BINARY FISSION IN BACTERIA

Much faster than mitosis in eukaryotic cells

Takes approx. 20 mins as opposed to several hours

If resources were available once cell could become 16,000,000 in 8 hours!

BINARY FISSION IN BACTERIA

BINARY FISSION IN BACTERIA

Bidirectional replication of circular DNA

BINARY FISSION IN BACTERIA

DNA moves to poles

BINARY FISSION IN BACTERIA

Cell elongates

Cleavage furrow forms

BINARY FISSION IN BACTERIA

Cell divides

THE CELL CYCLE – EUKARYOTIC CELLS

Final checkpoint at the end of metaphase

ANIMATIONS

Animation 1

Animation 2

THE CELL CYCLE – G1

The first phase is a growth phase (G1) The new cell starts growing and

replicating its organelles At the end of G1, the checkpoint is to

gauge if it is a good time for cell to keep growing or dividing or if a delay is necessary.

If the cell is under any sort of stress, it will pause at this stage.

THE CELL CYCLE - S

During the synthesis (S) phase, DNA replication occurs It is at this point that the diploid cell (2n) doubles to 4n, so

that when it divides there will be two complete copies of the DNA

DNA replication is semi-conservative – every new molecule consists of one original strand and one new strand.

DNA replication involves numerous enzymes Helicase and gyrase unwind and unzip to strands of DNA DNA polymerase continuously adds nucleotides to the leading

strand in a 5’ to 3’ direction RNA primase makes an RNA primer on the lagging strand DNA ligse sticks together prefabricated ‘Okazaki’ fragments

made by DNA polymerase to replace the RNA

THE CELL CYCLE – G2

During the next growth phase (G2), the cell continues to grow in preparation for division

The checkpoint during this phase is extremely important.

If any errors have occurred during transcription, the cell must undergo apoptosis (programmed cell death)

THE CELL CYCLE - M

During Mitosis (M), the single parent cell gives rise to two identical daughter cells.

Another checkpoint occurs at the end of metaphase to ensure that chromosomes are aligned and that spindle fibres are attached corectly.

A negative result at the checkpoint here will result in apoptosis.

THE CELL CYCLE - C

Cytokynesis (C) is actually the last part of Mitosis

At this point the cell, which momentarily has twin nuclei, will divide in to two new cells (2n) and the process will begin again.

WHAT HAPPENS IF THE CHECKPOINTS DON’T WORK?

If cell cycle checkpoints are operated by enzyme proteins.

These enzymes can be prevented from carrying out normal function by inhibitors.

These enzymes could also be stimulated to maintain an active state permanently.

Either way can lead to uncontrolled and unmonitored cell division, this is commonly known as ….

CANCER

Cells multiply uncontrollably and regardless of accumulating errors will never apoptose.

Does not form useful cells as cells spend far more time in division rather than growth (a reversal of the normal situation).

MITOSIS

INTERPHASETHE CELL PREPARES FOR DIVISION

Animal & Plant Cells Nucleus clearly

defined Chromosomes not

yet visible

DNA replicated Organelles

replicated Cell increases in

size

INTERPHASE

Animal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

PROPHASETHE CELL PREPARES FOR NUCLEAR DIVISION

Animal & Plant Cells DNA packaged in

to chromosomes Nuclear envelope

disappears

Centrioles move to opposite sides of the cell

PROPHASE

Animal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

METAPHASETHE CELL PREPARES CHROMOSOMES FOR DIVISION

Animal & Plant Cells Chromosomes

line up at the center of the cell

Spindle fibres attach to centromeres on the chromosomes

Centrioles reach the opposite poles of the cell

METAPHASE

Animal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

ANAPHASETHE CHROMOSOMES DIVIDE

Animal & Plant Cells Spindle fibers

contract and pull chromosomes apart

½ of each chromosome (called a chromatid) moves to each hemisphere

Unattached spindle fibres push against each other or the sides of the cell, and it begins to elongate

ANAPHASE

Animal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

TELOPHASETHE CYTOPLASM DIVIDES

Animal Cell DNA spreads out 2 nuclei form Contractile ring

causes a cleavage furrow to form the 2 new daughter cells

Plant Cell DNA spreads out 2 nuclei form Cell plate forms

new cell wall between the nuclei to form the 2 new daughter cells

TELOPHASE

Animal Cell Plant Cell

Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm

MITOSIS ANIMATIONS

Mitosis.exe

Mitosis Animation

Animal Mitosis -- Review

Interphase

                                              

              

Prophase

                                             

               

Metaphase

                                              

              

Anaphase

                                             

               

Telophase

                                              

              

Interphase

                                             

               

Plant Mitosis -- ReviewInterphase

                                                        

    

Prophase

                                                       

     

Metaphase

                                                        

    

Anaphase

                                                       

     

Telophase

                                                        

    

Interphase

                                                       

     

SUMMARY OF MITOSIS

Prophase: Chromosomes condense Nuclear envelope disappears centrosomes move to opposite sides of the cell Spindle forms and attaches to centromeres on the

chromosomes

Metaphase Chromosomes lined up on equator of spindle centrosomes at opposite ends of cell

Anaphase Centromeres divide: each 2-chromatid chromosome

becomes two 1-chromatid chromosomes Chromosomes pulled to opposite poles by the spindle

Telophase Chromosomes de-condense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

MEIOSIS

RECOMBINATION THROUGH MEIOSIS

DNA replicates Crossing over

Random assortment

First division

Second random assortment

Second division

MITOSIS VS. MEIOSIS

GETTING THE NAMES RIGHT

Kinetochore

Centromere

Sister chromatids

Spindle fibres / microtubules

RECOMBINATION = VARIATION

2 pairs of chromosomes = 4 possibilities 4 pairs of chromosomes = 8 possibilities What are the chances of identical offspring?

2n = 46 (not incl. twins) 1 / 16,777,216

DNA Replication

DNA is uncoiled and unzipped by helicase & gyrase

The original strands are called the template strands

The new strands are called the complementary strands

The 3’ to 5’ template strand is the leading strand

The 5’ to 3’ template strand is the lagging strand

The complementary strand can only be written 5’ to 3’

DNAi ANIMATIONS x 2

The leading strand

An RNA primer is put down by RNA polymerase

Complementary DNA is put down base-pair by base-pair by DNA polymerase

3’ 5’

5’ 3’

Rp Dp

DIRECTION OF REPLICATION

The lagging strand

RNA polymerase puts down primers along the template strand of DNA

DNA polymerase III builds short ‘Okazaki’ fragments from each primer

DNA polymerase I removes the RNA and replaces it with DNA

DNA ligase sticks the fragments together

The lagging strand

5’ 3’

3’ 5’

RNA Polymerase

DNA Polymerase III

DNA Polymerase I

DNA Ligase

DIRECTION OF REPLICATION

ADDITIONAL INFO ON CELL REPRODUCTION

Our cells can be categorised in to two types Somatic cells (diploid) Germline cells (diploid but give rise to

haploid gametes – sperm & eggs)

Biozone Fun! DNA replication

pp. 203-25 Cell cycle, mitosis, apoptosis

pp. 209-212 Meiosis

pp. 287-290