Cell Biology Revision Questions and Answers

QUESTIONS AND ANSWERS
SECTION A: SIMILARITIES, DIFFERENCES, AND DEFINITIONS
Gametes are mature haploid reproductive cells that unite to form a zygote that develops into a new individual. Gametes are also called sex cells. An egg or ovum is a haploid female reproductive cell or gamete whereas a sperm is a male haploid reproductive cell.
A zygote is the initial cell that is formed as a result of the union of a male and a female gamete by means of sexual reproduction. The two haploid cells merge to give rise to a diploid cell which marks the first stage of development.
An allele is an alternative form of a gene and is located at a specific position in a specific chromosome. Usually, two contrasting alleles exist for each trait. A gene is a molecular unit of heredity. They reside on stretches of DNA that code for proteins of living organisms. DNA refers to Deoxyribonucleic acid. It is a nucleic acid that contains hereditary information and is necessary for the growth and development of an organism.
Natural selection is a mechanism that causes evolutionary change in populations. It produces changes in the genetic composition of a population from one generation to the other. Organisms hence become better adapted to their environment. Artificial selection refers to an intentional reproduction of individuals that have desirable traits. This could be done by breeding or genetically modifying the organisms.
Unicellular organisms are those organisms that are microscopic and comprise of only one cell, whereas a multicellular organism consist of many cells. Prokaryotes, protists and several fungi are unicellular. They absorb nutrients directly through their body surfaces and excrete them by use of pseudopodia. Gaseous exchange is through their body surfaces. Multicellular organisms on the other hand have their cells organized into complex tissues and organs and body processes like respiration and excretion occurs in this organs.
Photosynthesis is the process in which plants use energy from the sun to break down a water molecule for use in production of simple sugars. Hydrogen obtained from water is combined with carbon from carbon dioxide to form carbohydrates. Oxygen is given off as a byproduct. Cellular respiration on the other hand refers to a set of metabolic reactions that take place in a living cell. Oxygen is required to convert energy from nutrients like sugars into Adenosine Triphosphate. Carbon dioxide is given off as a byproduct in these reactions.
Plant cell’s structure consists of a cell wall which lacks in the animal cell. Plant cells have large vacuoles that are centrally placed whereas animal cells have small vacuoles in the cytoplasm. Plant cells have chloroplasts that serve in trapping of energy from the sun for photosynthesis while animal cells lack.
Lipids are a group of organic compounds that are hydrophobic and include fats, oils , waxes and phospholipids. Lipids are vital in energy storage, in structure of the cell membrane and as signaling proteins. Phospholipids are a class of lipids that form a major part in cell membranes as lipid bilayers. They consist of a simple molecule, a phosphate group and glycerol.
Mitosis is a process of nuclear division which involves the division of the cytoplasm in somatic cells. The process produces two identical daughter cells. The stages involved are interphase, prophase, prometaphase, metaphase, anaphase, and telophase. Meiosis is a type of cell division that produces gametes, with half as many chromosomes as the parent cell. This division results in four daughter cells. Meiosis has 10 stages: prophase 1, metaphase 1, anaphase 1 and telophase 1, followed by prophase 2, metaphase 2, anaphase2, and telophase 2. During meiosis, crossing over occurs during prophase 1 of meiosis in a process called synapsis allowing for the exchange of genetic material. This process does not occur in mitosis.
Ribose is found in RNA and is a normal sugar with an OH- group attached to the 2’ carbon ring. Deoxyribose sugar on the other hand is found in DNA and is a modified sugar that lacks the OH- on the 2’ carbon ring.
Adenine, Guanine, Cytosine, and Thymine are the four nucleobases found in DNA. In RNA, uracil replaces thymine. In DNA, thymine pairs with adenine by the formation of 2 hydrogen bonds. Guanine pairs with cytosine by 3 hydrogen bonds. Adenine and Guanine belong in a class of larger bases called purines that have a double ringed chemical structure whereas Thymine and Cytosine belong to the class of pyrimidines which are single-ringed chemical structures. mRNA, tRNA, and rRNA are the different forms of RNA which carry out the different functions of RNA. mRNA carries the genetic information required in the synthesis of proteins. It directs the assembly of proteins on ribosome. tRNA is used to deliver amino acids to the ribosome during this process of protein synthesis called translation. rRNA links the amino acids together to form proteins.
A chromosome is a thread like structure found in the nucleus of a cell in which DNA molecule ids tightly packaged. Chromosomes occur in pairs and are only visible under the microscope only when the cell is dividing. A centromere is a constriction found on each chromosome that divides the chromosome into a short and a long arm, and also holds the two sister chromatids together.
