ASVAB · General Science · 6 min read · Open lesson
Cells, Genetics and Heredity
- Identify the primary structures and functions of prokaryotic and eukaryotic cells, including key differences between plant and animal cells.
- Contrast the cellular processes of mitosis and meiosis, explaining their roles in growth, repair, and sexual reproduction.
- Describe the structure and function of DNA and RNA, and use Punnett squares to predict inheritance outcomes.
Cell Theory and Cellular Organization
All living things are made of cells; some cells have specialized jobs, while others are single-celled organisms that carry out all the functions needed for life. The cell is the basic structural and functional unit of all living organisms, meaning everything an organism does ultimately happens at the cellular level. Scientific cell theory establishes three foundational ideas: all living organisms are composed of one or more cells, the cell is the fundamental unit of life, and all cells arise from preexisting cells through the process of cell division.
Scientists divide living cells into two major categories: prokaryotic and eukaryotic. Prokaryotic cells are structurally simpler and lack a membrane-bound nucleus and membrane-bound organelles. Bacteria are familiar examples of prokaryotic organisms. Prokaryotic cells have no membrane-bound nucleus, but they still have organized internal structures, including ribosomes and a region containing their DNA.
In contrast, eukaryotic cells contain a membrane-bound nucleus and specialized organelles that act like microscopic rooms handling distinct tasks. Animals, plants, fungi, and protists are all composed of eukaryotic cells. In these cells, the nucleus houses most of the cell's DNA and helps direct cellular activities. Surrounding the nucleus is the cytoplasm, which includes the gel-like fluid called cytosol and the organelles it surrounds, but not the nucleus itself. Enclosing every cell is the cell membrane, a selectively permeable phospholipid bilayer that regulates the entry and exit of substances.
Organelles and Plant Versus Animal Cells
Inside eukaryotic cells, specialized internal structures carry out everyday metabolic duties. Mitochondria generate chemical energy in the form of ATP through cellular respiration, supplying the power needed for cellular work. Ribosomes are the cellular structures responsible for synthesizing proteins. These proteins are often processed by the endoplasmic reticulum; the rough endoplasmic reticulum is studded with ribosomes and assists in protein folding and transport, while the smooth endoplasmic reticulum synthesizes lipids and aids in the detoxification of chemicals. The Golgi apparatus then modifies, sorts, and packages proteins for secretion or delivery within the cell.
Cells must also store materials and dispose of waste. Lysosomes contain digestive enzymes that break down cellular waste and foreign invaders, functioning like internal cleanup crews. Vacuoles serve as storage compartments for water, nutrients, and waste products inside the cell. While both plant and animal cells share many of these vital components, their overall designs show distinct structural differences suited to their lifestyles.
Plant cells have cell walls, and many have chloroplasts; animal cells have neither. A plant cell wall provides rigid structural support and protection outside the cell membrane and is primarily composed of the carbohydrate cellulose. Many plant cells have chloroplasts, which use light energy to help make sugars through photosynthesis; a large central vacuole helps maintain turgor pressure and keeps many plant tissues firm. In contrast, many animal cells have centrioles that help organize cell division, while most plant cells lack them.
| Cell Feature | Plant Cells | Animal Cells |
|---|---|---|
| Cell Wall | Present (composed of cellulose) | Absent |
| Chloroplasts | Present in many cells (contain chlorophyll) | Absent |
| Vacuole Structure | Typically a large central vacuole | Smaller vacuoles |
| Centrioles | Absent in most plant cells | Present in many animal cells |
Cell Division: Mitosis and Meiosis
Eukaryotic cells use mitosis to make new cells for growth and repair and meiosis to make cells used in sexual reproduction; prokaryotes such as bacteria generally divide by binary fission. Before mitosis or meiosis I, a eukaryotic cell typically goes through interphase, when it grows and copies its DNA; it does not copy its DNA again between meiosis I and meiosis II. Mitosis, followed by division of the cytoplasm, produces two daughter cells with the same chromosome number as the parent cell. If the parent cell is diploid, meaning it contains two complete sets of chromosomes, the resulting daughter cells are also diploid. Mitosis helps multicellular organisms grow and repair tissues and supports asexual reproduction in some organisms.
Mitosis progresses through four sequential stages in order: prophase, metaphase, anaphase, and telophase. During metaphase, duplicated chromosomes line up along the center of the cell before being pulled apart. The physical separation of the remaining material is called cytokinesis, which is the division of the cytoplasm into two cells. Cytokinesis usually begins during or after the final stages of nuclear division, completing the journey from one parent cell to two genetically identical daughter cells.
