Cell Biology 1

Cell Biology Lecture 1: Cell Size, Type, and Cycle

• The lecture aims to cover the diversity of cellular shapes and functions
• It will look at the importance of scale and measurement in understanding cells
• Different types of microscopes, limitations, and limitations of 2D images for 3D reality are to be explored
• The basics of the cell cycle and mitosis are also to be covered in this lecture

The Cell

• The structural and functional unit of living organisms
• All human cells originate from a single fertilized egg
• Development involves cell replication and specialization

Types of Tissues

• Four main types of tissues exist in humans
• These are: epithelium, muscle, connective, and nervous tissue
• Epithelial cells create the epithelium
• These cells originate from body surfaces like skin, digestive tract, and organs

Cell Shapes

• Cells have a diversity of shapes, including:

  • Squamous: thinnest cell in the body, molecules can diffuse across easily
  • Spheroid: Smooth surfaces allow easy movement; it's a type of white blood cell
  • Polygonal: Epithelial, has more connections
  • Discoid: smooth surface allows easy movement through capillaries, red blood cells
  • Cuboidal: Found in the ovary
  • Fusiform (spindle shaped): Elongated cells fused together to contract
  • Columnar: Intestines that absorbs nutrients from the lumen of the gut, strong polarity
  • Fibrous: secretes molecules of the connective tissue in the body e.g. tendons
  • Stellate: Neurons or osteocytes that coordinate regrowth and repair of body

• Squamous, polygonal, and cuboidal cells are found in the epithelium, but have different roles

Cell Diversity Determination

• Genes lead to proteins, which determine cell structure and function
• Each cell expresses only a subset of its genes
• All cells have the same DNA at the start, but mutations arise in DNA as we age, cells are not identical

Number of Cell Types

• A textbook may say that there are 200 cell types, but new cell types are still being discovered and cellular states being analyzed
• The mission of www.humancellatlas.org is to create comprehensive reference maps of all human cells
• It serves as a basis for understanding human health, diagnosing, monitoring, and treating diseases

Pulmonary Ionocyte

• Pulmonary ionocytes are similar to salt-balancing ionocytes in fish gills or frog skin
• These account for 1% of airway epithelial cells
• They are the main source of CFTR expression involved in cystic fibrosis

Importance of Size

• Typical cells measure 5-20 μm
• Micro (µ) is 10^-6, or 1/1,000,000 (one millionth)
• One micrometre (µm) is 1/1,000,000 of a metre, or 10^-6 metre; it's also 1/1000 of a millimetre (10^-3 millimetre)

Measurements

• 1 millimetre (mm) equals 1/1000 of a metre, or 10^-3 m
• 1 micrometre (µm) equals 1/1000 of a mm, that is 10^-6 m
• A human egg is 100 um
• 1 nanometre (nm) equals 1/1000 of a µm, that is 10^-9 m
• An actin subunit is 7 nm

Cell Size

• Cell sizes range from 5 to 100 µm
• An average cell nucleus has a diameter around 5 µm
• A red blood cell is about 8 µm in diameter

Viewing Cells: Resolving Power

• The human eye can resolve down to 0.2 mm
• Light microscopes use light wavelengths and can resolve down to 0.2 µm
• Electron microscopes shine electrons at a sample and can resolve down to 0.2 nm
• Super-resolution microscopy can resolve down to 20nm

Electron Microscopes

• Transmission electron microscopes

• Electrons pass through the sample

• Suited to resolving cell organelles and cell contacts

• Scanning electron microscope

• Electrons bounce off a sample

• Suited to resolve the surface morphology of cells and structures

Viewing Cells: 3D to 2D

• A 2-dimensional view is what we normally see via a light microscope
• Three-dimensional views need to be constructed in our minds
• Compared to a light microscope, a Scanning EM gives a 3D perception of cells

Cell Division – Mitosis

• Mitosis is needed for growth and repair
• Some cells are too specialized to divide while some divide very rapidly.

The Cell Cycle

• Most cells spend time in the G1 phase for growth and normal metabolism functions.
• When a cell chooses to divide, it enters the S-phase where it doubles all of its DNA.
• It then prepares to divide in G2.

Chromosomes

• Normal human cells contain 46 chromosomes, or 23 pairs
• Chromosomes come from our mothers and fathers
• In the S phase, each chromosome doubles its DNA content, forming 2 chromatids joined by a centromere
• A diploid cell has the full set of chromosomes, whilst a haploid cell has half (only the eggs)

Cell Cycle Phases – Interphase

G1 phase (gap phase 1) - phase of growth and normal cellular activities S Phase, or synthetic phase - DNA replication stage taking 6-8 hours G2 phase (gap phase 2)

• short phase that prepares for cell division
• takes 3-4 hours

Cell Cycle Phases - Mitosis

 - Prophase: Chromosomes condense and become visible, the nuclear membrane disappears, and the mitotic spindle starts to form from centrioles
 - Metaphase: chromosomes line up on the equator and the spindle is fully formed
 - Anaphase: the chromosomes split at the centromeres with each half of the chromosomes going to the different pole to the different centriole
 - Telophase: decondensation of chromatin, reformation of nuclear envelope, and cytokinesis (where cells begin to separate into 2 daughter cells)