Replication is a process that occurs in the DNA of all living organisms and serves in inheritance purposes. Replication begins by uncoiling of the helix, followed by strand separation by breaking of the hydrogen bonds between the complementary strands, and synthesis of two new strands by complementary base pairing.The process begins at a specific site in the DNA called the origin of replication. Replication is bidirectional from the origin of replication. Unwinding enzymes called DNA helicases unwind and separate the two DNA strands at the origin of replication to form two "Y-shaped" replication forks. These replication forks are the actual site of DNA copying. Helix destabilizing proteins bind to the single-stranded regions so the two strands do not rejoin. Enzymes called topoisimerases produce breaks in the DNA and then rejoin them in order to relieve the stress in the helical molecule during replication. As the strands continue to unwind and separate in both directions around the entire DNA molecule, the hydrogen bonding of free DNA nucleotides with those on each parent strand produce new complementary strands. As the new nucleotides line up opposite each parent strand by hydrogen bonding, enzymes called DNA polymerases join the nucleotides by way of phosphodiester bonds. As the phosphodiester bond forms between the 5' phosphate group of the new nucleotide and the 3' OH of the last nucleotide in the DNA strand, two of the phosphates are removed providing energy for bonding. Finally each parent strand serves as a template to synthesize a complimentary copy of itself, resulting in the formation of two identical DNA molecules.
Transcription is the transfer of genetic material from DNA to RNA. For genes in organisms to produce proteins, DNA must first be transcribed to RNA. The process occurs in the nucleus. The steps include initiation, elongation and termination. One of the DNA strands is used as a template for the synthesis of a complimentary strand of RNA. The enzyme RNA polymerase binds at a promoter site of the DNA, and unwinds the DNA. A complimentary RNA strand is formed downstream of the RNA polymerase. The process of elongation then follows. The RNA polymerase matches a base on the DNA to an RNA nucleotide by complimentary base pairing. Nucleotides are added on the 3’ hydroxyl end of the growing RNA strand. Termination occurs when the DNA template reaches a signal that indicates it should stop copying.
Translation is the process by which amino acids are joined to form proteins. The process begins by binding of a ribosome to a site on the 5’ side. After encountering a start codon AUG, it is joined by a large subunit and a special initiator, tRNA. tRNA binds to the P site on the ribosome. A tRNA bound to its correct amino acid is able to form a base pair with the next codon on the mRNA arrives at the A site. The process continues until the tRNA encounters a stop codon (UAA, UAG or UGA)
Covalent bonds are very strong and stable bonds that form between two carbon atoms by the sharing of one or more electrons. Hydrogen bonds on the other hand are weaker compared to covalent bonds. They are attractive interactions of hydrogen atoms with electronegative atoms such as nitrogen and oxygen. These bonds are formed covalently.
Structural adaptations refer to modifications on the body of animals or of plants that enable that organism to survive in a specific environment. Behavioral adaptations refer to the change of patterns in behavior in different environmental conditions. These include basking, wearing warm clothing, seeking shelter. Functional adaptations refer to the physiological changes that occur in an organism’s metabolism in response to change in its environment. This includes hibernation and aestivation.
Cross pollination refers to the transfer of pollen grains from the anthers to the stigma a different plant of the same species. Self pollination refers to the transfer of pollen grains from the anthers to the stigma of the same plant.
A mutation refers to a change in the genetic makeup of an organism brought about by a change in the nature of the chromosomes. Mutations may arise due to deletions, insertions or alternations in the arrangement of genes in chromosomes. Mutagens are the compounds responsible for triggering the mutations.
Aerobic respiration is the breakdown of simple sugars by the cells in the presence of oxygen to produce energy whereas anaerobic respiration is the breakdown of these sugars in the absence of oxygen. In both processes, carbon dioxide is released.
Autotrophs are organisms that produce complex organic compounds such as carbohydrates from simple inorganic molecules using light from the sun in a process called photosynthesis. Heterotrophs are organisms that cannot produce simple sugars from inorganic compounds and thus uses organic carbon for growth.
Codons are sequences of three nucleotides which specify a particular amino acid to be added during protein synthesis. An anti codon is a unit made up of three nucleotides that correspond to the bases on the codon of the mRNA.
Homologous chromosomes are two different copies of a chromosome of approximately the same length, centromere position and staining patterns. They have genes for the same characteristics at the same loci. They pair during mitosis and meiosis and are inherited each from one parent. Sister chromatids refer to two identical copies of a chromatids joined together by a centromere. Sister chromatids contain the same genes and the same alleles but homologous chromosomes contain the same gene but different alleles.
Aquatic means relating to water: living in or near water, or taking place in water. Terrestrial means relating to the land. Living on land, or found on the land.
Recombinant DNA cloning refers to methods in molecular cloning used to bring together sources of genetic information from different pieces of DNA.

