into host cell

3. Host cell transcribes and translates viral genes

4. These proteins form new virus particles

5. Virus particles burst out of host cell, so host cell is destroyed

Adenoviruses

• Cause a wide range of illnesses e.g. fever, sore throat etc

• Contagious » transmitted via droplets

Influenza virus

Structure of the virus particles

• Spherical shape

• 2 antigens on the surface (glycoproteins) » HA and NA

• These proteins determine the subtype of the influenza virus

Influenza virus

Why is there a new flu vaccine each year

• Due to antigenic drift

  » Mutations in the HA and NA antigens

Influenza virus

How does the influenza virus infect a host cell and replicate?

1. The virus’ hemagglutinin (HA) glycoprotein binds to sialic acid receptors on the cell’s surface.

2. The cell engulfs the virus by endocytosis

3. This vesicle then fuses with a lysosome » contains digestive enzymes and is acidic

4. The acidity inside the lysosome breaks down the virus’ coat.

5. The hemagglutinin protein changes shape and inserts itself into the membrane of the vesicle

6. The viral membrane fuses with the vesicle membrane, allowing the virus to release its RNA into the cell.

7. Host cell transcribes and translates viral RNA. These proteins form new virus particles

8. A enzyme neuraminidase helps cuts the sialic acid from the cell membrane so the new virus particles can escape

How does HIV infect a host cell and replicate?

1. The HIV's attachment proteins binds to the CD4 receptor proteins on the surface of the t helper cell

2. The virus's lipid envelope fuses with the cell membrane of the Th cell.

3. The protein capsid breaks down

4. RNA and enzymes (e.g. reverse transcriptase) of the virus are now released into the cytoplasm of the host cell.

5. Reverse transcriptase converts the viral RNA to DNA.

6. The viral DNA is incorporated into the cell's DNA.

7. The viral DNA can now be transcribed into mRNA

8. Viral mRNA passes through the nuclear pore and attaches to a ribosome

9. Viral mRNA is translated into viral proteins that can be assembled into new HIV particles.

10. HIV particles bud off the Th cell (so that the Th cell's membrane forms the lipid envelope of the virus)

How does Herpes Simplex Virus (HSV) infect a host cell and replicate?

1. HSV uses the glycoproteins on its surface (gB and gC) to bind to the heparan sulfate proteoglycans on the host cell

2. The virus fuses with the cell membrane and releases its capsid into the cytoplasm

3. Capsid travels to nucleus and injects its DNA into nucleus

4. Transcription and translation occur producing new virus particles

5. Can cause lytic infection » virus bursts out of host cell, destroying it

6. Can cause latent infection » viral DNA can remain dormant in the cell until activated

Treating viruses

1. Vaccines » stimulates immune system, producing antibodies and memory cells

2. Antiviral drugs » inhibit viral replication

What to antiviral drugs target?

• Viral glycoproteins used to enter cells

• Reverse transcriptase used to incorporate viral DNA into host DNA

• Proteases used to assemble new virus particles

Antiviral drug targets should be…

Unlike any other proteins in humans as possible to reduce likelihood of side effects

Mitosis

• Cell division which allows organisms to grow and replace cells

• Produces 2 identical daughter cells

Meiosis

Division that results in 4 gametes with half the chromosome number

Chromosome

• Threadlike structure of nucleic acids and proteins found in the nucleus

• Consists of 2 sister chromatids joined by a centromere

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 maintains stability

Cell cycle

Series of events that occur when a cell divides and grows:

• Interphase

• Mitosis: prophase, metaphase, anaphase, telophase

• Cytokinesis

Label the structure of a chromosome

Explain the levels of DNA organisation

• DNA is associated with histone proteins forming chromatin

• Chromatin condenses to form chromosomes

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

Cell Cycle

Interphase

G1:

• Cell grows

• Protein synthesis

• Synthesis of RNA

• Replication of organelles

S:

• DNA decondenses (not visible)

• DNA is replicated (results in 2 sister chromatids attached by a centromere)

G2:

• Synthesis of organelles such as mitochondria

• Cell grows more and proteins for cell division made

• ATP increases

Cell Cycle

Mitosis: Prophase

• DNA condenses and becomes shorter and thicker » more visible

• Nuclear envelope breaks down

• Centrioles migrate to opposite ends of the cell

• Centrioles produce spindle fibres, made from microtubules

Cell Cycle

Mitosis: Pro-metaphase

• Phosphorylation of nuclear lamins (proteins that support the nuclear envelope) by M-CDK causes the nuclear envelope to break down completely

