CELL REPLICATIONCELL REPLICATION

INTRODUCTION

• The cell theory states that all cells are produced from pre-existing cells.

• PROKARYOTIC CELLS replicate by a process called Binary Fission.

• Binary Fission – the cell & its contents are divided into two.

• EUKARYOTIC CELLS replicate by a process called Mitosis.

Introduction Continued………

• Mitosis – the division of the nucleus followed by the splitting of the entire cell into two (cytokinesis).

• In both processes the parent cell divides to form 2 identical daughter cells. When cells replicate to form identical daughter cells, the resulting cells are called clones.

PURPOSE OF CELL REPLICATION

Replacement of damaged cells after injury.

Repair

Enables part of organism or whole organism to increase in size.

Growth

Replacement of old cells as they ‘wear out’.

Maintenance

ExamplesCell Replication Contributes To

GENETIC MATERIAL

• Eukaryotic cells possess a nucleus containing genetic material called DEOXYRIBONUCLEIC ACID (DNA).

• DNA is found in thread-like structures known as CHROMOSOMES.

• DNA influences the characteristics & controls all the functions that go on within an individual

Genetic Material Continued…..

Genetic Material Continued…….

• Prior to the replication of cells, DNA has to be replicated so that each of the two new cells – called DAUGHTER CELLS – contains the same amount and kind of genetic material.

• The correct distribution is vital because any error may result in serious defects & ultimately kill an organism.

• Mitosis ensures the same amount & kind of genetic material is transmitted from one generation to the next.

MITOSIS• Cell replication involves mitosis followed by

cytokinesis.

• This process is divided into phases.

• Interphase is followed by mitosis.

• Mitosis is divided into: prophase, metaphase, anaphase & telophase.

• Cytokinesis occurs at the end of telophase.

Mitosis Continued……..

• Before mitosis begins, chromosomes are too slender to be visible in a cell.

• As replication of genetic material begins, chromosomes become shorter & thicker & are more easily seen. From this point, their behaviour can be studied using a light microscope.

Mitosis Continued………..

• Each chromosome is single stranded & consists of 1 molecule of DNA. However at certain times during the cycle, a chromosome is double-stranded & consists of 2 molecules of DNA.

Interphase

• The cell is engaged in metabolic activity & preparing for mitosis.

• Chromosomes are not visible but their DNA has already duplicated. Chromosomes are cotton like chromatin threads.

• The cell may contain centrioles. In animal cells centrioles appear as rod like structures.

• Cell is in this phase most of the time.

Interphase

Prophase

Chromosomes are visible. They are double-stranded chromatids & are held together at the centromere.

Nuclear envelope dissolves & a structure called the spindle starts to form.

The spindle consists of fibres that radiate across the cell from centrioles that have moved to opposite end of the cell.

Prophase

Metaphase• Mitotic spindle fully formed between centrioles.

• The double stranded chromosomes (each strand called a chromatid) line up along the equator of the cell.

• Each chromosome attaches to a spindle fibre by its centromere.

• This organisation helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive only 1 copy of each chromosome.

Metaphase

Anaphase

• The spindle fibres contract, causing the centromeres to spilt, pulling the sister chromatids towards opposite poles.

• REMEMBER, each chromosome was replicated during interphase & the two copies of each have remain joined until now.

• The migration is orderly & results in one copy of each chromosome moving toward each end of the spindle.

Anaphase

Telophase• Chromatids arrive at opposite poles of the cell.

• Chromosomes become thinner & less obvious & are no longer visible under the light microscope.

• A nuclear membrane begins to form around each group of chromosomes.

• The spindle fibres disperse.

• This completes the process of mitosis.

Telophase

Cytokinesis • Division of the cytoplasm by a process called

cytokinesis is completed.

• New membranes form around each of the two new groups of chromosomes.

• These are now two new cells, which become interphase cells. This cell cycle takes

approximately 24hrs to complete in

mammalian cells

http://www.johnkyrk.com/mitosis.html

THE CELL CYCLE• Cell cycle – is the time it takes for a newly

formed cell to mature & then produce two new cells.

NOTE:

• Mitosis includes prophase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle

Cell Cycle

Interphase• Is the phase between successive mitoses.

• Includes the G1, S & G2 phases.

S Phase• ‘S’ Stands for synthesis.

• Is phase during which DNA is replicated.

• It takes about 10hrs to replicate the 3 billion bits of information contained in the nucleus of a single human cell.

Interphase Continued….

G Phase or Gap Phase• S phase is ‘sandwiched’ by the gap phases – G1 &

G2.

• These are the points at which cell growth takes place.

• Are also times during which the cell checks its DNA for mistakes. These are shown as ‘checkpoints’.

• If a cell does not receive a go-ahead signal at a checkpoint, it exits the cycle.

G Phase Continued………• G1 checks for mistakes in DNA that may have

occurred during cell replication.

• In G2 cells check for mistakes that may have occurred during the synthesis of DNA in the S phase.

G2 PHASE• Is a very active time for the cell.

• For example, proteins used during mitosis have to be synthesized & membrane materials for new daughter cells must be packaged in vesicles ready for use.

G Phase Continued………

G1 PHASE• G1 cells carry out basic cell functions.

• G1 is a crucial decision point. It is the most variable phase of the cell.

• Some cells exit the cell cycle & become dormant; other cells may speed through G1 to divide again

HOW MANY CHROMOSOMES??

• Each species has a characteristic number of chromosomes in each of its body cells.

• Human body cells have 46 chromosomes – 23 pairs

• The collection of chromosomes includes a pair of sex chromosomes. Males have 1 X & 1 Y sex chromosome. Females have 2 X chromosomes.

How Many Chromosomes Continued…..

• The remaining 44 chromosomes in body cells are called autosomes.

• Autosomes comprise 22 pairs of chromosomes, & each pair is identified by a number from 1 – 22.

• Each pair of autosomes makes up a homologous pair. The pair are alike in size & shape & carry genetic material that influences the same characteristics.

ASEXUAL REPRODUCTION IN PLANTS

• The ability to asexually reproduce is economically important.

• Forms of vegetative propagation include:

e. Cuttings – piece of parent plant can be planted and new plants will grow from it e.g. roses

g. Horizontal Stems – ‘runners’ e.g. strawberries and Rhizomes e.g. ferns

i. Suckers – plants that sprout from the roots of the parent plant

Asexual Propagation in Plants Continued…………

a. New Plants From Leaves – e.g. ‘piggy-back’ plant. These are new small plants that drop off the parent and can strike.

c. Gemme – small multicellular bodies produced in cup-like structures e.g. liverworts

Examples: Asexual Reproduction in Plants

ASEXUAL REPRODUCTION IN ANIMALS

• Some animals are capable of reproducing both asexually and sexually depending on the environmental conditions.

• Forms of asexual reproduction are:

e. Regeneration – parent breaks into 2 or more pieces and a new individual will arise from the piece e.g. planaria

g. Splitting in Half – e.g. amoeba

Asexual Reproduction in Animals Continued….

– Budding – small groups of cells may bud or break away from the main organism and then settle on a substrate and reproduce mitotically to produce a new organism e.g. sponges

– Parthenogenesis – ‘virgin birth’. Young are produced from unfertilised eggs. Offspring are identical to female parent e.g. honey bee