SECTION B
Labeled diagram of plant cell organelles as seen under the light microscope: Labeled diagram of plant cell as seen under the electron microscope:

Labeled diagram of an animal cell as seen in light microscope: Labeled diagram of an animal cell as seen in the electron microscope: Functions of plant and animal organelles:
The cell wall is found in plant cells only. Its functions include providing support to the plant, protection from mechanical damage and entry of pests and diseases and allow water, carbon dioxide and oxygen to pass in and out of the cell.
The cell membrane is found in both animal and plant cells. It provides support, protection, controls movement of materials in and out of the cell and acts as a barrier between the cell and its environment.
The nucleus is found in both plant and animal cells. It carries the genetic information of a cell and controls all the cell activities.
The nuclear membrane is found in both plant and animal cells and it controls the selective movement of materials in and out of the cell.
The cytoplasm is also found in both plant and animal cells and it supports and protects the cell organelles.
Endoplastic reticulum is found in plant and animal cells. Its function is to carry materials through the cell, for example synthesized proteins. Rough endoplastic reticulum bears ribosomes during protein synthesis.
The mitochondrion is found in both plant and animal cell and is the site in which respiration takes place.
Chloroplasts are found in plant cells only and are the site in which light from the sun is absorbed and subsequent formation of simple sugars from organic carbon proceeds. This process is called photosynthesis.
Lysosomes are also found in plant cells only and are involved in the breakdown of larger food molecules into smaller molecules, and digest old cell parts.
Structure and function of DNA: Key: P-Phosphate group S-Sugar(deoxy-ribose)
DNA is a double helix structure with two anti-parallel strands. Each chain is made of a polymer of subunits called nucleotides. Each strand is made of deoxy-ribose sugar molecules linked together by phosphate groups. Each sugar is covalently linked to one of the four bases. In double stranded DNA, two strands run in opposite directions and are joined by base pairs that are linked to each other by hydrogen bonds. Cytosine and Guanine are linked by three hydrogen bonds whereas Adenine and Thymine are linked by two hydrogen bonds.
The structure of RNA:
RNA is a single stranded polynucleotide with a sugar phosphate backbone made of ribose sugar. To each sugar, one of the four types of bases is attached. These bases include Uracil(in place of Thymine), Guanine, Cytosine and Adenine.

Section C
How is the nucleus (& nucleolus) involved in cell division (both types), ribosome assembly, and protein synthesis?
The nucleus is involved in all forms of cell replication in that it houses the nucleic acids that are fundamental in cell division, ribosome assembly and protein synthesis. DNA controls all cell metabolisms through action genes. Genes code for all genomic material in the cell. They are transcribed to RNA then RNA translates them into proteins in the ribosome with the help of transfer RNA. In mitosis and meiosis, DNA plays its role in form of chromosomes that control replication when they form homologous chromosomes after uncoiling (for chromosomes to be visible, they super coil).
Compare and contrast DNA and RNA (structure, components, function); where are hydrogen bonds found?
DNA is a double stranded deoxyribonucleic acid whereas RNA is ribonucleic single stranded acid.
DNA RNA
It is double stranded. (major) It is single stranded
It houses the genes It does not contain any genes
Nitrogenous bases include thymine. Nitrogenous bases include uracil
It has a deoxyribose sugar. (major) It has a ribose sugar.
Associated with translation of genes. It is associated with translation of peptides to proteins.

Processes and organelles involved in protein synthesis
The organelles involved are ribosome, nucleus, golgi bodies and the endoplastic reticulum. The processes involved are protein synthesis, protein coding, protein packaging and protein transportation respectively.