• Spindle fibres attach to each chromosome at its kinetochore (proteins located at the centromere)

Cell Cycle

Mitosis: Metaphase

The spindle fibres move the chromosomes so that they line up along the equator of the cell

Cell Cycle

Mitosis: Anaphase

• Cohesin which joins the sister chromatids is broken down by enzymes, causing the centromeres to split

• Spindle fibres contract and shorten

• This separates the sister chromatids, pulling them to opposite poles of the cell

Cell Cycle

Mitosis: Telophase

• 2 new nuclear envelopes form around the 2 new sets of chromosomes

• Chromosomes decondense

• Nucleoli reform

• Nucleus takes on a granular appearance

Cell Cycle

Cytokinesis

• The cell membrane between the 2 nuclei separates

• Cytoplasm separates

• Forms 2 identical daughter cells

How many chromosomes do human cells contain

• 46 chromosomes

• 23 homologous pairs » one of the pair from mother, the other from father

What are homologous pairs of chromosomes?

Chromosomes that have the same genes at the same loci

What is different about a female’s chromosomes?

Females have 2 X chromosomes as the 23rd homologous pair

Why is it so important that gametes are haploid?

To maintain chromosome number at fertilisationd

Key parts of meiosis

2 divisions:

1. Separates homologous pairs

2. Separates sister chromatids

Describe the process of meiosis I

1. Interphase: DNA unravels + replicates so there is 2 copies of each chromosome (2 x 2n)

2. Prophase I:

   • Chromosomes condense and become visible >> each chromosome has a pair of sister chromatids joined by centromere

   • Homologous chromosomes pair up forming bivalents (2 x 2n) » this is called synapsis

   • Crossing over occurs

3. Metaphase I:

   • Spindle fibres join to the centromeres causing homologous pairs of chromosomes to line up next to each other along equator of cell (2 x 2n)

   • Independent segregation occurs

4. Anaphase I: spindle fibres contract pulling one of each pair of homologous chromosomes to opposite poles (2 x 2n)

5. Telophase I:

   • 2 new nuclear envelopes form

   • Chromosomes uncoil, nucleoli reform, nuclei take on granular appearance

6. Cytokinesis: cytoplasm divides to form 2 daughter cells (2n)

What is synapsis?

When homologous chromosomes pair up to form bivalents

What are the 2 factors which cause gametes to vary (genetic variation)

• Independent segregation

• Crossing over

How does crossing over occur during Prophase I?

• Homologous pairs of chromosomes pair up forming bivalents

• Chromatids twist around each other forming crosslinks » called chiasmata

• This leads to genetic material being exchanged

• So chromatids now contain a different combination of alleles

• Each of the 4 daughter cells will contain chromatids with a different combination of alleles

» results in shuffling of genes by RECOMBINATION

(E.g. if initially one chromatid in the pair was Ee and the other was Ff, due to crossing over one will now be Ef and the other eF)

How does independent segregation occur during Metaphase I?

• The homologous chromosomes in each bivalent are arranged along the equator of the cell

• The alignment of these homologous chromosomes is random (e.g. chromosome from mother could be on left and chromosome from father could be on right or vice versa and this can happen for all the homologous pairs)

• When the homologous chromosomes are separated into two daughter cells during anaphase I, the combination of chromosomes in each daughter cell is a random mix of chromosomes from both parents

  » results in shuffling of genes by REASSORTMENT

ALL GENETIC VARIATION OF GAMETES ONLY OCCURS DURING

MEIOSIS I

Describe the process of meiosis II

7. Prophase II: centrioles move to opposite ends of the cell and produce spindle fibres

8. Metaphase II: spindle fibres attach to centromeres of the chromosomes

9. Anaphase II: spindle contracts and sister chromatids pulled to opposite poles

10. Telophase II: chromatids reach opposite poles and decondense, nuclear envelope forms around each chromatid

11. Cytokinesis: cytoplasm divides

12. 4 haploid daughter cells produced

Why does sexual reproduction not result in an organism that is exactly the same as the parents?

Because of recombination and reassortment, gametes have a varied combination of genes