Prokaryotes generally have cell walls and some organelles such as ribosomes. However, their unique feature is that they do not have *membrane-bound* organelles such as mitochondria.

Since they do not have membrane-bound organelles, they also *do not have nuclei*. Prokaryotes still contain DNA that consists of nucleic acids, but they float freely in their cytoplasm.

A

Retroviruses and bacteriophages are types of viruses, while protists are eukaryotic organisms. These organisms are not considered as archaebacteria, which consists of prokaryotes.

**Methanogens** are examples of archaebacteria that live in oxygen-free environments. They are prokaryotes that produce methane as a by-product of metabolism, hence their name.

A

Bacteria can be classified based on their cell shape. A *bacillus* is a rod-shaped bacterium. Among the given choices, letter **B** illustrates bacillus bacteria due to their rod-shaped cells.

Letter A illustrates bacteriophage viruses. Letter C illustrates coccus bacteria (spherical-shaped). Finally, letter D illustrates spirillum or spirochete bacteria (spiral-shaped).

B

**Chlorophyll a** is a specific kind of chlorophyll responsible for the absorption of light in the orange and violet spectrum so it can be utilized for photosynthetic reaction. This chlorophyll is the main pigment for photosynthesis. Among the given choices, only **cyanobacteria** is capable of this process. Hence, the correct answer is B.

B

Bacteria reproduce through the process of asexual reproduction, where a single cell splits into two identical daughter cells, having the same genetic makeup. This process is specifically called **binary fission**. During binary fission, the cell grows, and its genome replicates to that each daughter cell will get a copy of the original DNA material. Hence, the correct answer is **A**

A

Conjugation is a method of “reproduction”, sterilization is a process to kill bacteria, and **decomposition** is the breakdown of a substance including the breakdown of nitrates or nitrites into a useable form of Nitrogen. Hence, the correct answer is **D**.

D

Bacteria can reproduce without infecting other cells. The capsid is just a protein shell of a virus, Prophage is the viral genome that incorporates itself in host cell DNA. Hence, the correct answer is **D**

D

A **virus** is an infectious particle consisting of a nucleic acid surrounded by a protein coat called a *capsid*. Viruses do not have the ability to replicate alone, instead, it needs a host to enable replication. Hence, the correct answer is **C**

C

retroviruses uses reverse transcriptase to make DNA from its RNA genome

Cocci and bacilli are two types of bacterial cells that are classified based on their shapes, while archaebacteria are prokaryotes that are known to survive in most extreme environments.

Not all cocci, bacilli, and archaebacteria are able to cause disease. However, all disease-causing organisms are collectively known as **pathogens** which include several microbes.

C

cell wall contains a peptidoglycan layer

No membrane bound organelles

Its DNA and ribosomes are free floating in the cytoplasm

Please see explanation

Prokaryotes such as bacteria can be classified based on the shape of their cells. There are three main cell shapes of prokaryotes, which include: **bacillus**, **coccus**, and **spirillum**.

Bacillus refers to a “rod-shaped” prokaryote, while coccus refers to a “spherical-shaped” prokaryote. Lastly, spirillum (also known as spirochete) refers to a “spiral-shaped” prokaryote.

The main difference between Gram-positive and Gram-negative bacteria is their peptidoglycan layer. Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a relatively thin peptidoglycan layer.

Scientists are able to distinguish between Gram-positive and Gram-negative bacteria through staining techniques such as the Gram stain. In this method, Gram-positive cells can be distinguished from Gram-negative cells through differences in the color of the retained stains.

In the Gram stain method, Gram-positive cells appear purple-blue since they retain the primary stain (crystal violet). On the other hand, Gram-negative cells do not retain the primary stain but rather the counterstain (safranin) and thus appear reddish-pink.

Prokaryotes can be described based on how they move. Some of them cannot move at all, but others may make use of specialized structures for locomotion or other methods.

Some examples of locomotive structures in prokaryotes include cilia and flagella. These structures generally appear as projections that beat back and forth, causing the cell to move.

Prokaryotes can also make use of a slime-like secretion to move on a surface. These cells slowly glide along a slime-like substance that they secrete on a surface, allowing them to move.

On the other hand, **photo**autotrophs are different from **chemo**autotrophs with regards on how they obtain energy. Photoautotrophs derive their energy from sunlight, while chemoautotrophs derive their energy from chemical substances in the environment.

Since photoheterotrophs and chemoheterotrophs are both heterotrophic prokaryotes, they consume organic molecules such as carbon for their source of energy since they are unable to produce their own energy.

An **obligate aerobe** is an organism that requires oxygen as a final electron acceptor to produce energy. This is because they cannot ferment or respire anaerobically. They have to make use of aerobic respiration, which requires oxygen, in order to produce energy.

On the other hand, an **obligate anaerobe** is an organism that does not require oxygen to produce energy. In fact, oxygen is considered to be toxic to them and would be easily killed in its presence. They produce energy either by fermentation or anaerobic respiration.

Faculative anaerobes can survive in any condition. They can live in an open or closed environment.

see explanation for solution

Since plants are unable to use nitrogen gas as it is, it must be converted into other forms such as ammonia, nitrates, and other nitrogen compounds. Nitrogen-fixing bacteria are the ones responsible for converting nitrogen gas into a form that the plants are able to use. Examples of nitrogen-fixing bacteria are Rhizobium, which is associated with beans and other leguminous plants; Frankia, which helps angiosperms and woody shrubs; and Azospirillum, which helps cereal grasses.