How is mitosis different from meiosis? Why are both necessary for life?
Mitosis is a process for growth without any genetic variation. It is controlled by hormones and growth factors. Meiosis is form of replication with major genetic variation. After spindle formation in meiosis, different homologous chromosomes pair up at the newly formed nuclei. This ensures the variation. It is how gametes are formed. Both processes are necessary for life as the body organs and development is through mitosis whereas sexual reproduction is through meiosis.
When are chromosomes visible? When do they replicate? How do they replicate?
Chromosome are visible when they supercoil during the prophase stage. To replicate, chromosomes first uncoil to form sister chromatids. The DNA initiates replication in the 3’ – 5’ direction so as to form a replica of the other chromatid. This happens spontaneously to the sister chromatid therefore the end product is to similar chromosomes.
What is crossing-over? Crossing over refers to the exchange of materials between two chromosomes through the exchange of part of their segments. This occurs when chromatids of homologous chromosomes pair up in a process called synapsis. This occurs during the prophase 1 stage of meiosis.
What are 4 sources of variation in organisms, that is, what leads to differences in individuals or species?
Variation can be caused by mutation. This is the expression of certain genes. Errors during replication can also be a source of variation, for example during intron removal or wrong pairing in the nitrogenous bases. Chromosomes can also break during replication and as the repair mechanism works to restructure the break, it fixes it in a wrong manner, for example upside down or on another chromosome.
Basic components of Darwin's theory of evolution by natural selection
In any population, the birth rate often exceeds the death rate
The population cannot continue to increase indefinitely
There is therefore competition for survival and mating opportunities within any population.
Within the population, organisms have variations in their traits. They are not all similar
Traits are inherited within a population
Therefore, those traits that provide some advantage in competition among individuals will tend to be passed on through generations. Hence, nature selects the best suited to survive.

What do plants need for growth? From where do each of these resources or conditions come?
Plants grow well in optimal temperatures. Too high or too low temperatures are inappropriate because reactions are catalyzed by enzymes.
Light-plants require light for the purpose of photosynthesis. This is obtained from the sun.
Water-it is needed for the process of photosynthesis, for transport of manufactured food and support of the plant. Water is obtained from rainfall or irrigation from rivers and lakes.
Oxygen is required for respiration. It is obtained from the atmosphere and is taken into the plants through the stomata.
Mineral salts are also required for the growth and development of the plant. They are absorbed by the root hairs of the roots.
Support is vital for the growth of plants and is provided. The soil provides the support needed. Carbon source-is provided by carbon dioxide from the atmosphere.
What is the "endosymbiosis theory" for the origin of the eukaryotic cell? What is the supporting evidence?
The endosymbiosis theory explains the origin of chloroplasts and mitochondria and their double membranes. The theory postulates that they are the result of years of evolution initiated by endocytosis of bacteria and blue green algae.
Evidence
-New mitochondria and plastids are formed through a process similar to binary fission.
-They are surrounded by two or more membranes and the innermost shows differences in composition from the other membranes.
-Both mitochondria and plastids contain DNA different from that of the cell nucleus but similar to that of bacteria.
-The ribosomes of mitochondria and chloroplasts are like those found in bacteria.
-Mitochondria and plastids are similar in size to bacteria.
-Mitochondria have several enzymes and transport systems similar to those in bacteria.

How are prokaryotic cells different from the eukaryotic cells that we emphasized?
Prokaryotes differ from eukaryotes in their genetic makeup and structure. Eukaryotic cells have multiple chromosomes enclosed within a nuclear membrane whereas prokaryotes have a single chromosome that is suspended within the cytoplasm.
The levels of organization in nature (in order) (hierarchy of life HO)
Kingdom
Phylum
Order
Family
Genus
Species
The general relatedness and differences of photosynthesis and aerobic respiration (organelles, raw materials, products, conditions) (see PPT); How does the carbon move from the air, to organisms, back to the air again? how is carbon related to productivity?
Carbon moves from the atmosphere into the biosphere through photosynthesis to produce sugars. It moves back to the air through respiration and combustion. Carbon relates to productivity in that it forms the backbone if amino acids and proteins.
What are the repeating subunits in proteins? Amino acids ……nucleic acids? Nitrogenous bases …..In many polysaccharides? Monomers.
What are the processes by which molecules are put together and broken apart?
Metabolism is the biochemical processes that occur within living organisms to maintain life. They allow us to grow, reproduce, repair damage and respond to the environment. Catabolism and anabolisms are the processes involved in metabolism.
Catabolism is a series of degradative chemical reactions that breakdown complex molecules into smaller units releasing energy.
Anabolism is a succession of chemical reactions that synthesizes molecules from smaller components and requires energy.
4 kinds of macromolecules, elements in each, chemical (or physical) tests, general formulae (where appropriate)
Proteins are made up of amino acids
Polysaccharides are made up of carbohydrates.
Nucleic acids are made up nitrogenous bases held by a sugar – phosphate backbone.
Phospholipids are made up of fatty acids and phosphorous.

