Cell Division and DNA Replication

Cell Division Basics

• All cells originate from pre-existing cells
• Cells must replicate precisely for growth, repair, or reproduction
• The two main processes in cell division are Mitosis (division of the nucleus) and Cytokinesis (division of the cell)

The Cell Cycle

• Interphase is the preparation phase where the cell grows, prepares for replication, and becomes biochemically active
  • During the S phase (synthesis phase) DNA replicates in the nucleus
  • During the G2 phase (second gap phase) the cell continues to rapidly grow, exhibit biochemical activity, and prepares for Mitosis
• M phase (Mitosis) is where the nuclear division occurs
• Cytokinesis involves division of the cellular content
• Interphase (G1, S, G2) takes around 23 hours
• Mitosis (M phase) takes about 80 minutes
• The cell completes its cycle in about 24 hours

DNA and Chromosomes

• DNA is structured into chromosomes
• Humans have 46 chromosomes or 23 pairs
• After DNA replication in the S Phase, there are 92 chromatids (46 chromosomes duplicated)
• Chromosomes carry genetic information which is essential for cell function

Mitosis (M Phase) - Stages of Nuclear Division

• The acronym PPMAT is used for the stages of Mitosis (Prophase, Prometaphase, Metaphase, Anaphase, Telophase)
• Prophase (Longest Phase): the chromosomes shorten and thicken
• Sister chromatids hold together at the centromere and the nucleolus breaks apart while spindle fibers start forming
• Prometaphase: Chromosomes move towards the cell's equator, the nuclear membrane dissolves and microtubules attach to kinetochores (proteins assemble on the centromere, linking it to microtubule polymers
• Metaphase: Chromosomes align at the equator of the cell.
• Anaphase (Shortest Phase): Sister chromatids separate and move toward opposite poles.
• Telophase: Chromosomes de-condense and become longer and thinner, new nuclear membranes and nucleoli form
• Mitosis is complete but not cell division

Cytokinesis - Final Stage of Cell Division

• Cytoplasm divides, creating two new daughter cells
• Each new cell enters G1 and the cycle repeats

Sexual and Asexual reproduction

• Sexual reproduction involves the offspring inheriting unique traits from parents allowing for better environmental adaptation
• Asexual reproduction is common in plants and microorganisms, its faster and safer but provides less genetic diversity

Chromosomes, Ploidy and Meiosis

• Diploid (2n) consists of 2 sets of chromosomes (Humans have 46 chromosomes)
• Haploid (n) consists of 1 set of chromosomes (Human sperm & egg: 23 chromosomes)
• Meiosis is the process that creates haploid sperm and egg cells

Meiosis vs Mitosis

• Mitosis occurs in almost all cells vs meiosis which only occurs in sex organs (ovaries & testes)
• Mitosis produces 2 identical daughter cells vs meiosis producing 4 genetically unique haploid cells
• Mitosis chromosome number stays the same (2n → 2n) vs meiosis reducing (2n → n)
• Mitosis gives no genetic variation vs meiosis which gives high genetic variation

Genetic Disorders from Chromosome Abnormalities

• Non-disjunction: Failure of chromosomes to separate during meiosis
• Examples of Chromosomal Disorders: Down Syndrome (Trisomy 21) - Occurs from an Extra chromosome 21, Turner Syndrome (XO) from a missing an X chromosome, Klinefelter Syndrome (XXY) from an extra X chromosome in males, Jacob's Syndrome (XYY) from an extra Y chromosome in males

Mendelian Genetics & Patterns of Inheritance

• Gregor Mendel (1822-1884) is considered the father of genetics
• Gene: A section of DNA that codes for a protein
• Allele: Different versions of a gene
• Dominant Gene: Expressed when present
• Recessive Gene: Expressed only when two copies are present

DNA Structure

• Nucleotides are the building blocks of DNA
• the 4 bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
  • Adenine (A) pairs with Thymine (T) while Guanine (G) pairs with Cytosine (C)
• Chargaff's Rules: Amount of A = T and G = C
• Watson & Crick discovered the Double Helix Model in 1953
• DNA stores genetic information, replicates, and mutates to create genetic diversity

Molecular Genetics: How Proteins Are Made

• Transcription occurs in the nucleus where DNA unzips and forms mRNA with the aid of RNA polymerase to synthesize mRNA.
• mRNA codons are a set of 3 nucleotides coding for amino acids.
• Translation occurs in the cytoplasm when mRNA moves to ribosomes

Types of RNA

• mRNA (Messenger RNA) carries the blueprint for protein synthesis to ribosome (has codons)
• rRNA (Ribosomal RNA) is responsible for protein synthesis
• tRNA (Transfer RNA) transports amino acids to the ribosome (Has anticodons)
• tRNA carries amino acids, and rRNA then helps link those amino acids into a protein chain

Genetic Code & Mutations

• DNA is read in triplets (codons)
• Mutations: Can be substitutions where one base is changed or Insertions/Deletions where extra or missing bases are added, causing a frameshift
• Mutations can lead to genetic variation being either beneficial, harmful, or neutral.