All viruses consist of a nucleic acid, which can either be DNA or RNA, that is enclosed by a protein coat called capsid. They all affect another organism by replicating itself inside the cells of the host.

The capsid is the protein coat around a virus. It allows the virus to trick the cell into thinking it’s safe to let the virus enter, thereafter the virus eventually destroys the host (under most circumstances).

During a lytic infection, the virus first attaches to a susceptible host cell by way of surface proteins. Depending on its nature, the virus then penetrates into the host cell or injects its viral genetic material into the host cell.

Once the viral genetic material is inserted into the host cell, it then becomes incorporated into the host’s genome. The host cell then begins replicating and expressing the viral genome, which produces more viral particles.

The viral particles are then assembled into new viruses, which then causes the host cell to lyse or burst. This causes the host cell to die and release the new viruses, which are able to infect another host cell and repeat the process.

When a cell is attacked by a lysogenic virus, the virus attaches itself to the cell’s DNA. The cell’s DNA will be replicated along with the DNA of the virus. The replication may take for many generations and once it’s done, the cell goes into a lytic cycle wherein the cell is broken down or destroyed.

The best way to protect against most viral diseases is in prevention, often by vaccines. Usually it only works if used before infection.

Scientists discovered that viruses bind to the proteins on the cells and they use its genetic information to replicate. In this case, if a virus infects a cell, the cell must precisely transcribe and translate the genetic information of the virus. Because of this, only plant cells are affected by plant viruses, whereas animal viruses only attack certain animals. On the other hand, bacteriophages are the type of virus that attacks a bacterial cell.

If bacteria lost the ability to fix nitrogen, the plants that need the fixed nitrogen (ammonia) to grow would be affected most, and then it would affect other animals that feed on the plants

Unlike bacteria that can be grown in an artificial or synthetic media, viruses such as bacteriophages are unable to grow on a synthetic media. This is because viruses can only grow and replicate inside the living cells.

It is because the acids produced by the bacteria erode the outer layer of the tooth. Brushing regulary will remove these bacteria and acids.

When classifying organisms it is important to look first at their distinguishing characteristics:

1. It is unicellular
2. It has a peptidoglycan cell wall
3. It is composed of circular DNA instead of a linear DNA
4. It does not have a true nucleus.

Based on the given premises we can conclude that the organism is a prokaryote, but it specifically belongs to Kingdom **eubacteria**. This is because the organism is composed of a peptidoglycan cell wall, a characteristic that is **absent** in archean cells.

**Antibiotics (B) and (C)** did not inhibit the growth of the bacteria at all, based on the experiment results. These bacteria developed a resistance to the antibiotics.

B and C.

The most effective antibiotic is (D). Based on the experiment, the growth of the bacteria stopped within 2 mm diameter of the petri dish

D

Prokaryotes

Structure:
A. unicellular
B. lacks a nucleus
C. lacks membrane-bound organelles
D. DNA is in the cytoplasm
E. has a circular chromosome (DNA)
F. has a cell wall
G. different shapes: rod-shaped, spiral, or spherical

Function:
A. reproduces asexually (binary fission, conjugation, spore formation)
B. releases energy through fermentation, cellular respiration, or both
C. movement involves surface appendages like flagella and pili

Viruses that affect bacteria are called bacteriophages.

Eukaryotes

Structure:
A. unicellular or multicellular
B. has a nucleus and membrane-bound organelles
C. has mitochondria and chloroplasts
D. DNA is in the nucleus
E. has linear chromosomes (DNA)
F. has a cell wall in the plant cell and cell membrane in the animal cell

Function:
A. reproduces sexually (mitosis, meiosis, and cytokinesis)
B. releases energy through fermentation, cellular respiration, or both
C. movement uses flagella that resemble the cilia

Eukaryotes are affected by plant viruses and animal viruses.

Binary fission differs from conjugation and endospore formation in terms of its outcome. In binary fission, two identical cells are produced. On the other hand, conjugation and endospore formation do not produce another bacterial cell. In conjugation, hollow bridge between two bacterial cells are formed in order to exchange genetic material with each other, while endospore formation produces a thick wall which encloses the DNA of the bacteria in order to survive harsh external conditions.

This experiment will test whether the contact of an agar plate with a finger would result in more bacterial growth than exposing the agar plate to air.

Method:
1. Prepare the two petri dishes. The media should be in the upper dish and the cover is on the bottom part.
2. Once the Petri dishes are ready, they should be in a room temperature that is around 20 to 25 degrees Celsius.
3. In the first petri dish, gently press your fingers in the surface without tearing the agar.
4. Cover the dish and label it. Store it upside down and ensure that it would be left undisturbed.
5. In the second petri dish, place it in an ambient room.
6. Leave the petri dish open so it will be exposed to the air.
7. Do not expose the petri dish to direct sunlight or to a cold temperature.
8. After three days, observe the growth of bacteria in the two petri dishes.
9. Check its smell and compare the bacteria growth.
10. Record the observation.

Since bacteria is transferred directly to the prepared petri dish via direct contact, the first petri dish has more bacterial growth compared to the second petri dish.

Yes, if the agar plates are contaminated beforehand, it would have affected the bacteria growth. Because of this, the results will be compromised and it would not give an accurate data. For the results to be valid, the agar plates should have no contact with bacteria beforehand and the bacterial culture must be put in the three plates at the same time.