Types and functions of carbohydrates, including specific examples
Monosaccharide – glucose (simplest form of carbohydrate that can be assimilated into living systems easily. It is the major source of energy in most living systems after oxygenation)
Disaccharide – fructose (it is an intermediate form. They serve primarily as a source of energy as they are broken down into monosaccharide in cells.
Polysaccharide – Starch (storage unit of carbohydrates in most organisms. They also form part of the cell wall in plant cells and bacteria in the form of cellulose)
Types and functions of lipids, including specific examples
Triglycerides – are store in adipose tissue and are a major form of energy storage in animals.
Phospholipids – form part of the cell membrane as a lipid bilayer.
Fatty acids – they form a part of hormones.
Fat soluble vitamins A, D, E and K are isopropene based lipids stored in the liver and fatty tissues.
Types and functions of proteins, including specific examples
Types include;
Primary – it’s the amino acid sequence.
Secondary – these are regularly repeating sequences stabilized by hydrogen bonds.
Tertiary – these are relationships of the secondary structures stabilized by a hydrophobic core, salt bridges and disulfide bond. It controls the basic function of a protein.
Quaternary – it is formed by several protein molecules called protein subunits that function as a single protein complex.

Functions as;
Antibodies – proteins that defend the body from antigens e.g. IgA
Contractile proteins – they are responsible or locomotion. They are involved in muscle contraction and movement. E.g. actin and mysin
Enzymes – they are proteins referred to as biocatalysts because they speed up chemical reactions.eg pepsin
Hormonal proteins – these are messenger proteins that help coordinate body activities.eg estrogen
Transport proteins – these are carrier proteins that move molecules from one place to another e.ge hemoglobin.
What are some pros and cons of genetic engineering or modification? Are you for, against, or unsure?
In genetic engineering you can design characters or gene products that are desired thus gene efficiency and expression. The major problem is that this can lead to mutations with detrimental effects much later. Genetic engineering is subject to more research but I would not advocate for it. It is alteration of nature.
What are the three categories of adaptations (in plants as seen in video)? Know examples for each.(please refer to video shown by your lecturer)
What are some specific processes in living organisms that involve enzymes?
Digestion
Transcription
Translation
Discuss the unity and diversity of life (i.e. what makes it related?; what makes it varied and different?)
All live on earth is united in the principles of energy acquisition and cell metabolism as all living organism, unicellular or multi-cellular require energy to live. They all integrate molecules into their different metabolic pathways. Their difference at phenotypic level coincides with that in the genotype. This is to say they are different in genetic makeup.
What is the connection to inheritance (genetics) that each of the following has:
• Evolution/natural selection of mutations?
Those individuals with desirable characters due to mutation will reproduce offspring with these characters hence they will survive.
• Sex cells produced by meiosis and combined via sexual reproduction (pollination in plants followed by fertilization of egg by sperm)?
Each gamete here assumes a charge either positive or negative. They inherit this from the parent plant. There connection is that they are both gametes (haploid) that fuse to form a diploid cell from which its progeny will start fro.
• DNA/RNA functions in protein synthesis?
DNA codes for genes that are transcribed by RNA then translated into proteins. Both dictate the proteins to be synthesized.
Consider the interrelationships between all the topics we have covered: what does ecology have to do with genetics? What does cell structure have to do with inheritance? What does photosynthesis have to do with ecology? What does evolution have to do with inheritance? Etc.
Ecology dictates the conditions and adaptations of the organism with its genome adapting accordingly. All adaptations are passed to the next generation through genetic material. The cell structure and phenotype are passed on to the new generation. Photosynthesis is the most important ecological chemical process. This is because it the organic and inorganic carbons balance. All traits that an organism acquires over time are inherited by the subsequent generations.