While mitosis maintains chromosome counts for body maintenance, meiosis handles sexual reproduction. Meiosis produces haploid cells that are genetically different, carrying one single set of chromosomes instead of two. In humans, gametes are reproductive sex cells, such as sperm and egg cells. In human males, meiosis produces four haploid spermatids that later mature into sperm; in human females, meiosis produces one functional egg and smaller cells called polar bodies. During meiosis, crossing over exchanges genetic material between homologous chromosomes to increase genetic diversity. When fertilization occurs, two haploid gametes fuse to restore the diploid chromosome number.
DNA, RNA, and Protein Synthesis
Hereditary instructions in living cells are written in nucleic acids. DNA stands for deoxyribonucleic acid and carries hereditary genetic instructions. Structurally, DNA is a double helix consisting of two complementary strands wound together. Each strand is a chain of building blocks called nucleotides. A DNA nucleotide is composed of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
There are four nitrogenous bases in DNA: adenine, thymine, cytosine, and guanine. Under standard DNA base pairing rules, adenine pairs with thymine, and cytosine pairs with guanine. RNA differs from DNA in a few critical ways: RNA uses the nitrogenous base uracil in place of thymine and contains ribose sugar instead of deoxyribose. A gene is a segment of DNA that contains instructions for making a functional product, such as a protein or an RNA molecule. In eukaryotic cells, chromosomes are structures made of DNA packaged with proteins called histones. Human somatic body cells normally contain 46 chromosomes, arranged in 23 pairs, and human biological sex is typically determined by sex chromosomes, with XX for females and XY for males.
Cells turn genetic blueprints into working parts through two primary steps. First, transcription is the cellular process in which a DNA sequence is copied into messenger RNA. Next, translation occurs when ribosomes read messenger RNA to assemble an amino acid chain into a protein. Occasionally, changes in the nucleotide sequence of DNA take place; these changes are known as mutations. Mutations may affect protein function or traits, but some have no noticeable effect on the organism.
Genetics and Patterns of Heredity
Inherited traits depend on the specific genes passed from parents to offspring. Alleles are different variants or alternate forms of the same gene. An organism's genotype refers to the specific combination of alleles it carries, while its phenotype refers to the observable physical characteristics or traits expressed. An individual is homozygous when they carry two identical alleles for a specific gene, and heterozygous when they carry two different alleles for a specific gene.
In a simple dominant-recessive inheritance pattern, the dominant allele determines the phenotype when an organism has one dominant and one recessive allele. For a simple dominant-recessive trait in a diploid organism, the recessive phenotype appears only when both alleles are recessive. A Punnett square is a grid used to predict the probabilities of offspring genotypes in a genetic cross. For a single gene with complete dominance, crossing two heterozygous parents, such as Bb × Bb, gives an expected genotypic ratio of 1 BB to 2 Bb to 1 bb, which translates to an expected phenotypic ratio of 3 dominant to 1 recessive.
Inheritance does not always follow simple complete dominance. In codominance, both alleles are fully and simultaneously expressed in the heterozygous phenotype. In incomplete dominance, the heterozygous phenotype is intermediate between the phenotypes of the two homozygotes, producing a blended physical appearance rather than one allele completely masking the other.
Common mistakes
- Confusing genotype with phenotype: genotype is the internal genetic makeup or combination of alleles, whereas phenotype is the physical, observable trait.
- Assuming that crossing two heterozygous parents (Bb × Bb) results in equal numbers of dominant and recessive offspring: it yields an expected 3 to 1 ratio of the dominant phenotype to the recessive phenotype.
- Mixing up transcription and translation: transcription makes RNA from a DNA template, usually in the nucleus of a eukaryotic cell, while translation happens at ribosomes to build a protein chain.
- Believing that all mutations are harmful: while mutations can alter protein function and observable traits, some produce no noticeable effect at all.
Summary
- The cell is the basic unit of life; prokaryotes lack a membrane-bound nucleus, while eukaryotes contain a nucleus and specialized membrane-bound organelles.
- Unlike animal cells, plant cells have a cellulose cell wall and typically a large central vacuole; many, but not all, plant cells also have chloroplasts for photosynthesis.
- Mitosis normally produces two genetically identical daughter cells with the same chromosome number as the parent cell; its stages are prophase, metaphase, anaphase, and telophase.
- Meiosis produces genetically diverse haploid gametes through crossing over, and fertilization restores the diploid chromosome count.
- DNA is a double helix built from nucleotides with adenine pairing with thymine and cytosine pairing with guanine; RNA uses uracil instead of thymine.
- Protein synthesis involves transcribing DNA into messenger RNA and translating that RNA into an amino acid sequence at the ribosome.
- Genotypes describe allele pairings, and phenotypes describe expressed traits; for a single gene with complete dominance, a Bb × Bb cross gives an expected 3:1 dominant-to-recessive phenotypic ratio.
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