Summary of Key Concepts

• Cell cycle: G1 → S → G2 → M → Cytokinesis
• Mitosis: PPMAT - Prophase, Prometaphase, Metaphase, Anaphase, Telophase.
• Meiosis: Forms 4 unique haploid cells for reproduction
• Genetic disorders result from chromosome abnormalities.
• DNA Structure: Double helix with A-T, G-C base pairing.
• Protein Synthesis: DNA → mRNA (Transcription) → Protein (Translation)

Introduction: How Cells Acquire and Release Stored Energy

• Metabolism is the sum of all chemical reactions in an organism allowing it to obtain and use energy
• A Metabolic Pathway is a series of chemical reactions each catalyzed by specific enzymes to release stored energy

Enzymes

• Enzymes are proteins that speed up chemical reactions
• Adenosine Triphosphate (ATP) is the universal energy carrier in cells

ATP (Adenosine Troposphere)

• Composition is Adenosine (Adenine + Ribose Sugar)
• ATP has 3 phosphate groups
• ATP functions by storing and releasing energy by adding or removing phosphate groups
  • When ATP loses a phosphate (becoming ADP) energy is released
  • when ADP gains a phosphate (becoming ATP) energy is stored

Role of producers and consumers

• Producers (Autotrophs): make their own food using sunlight (e.g., plants, algae, some bacteria).
• Consumers (Heterotrophs): eat other organisms for energy.

Photosynthesis Overview

• Photosynthesis converts light energy into chemical energy in the form of glucose (C6H12O6) in chloroplasts
• Light energy is composed of 4% Ultraviolet (UV) light, 52% infrared light and 44% visible light used in photosynthesis

Pigments in photosynthesis

• Pigments are molecules that absorb specific wavelengths of light
• Chlorophylls are the primary pigments of which there are two types
  • Chlorophyll a (75% of total chlorophyll) is the primary pigment.
  • Chlorophyll b is an accessory pigment.
• Carotenoids:
  • Carotenes show as orange pigments.
  • Xanthophylls show as yellow pigments.
• Pigments absorb light energy and use it to power photosynthesis.

Photosynthesis process (two stages)

• Stage 1: Light Reactions (light dependent reactions) happen on thylakoid membranes inside chloroplasts
  • Sunlight hits chlorophyll which energizes electrons
  • Water (H2O) is split: Oxygen (O2) is released as a waste product, and hydrogen ions (H+) and electrons (e-) are used in the next stage
  • Electrons travel down the electron transport chain with ATP and NADPH is produced
• Stage 2: Calvin cycle (light-independent reactions / dark reactions) occurs in the stroma
  • Carbon fixation occurs where CO2 is captured from the atmosphere, and energy from ATP & NADPH is used to convert CO2 into glucose
  • The end product is glucose

Summary of Photosynthesis Equation:

• The reactants are carbon dioxide (CO2), water (H2O) and sunlight
• The products are glucose (C6H12O6), and oxygen (O2)

Overview of Cellular Respiration

• Cellular Respiration is a process of breaking down glucose to produce ATP
  • Cellular Respiration happens in the Mitochondria
  • Cellular Respiration requires oxygen (aerobic respiration), producing 38 ATP per glucose molecule

Three stages of cellular respiration

• Step 1: Glycolysis (anaerobic, does not require oxygen) happens in the cytoplasm
  • Glucose (C6H12O6) is broken down into 2 molecules of pyruvic acid which produces ATP and NADH.
  • Net ATP Production: 2 ATP.
• Step 2: Krebs Cycle (Citric Acid Cycle) happens in the mitochondrial matrix
  • Pyruvic acids enter mitochondria, carbon dioxide (CO2) is released, NADH with FADH2 are produced and ATP is generated
  • Net ATP Production: 2 ATP.
• Step 3: Electron Transport Chain (ETS) happens on the inner mitochondrial membrane
  • NADH and FADH2 donate electrons, oxygen (O2) combines with electrons and hydrogen ions to form water (H2O)
  • Electrons power ATP production through oxidative phosphorylation
  • Net ATP Production: 34 ATP

ATP production summary

• Glycolysis yields 2 ATP
• Krebs cycle yields 2 ATP
• Electron transport chain yields 34 ATP
• Total ATP per glucose molecule: 38 ATP

ATP Usage & Metabolism

• ATP is not stored long-term, rather it is constantly produced and used
• Cells generate 10^21 ATP per second
• The body uses its weight in ATP daily

Summary: Photosynthesis vs Cellular Respiration