The cytoskeletal system, cell cycle and DNA replication

Hilary MokYolande Leong

BMS/1M02Cheesecake!

The Cytoskeletal System

• An intricate network of protein filaments that extend throughout the cytoplasm

• Highly dynamic->continuously reorganised as a cell changes shape, divides and responds to its environment

Functions of the cytoskeleton• Establishes cell shape• Provides mechanical

strength• Locomotion• Chromosome separation

in mitosis and meiosis

Components of cytoskeletal system

1. Intermediate Filaments2. Microtubules3. Actin Filaments

• Each type of filament has distinct mechanical properties and is formed from a different protein subunit.

• Thousands of these subunits come together to form a fine thread of protein

Intermediate filaments (IF)• 8-10nm• Strong and Rope-like• Form a network throughout the cytoplasm of

most animal cells• Toughest and most durable of the three• Can survive concentrated salt solutions and

non-ionic detergents

Functions & Properties of IF

• Have great tensile strength for structural support

• Strengthens cells against mechanical stress when stretched

• Maintenance of animal cell shape• Stabilised and reinforced by accessory

proteins (e.g. plectin)

Categories of IF

Intermediate Filaments

Cytoplasmic

KeratinsVimentin and

Vimentin-related filaments

Neurofilaments

Nuclear

Nuclear Lamins

In (mostly) epithelial cells

In connective tissue, muscle cells and glial cells

In nerve cells In all animal cells

Cytoplasmic IF• Keratin

– Span interiors of epithelial cell from one side to the other– Forms a cable of high tensile strength which

distributes stress exerted on the skin cell

• Vimentin and Vimentin-related filaments– Maintain cell shape for glial cell– Provide structure support for contractile

machinery

• Neurofilaments– Supports axon growth

Nuclear IF

• Nuclear lamina– Just beneath nuclear membrane– Underlies and strengthens the nuclear envelope in

all eukaryotic cells– Other types extends across the cytoplasm, giving

cells mechanical strength

Hutchinson-Gilford Progeria Syndrome (HGPS)

• Caused by mutation of gene that encodes for Lamin A

• Not hereditary• Rare and fatal condition, no

cure• Cell nucleus has aberrant

morphology as compared to a normal cell nucleus

• Affects children of all ethnicity• Cause individuals to appear to

age prematurely• Signs: growth failure, loss of

body fat and hair, wrinkled skin, stiffness in joints, atherosclerosis, stroke

• Patients die young, ranging from 8-21, averaging at 13

Ashley Hegi, 16

Microtubules

• Long, hollow cylinder , made from the protein tubulin

• 25nm in diameter• More rigid than actin filaments• Normally have one end attached to a

centrosome. • 2 types : Axonemal microtubules and

Cytoplasmic microtubules

Axonemal microtubules• Highly organised, stable microtubules in

specific subcellular structure associated with cellular movements (E.g. cilia, flagella)

Cytoplasmic microtubules

• Loosely organised, dynamic network of microtubules

• Variety of functions: formed mitotic and meiotic spindles, required from movements of chromosomes during mitosis and meiosis

• Provides an organised system of fibres to guide movements on vesicles and other organelles

Actin Filaments (Microfilaments)• 2-stranded helical polymers of the protein actin• Thin and flexible structure, diameter 5 - 9nm• Generally unstable• Can form stable structure when associating with

other actin-binding proteins • Perform a variety of functions depending on the

protein it associated with

2 main functions of Actin filaments

• Muscle contraction

2. Cell locomotion (“crawling”)

E.g. amoebae

Cell Cycle• Duplication and division• Essential mechanism by which all living things

reproduce• Details vary from one organism to another,

occur at different times• Interphase (G0, G1, S and G2 phase) , M phase

(mitosis) and C phase (cytokinesis)

Cell cycle control system• Ensures that events of the cell cycle (DNA

replication, mitosis, etc) occur in a set sequence and that each process has been completed before the next begins

• Achieved by means of molecular brakes that can stop the cycle at various checkpoints: G1, G2, M (mitosis)

M phase (Mitosis)• A process of nuclear division• Replicated copies of a cell’s DNA are organised into

chromosomes• Identical copies of the DNA are then divided equally

between 2 daughter cells• Five stages: Prophase, Prometaphase, Metaphase,

Anaphase and Telophase

Prophase• Replicated chromosomes consisting of 2

closely associated sister chromatids condense• Nuclear envelope breaks down• Spindle fibres form as microtubules grow out

of the centrioles that move to opposite ends of the cell

Prometaphase• Starts abruptly with the breakdown of the nuclear

envelope• Chromosomes attach to spindle microtubules via

their kinetochores and undergo active movement

Metaphase• Kinetochores of the chromosomes line up along

the equator of the cell, moved by the spindle microtubules

• The spindle is now fully formed and the microtubules attach to each sister chromatid

Anaphase• Begins when the sister chromatids synchronously

separate• Centromere holding sister chromatids together

divides • Kinetochore microtubules get shorter and spindle

poles move apart, both contributing to chromosome segregation

Telophase• 2 groups of chromosomes reach the opposite poles of the

spindle• As a new nuclear envelope starts to form around each group

of chromosomes, they uncoil and the spindle disappears• Division of the cytoplasm begins with the assembly of the

contractile ring

C phase (cytokinesis)• Division of cytoplasm and organelles• Cytoplasm is divided into 2 by a contractile ring of

actin and myosin filaments• Cleavage furrow forms by action of contractile ring• Causes a pinch in the cell to create 2 daughters,

each with a nucleus

Meiosis I (reduction division)

Meiosis II (separation division)

Mitosis Meiosis

Occurs in somatic (body) cells Occurs only in reproductive (sex) cells

Diploid (2n) Haploid (n)

2 daughter cells (diploid) 4 daughter cells (haploid)

One cell division Two cell divisions

Genetically identical Genetically different

Difference between Mitosis and Meiosis

DNA Replication

• A process where DNA duplicates itself during interphase

• Also called semi conservative replication– Half of parent molecule retained by each daughter

molecule

Meet the proteins!Helicase:Uses energy from ATP hydrolysis to unwind DNA

DNA Polymerase III:Core replication enzyme of the cell

Sliding Clamp:Beta subunit of DNA Polymerase III; encircles and slides along the DNA

Single-strand DNA Binding Protein:Binds to single-stranded DNA

DNA Primase:An RNA polymers that generates a short RNA primer (oligoribonucleotide)

DNA Polymerase Removes the RNA primer and replaces it with DNA

DNA Ligase:Joins the gaps between newly synthesised DNA (Okazaki)fragments

Process of DNA replication

http://www.mcb.harvard.edu/losick/images/trombonefinald.swf

Alcohol metabolism causes DNA damage and triggers a breast cancer-related DNA damage response

• Ethanol is carcinogenic to human cells at several sites in the body

• Alcohol metabolism product, acetaldehyde causes DNA damage, chromosomal abnormalities and acts as an animal carcinogen

• Acetaldehyde acetate (relatively harmless)• By enzyme aldehyde dehydrogenase2 (ALDH2)

• 30% of East Asians are unable to metabolise alcohol to acetate due to a genetic variant in the ALDH2 gene

• Increased risk of oesophageal cancer from alcohol consumption

• findings show cells responded to DNA damage by activating Fanconi anemia-breast cancer (FA-BRCA) network –protects against breast cancer

Thank You!

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