2.2: Prokaryotic cells.
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a prokaryote is a cell with no membrane organelles, e.g. no nucleus, mitochondria or chloroplasts.
List some examples of prokaryotes.
Prokaryotes include bacteria and the archaea.
What is Escherichia coli?
E.coli is a very common bacteria found in the intestines of humans and other mammals. It is a gram-negative bacteria and is a member of the subgroup gamma proteobacteria. it is normally not pathogenic though some strains can cause food poisoning. Counts of E.coli in food and water are often used as an indication of pollution, e.g. caused by sewage contamination of water supplies or lack or proper hygiene in food preparation.
Distinguish between the structure and function of the bacterial nucleoid and a bacterial plasmid.
The bacterial nucleoid contains the bacterial genome of DNA. The nucleoid is one long, single molecule with the two ends of the double-stranded DNA bonded to form a physical and genetic circle. The nucleoid contains teh genetic information which controls cell metabolism and inheritance. plasmids are extra chromosomal genetic units that are nonchromosomal DNA. They are similar to the nucleoid in that the two ends bond to form a physical circle. Plasmids are not essential for growth, but can assist as they code for additional proteins, e.g. antibiotic resistance.
Many bacteria can form endospores. Outline the importance of this adaptation.
Endospores are formed when conditions become unfavorable. Endospores are highly resistant to many conditions, e.g. lack of water, high salt. The location within cells of endospores is species-specific and used in identifying bacteria.
Summarise the structure and function of the outer parts of a prokaryotic cell.
- Cell Wall: Bacterial walls contain peptidoglycan (sugar polymers cross-linked by short polypeptides). Archaean cell walls contain polysaccharides and protein with no peptidoglycan. - Protect cell, give cell shape.
- Capsule: Cell wall can be covered by a capsule layer of polysaccharide or protein. - Capsule allows bacteria to attach to other cells, e.g. to form a colony or to adhere to a substrate or pathogens attach to host.
- Plasma membrane: Phospholipid bilayer with wide variety of fatty acids present. - Assist attaching to surface and other cells. Some used for motility.
- Fimbriae: Protein tubes that extend from outer membrane. Usually short and present in large numbers.
- Pili: Protein tubes that extend from outer membrane. Usually longer and fewer in number than fimbriae. - Sex pili used during conjugation and transfer of DNA.
- Flagella: Protein structure with one end embedded in cell wall and plasma membrane. Can be concentrated at the end(S) or scattered over the surface. - Used for motility.
Define binary fission.
Binary fission is a type of cell division in which unicellular organisms reproduce asexually. each daughter cell receives an identical copy of the genetic information.
Describe the location of genes in a prokaryotic cell.
Most genes in a prokaryotic cell are located on a single bacterial crhomosome which consists of DNA and protein. Bacteria are haploid for most genes. Bacteria can have small circular DNA called plasmids that are accessory chromosomes. Some genes on the plasmid are also found on the single main chromosome making the bacteria diploid for those genes. Plasmids can reproduce independently of the main chromosome and move between cells or between species by a process called conjugation.
Explain what is happening in the diagram below.
- 1. Parent cell, e.g. prokaryote.
- (Binary fission begins)
- 2. Origin of replication.
- (Nuclear material has replicated, spearated and parent cell begins to split).
- 3. Two identical daughter cells are produced.
The replication of DNA begins at a specific place on the chromosome. Name this site and outline how it is involved in the process of binary fission.
The site is called the origin of replication. As the chromosome replicates, the two origins move to the opposite ends of the cell and when complete there is a copy in each new forming cell.
Suggest an advantage and a disadvantage of binary fission as a means of reproduction.
Binary fission produces two daughter cells identical to the parent. It is a form of asexual reproduction and is energy efficient (no energy is needed to find a mate) and can quickly produce many numbers. This is advantageous in favourable conditions. For organisms reliant on binary fission, genetic diversity occurs due to mutations. However, if the environment changes quickly and there has been little genetic change due to mutation, the small gene pool can lead to extinction of a particular species.
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