reproduction in bacteria

BY

A.BHARANIDEEPAN

13-513-301

REPRODUCTION IN BACTERIA

• Bacteria reproduce by Vegetative, asexual and sexual methods.

• Vegetative reproduction includes Budding, Fragmentation and Binary fission

REPRODUCTION TYPES

• Bacteria reproduce by budding process in which small bud develops at one end of cell, this enlarge and developed into new cell and separate from the mother cell.

• Budding has been observed in some Gram-Positive Bacteria e.g. Planctomycetes, Cyanobacteria.

Budding

• Binary fission =“division in half”

• Two step process

1. DNA (circular chromosome) must be copied prior to cell division

2. Each chromosome is separated and distributed to each daughter cell

Binary fission

• Bacteria reproduce asexually by cell division called binary fission

• First, the bacterial chromosome begins to replicate, starting at the origin of replication

• Replication continues, one origin moves to the opposite side of the cell, and the cell elongates

• Replication finishes, the plasma membrane grows inward, and a new cell wall is laid down

• Produces two genetically identical daughter cells – clones

• Binary fission is the evolutionary precursor to mitosis

• Most cells produced by binary fission are identical.

• Variation in bacteria results from mutation and genetic recombination

• Spontaneous mutations: once in every 10 million divisions. In 12 hours E.. coli produces 10 to 100 million cells

• It is the time, covering the beginning of division of the mother cell up to the formation of two new cells.

• The average generative time is about 20 – 30 minutes

Generative (or doubling) time

• Mostly during unfavorable conditions, bacterial protoplasm undergoes compartmentalization and subsequent fragmentation, forming minute bodies called gonidia.

• Under favorable conditions, each gonidium grows to a new bacterium.

• It becomes apparent that prior to fragmentation the bacterial genome has to undergo repeated replication so that each fragment gets a copy of it.

Fragmentation

• Endospore are resting spores formed in some gram positive bacteria (Bacillus and Clostridium) during unfavourable conditions.

• They are formed within the cells.

• During this process a part of the protoplast becomes concentrated around the chromosome.

Asexual Reproduction By Endospore Formation

• A hard resistant wall is secreted around it. The rest of the bacterial cell degenerates.

• Endospore are very resistant to extreme physical conditions and chemicals.

• During favourable conditions the spore wall gets ruptured and the protoplasmic mass gives rise to a new bacterium.

• occurs in the form of genetic recombination.

• There are three main methods of Genetic Recombination1. Transformation

2. Transduction and

3. Conjugation.

Sexual reproduction 

The direct transfer of genetic material between two bacteria cells that are temporarily joined

DNA transfer is one-way, from “male” to “female”

The donor (“male”) uses an appendage called the sex pilus that forms a cytoplasmic mating bridge

DNA gets transferred via this bridge in the form of a plasmid

The plasmid encodes the ability to mate as well as other traits such as antibiotic resistance

Conjugation - Lederberg and Tatum, 1946

The alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

Transformation was first demonstrated in 1928 by British bacteriologist Frederick Griffith.

Many bacteria possess cell surface proteins that facilitate transformation in natural populations

E. coli is used in biotechnology applications of genetic recombination (genetic engineering) Cells are cultured in high CaCl2 to become “competent”

Cells are then transformed with human genes that code for proteins such as insulin or growth hormone that are needed in large amounts

Transformation

• Zinder and Lederberg(1952)

• Phages (viruses that infect bacteria) carry bacterial genes from one host cell to another as a result of mistakes in the phage reproductive cycle.

Transduction