Cells and Genetics


Cell Structure

You’ll usually see animal cells shown as round cells, and plant cells shown as rectangular cells.


  • Nucleus: stores DNA

  • Golgi apparatus: packages proteins + lipids in vesicles and transports them

  • Rough endoplasmic reticulum (rough ER): has ribosomes on its surface, makes proteins

  • Smooth endoplasmic reticulum (smooth ER): makes lipids

  • Mitochondrion: site of cellular respiration, creates ATP which can be used for energy

  • Vacuole(s): hold water, nutrients, and waste

  • Lysosomes: break down waste

  • Ribosomes: make proteins

  • Microtubules: like a railroad, transports molecules

The only membrane-enclosed organelles are the ribosomes.

Plant/Animal Cells

  • Cell wall (only plants): makes cells rigid

  • Central vacuole (only plants): maintains turgor pressure

    • Animal cells have many small vacuoles

    • Turgor pressure: contributes to rigid structure by applying pressure to inside of cell wall

  • Lysosomes (only animals): breaks down molecules

  • Centrosome (only animals): made of two centrioles, organizes microtubules during mitosis

Development of Cell Theory

  1. Robert Hooke made a microscope and observed cells in pond water

  2. Robert Schleiden suggested that all plant tissues are made of cells

  3. Theodor Schwann suggested that all plant tissues and animal tissues are made of cells

  4. Rudolf Virchow suggested that cells must come from other cells

Cell Division

There are two main forms of cell reproduction—sexual reproduction and asexual reproduction.

Mitosis is when a single parent cell divides to form daughter cells.

Meiosis is when cells are formed from the genes of two different parent cells.




DNA is a nucleic acid that has the shape of a double helix:

The DNA is stored in the nucleus so that it does not get damaged.

Transcription occurs in the nucleus—this involves converting the DNA to mRNA. The mRNA is transported to the cytoplasm, where the mRNA sequence is used to create proteins. This is why we say that DNA is the genetic material—DNA codes for proteins, which conduct many cellular processes.


Chromatin wraps around proteins called histones to form chromatin, which coils further to form chromosomes:

Heterochromatin is very compact, and generally not usable. In order for it to be used, it must be converted to euchromatin which is less compact. Chromosomes can be either heterochromatin or euchromatin, depending on the stage of the cell cycle.


The genotype is what’s coded for by the DNA. The phenotype is the physical trait.

For example, suppose a pea is round. From experiments, we know that for the corresponding gene, there are two alleles: alleles (a form of a chromosome) for this gene: R and r. Capital letters are typically used to denote dominant alleles while lowercase letters are used to denote recessive alleles. If an allele is dominant, the phenotype for that allele will be expressed, regardless of the other alleles.

In this example, R codes for roundedness and r codes for wrinkledness. Since r is recessive, a pea will only be wrinkled if it has the genotype rr. Since the pea in question is round, we know it must have the genotype Rr or RR.

A Punnett square can be used to predict the ratio of genotypes and phenotypes in the offspring of two organisms. For example, consider the offspring of a RR pea and an Rr pea.

From this, we can estimate that if the two peas were to have 100 offspring, 50 of the offspring would have the genotype RR and 50 would have the genotype Rr. Likewise, since R is dominant, all of the offspring are expected to be round.

Here’s every possible Punnett square for our example. Don’t memorize this! Over time, you will learn common patterns.

Historical Contributions






Mendelian laws



Chromosome mapping in fruit flies

Selected from https://sites.ualberta.ca/~pletendr/tm-modules/genetics/70gen-table1-1.html.