DNA Replication and Binary Fission

DNA Replication

• DNA must be copied or replicated before cell division
• S phase defines DNA copying before cell division.
• S phase is semi-conservative

Key Enzymes Involved:

• Helicase unwinds DNA.
• Topoisomerase relieves supercoiling during DNA replication.
• SSBs stabilize DNA strands during replication.
• Primase adds RNA primers to start replication.
• DNA Polymerase extends the DNA strand.
• Ligase joins DNA fragments together.

Strands

• Leading strand is continuous.
• Lagging strand consists of Okazaki fragments and is discontinuous.

Repairing

• DNA polymerase fixes mismatch errors that may occur during replication.
• Mismatch repair corrects remaining mistakes.

Binary Fission

• Binary fission is asexual reproduction in prokaryotes.
• Binary fission leads to identical daughter cells.
• A single circular chromosome replicates
• Cell membrane separates the two DNA Copies

Steps of Binary Fission

• DNA replication, chromosome segregation, and then cytokinesis
• Binary fission is simpler in prokaryotes, whereas mitosis is more complex and occurs in eukaryotes

Prokaryotes and New DNA

• Prokaryotes such as Bacteria and Archaea divide asexually through binary fission
• Prokaryotes can get new DNA via sex pilus, a hairlike appendage
• A donor cell can transfer genes directly to a recipient cell
• Healthy cells can absorb DNA from dead and dying cells and incorporate it

Mitosis

• Mitosis is for growth, repair, and asexual reproduction in diploid daughter cells
• The cell cycle in mitosis moves from interphase (G1, S, G2) to the mitotic phase (M)

Mitosis Stages

• Prophase involves chromosome condensation and spindle formation.
• Metaphase involves chromosome alignment.
• Anaphase involves separation of sister chromatids.
• Telophase involves reformation of the nuclear envelope
• Cytokinesis involves cleavage furrow in animal cells and cell plate formation in plant cells.
• Key aspects consist of growth, tissue repair, and asexual reproduction

Sexual Life Cycle

• Production of of somatic cells, where each mature individual produces sex cells called gametes, sperm, and egg cells with a cell division call meiosis.
• Fertilization consists of the fetus grows and develops into a mature adult with countless cells via mitosis.
• Meiosis occurs only during reproduction.
• Gametes, or sex cells, contain half the DNA of somatic cells and are genetically different from their parents.
• The zygote inherits DNA from both gametes at fertilization.

Apoptosis

• Both mitosis and apoptosis are necessary for growth and development of an organism
• Apoptosis limits growth and replication of cells to maintain homeostasis, balance, and to get rid of cancer and old cells
• Apoptosis carves out distinctive structures during development.

DNA Replication Particulars

• cells must copy (replicate) their DNA before they can divide
• DNA replication has to happen before cell division can
• The entire genome is replicated to ensure each daughter cell gets a complete copy of the DNA
• DNA is made of two chains of nucleotide strands
• Each strand acts as a template to build a new molecule of DNA.
• DNA has complementary base pairs
• DNA in short is replication is semiconservative

Nucleotide

• A nucleotide is an organic molecule that is the building block of DNA and RNA
• A nucleotide comprises a phosphate group, a 5-carbon sugar, and a nitrogenous base.
• DNA molecules have one parental strand and one daughter strand after replication

DNA and Replication

• Helicases unwind the DNA double helix

Proteins

• Single-Strand Binding Proteins (SSBP; BP) hold the strands apart
• RNA primers match up with DNA

RNA Primase

• An RNA primase enzyme adds a short RNA strand to each template strand
• This starting point is for DNA polymerase
• DNA synthesis requires a pre-existing strand.
• DNA polymerase synthesizes new DNA strands, using RNA primers as a starting point.
• Nucleotides are added to the 3-prime end.

3-Prime

• The RNA primers provide 3 prime ends, where DNA polymerase adds the nucleotides

Leading/ Lagging

• Leading strand synthesis is continuous, while lagging strand synthesis is discontinuous
• DNA polymerase follows helicase as it unzips the DNA (leading strand)
• On the other strand, DNA polymerase adds nucleotides in the opposite direction from helicase movement (lagging strand).

Ligases

• DNA ligases fill in the gaps

DNA Ligase

• DNA polymerase follows helicase and synthesizes new DNA in the 5' to 3' direction
• DNA polymerase moves in the opposite direction to helicase on the other strand
• DNA polymerase synthesizes DNA in short segments, called Okazaki fragments, and has to work in short segments due to only being able to synthesize in 5' to 3'
• Each Okazaki fragment starts with an RNA primer laid down by RNA primase.
• DNA polymerase extends the fragment by adding DNA nucleotides. DNA polymerase I removes the RNA primers - then DNA ligase joins the Okazaki fragments together

DNA Origins

• DNA replication begins simultaneously at multiple spots called origins of replication, along the chromosomes.
• a replication bubble is an unwound and open region of DNA where DNA replication occurs
• Occurs simultaneously in both directions taking about 8-10 hours: It is semiconservative because each strand retains one strand. Any change is called (mutation).

Differences in Helicase and RNA Polymerase

• Helicase unzips the DNA double helix for DNA replication.
• RNA Polymerase synthesizes RNA from DNA during transcription.
• Check chart provided above.

Eukaryotic Cell Division

• Eukaryotic cells do not divide by binary fission but by mitosis, which is more complex
• Replicated chromosomes condense before eukaryotic cell division.
• When the cell is not dividing, its DNA is loose in the nucleus
• the cell has condensed into visible chromosomes when the cell begins to divide its DNA becomes packaged to protect it.

Chromosomes

• Chromosomes can be called "Condensed Chromatin", Either 2 sister chromatids or 1 chromatid
• Are chromosomes are visible during cell division

Cell Cycle

• The cell cycle includes interphase and mitosis and is a repeated series of events.
• It lasts from one cell division until the next and includes the time when cells divide and when cells do not

Interphase

• Interphase includes G₁, S, and G2 phases

G1 Phase

• The cell grows, functions normally, and synthesizes proteins
• moves to the S phase if preparing for division, remaining in the G1 if otherwise.

G0 Phase

• A resting state which has the cell's functions but does not replicate DNA or divide. Most cells are in G0

S Phase

• DNA replication occurs that doubles the chromatids, for a total of 92 (46 pairs of sister chromatids)
• Once begins, the cell is committed to division

G2 Phase

• The cell produces proteins essential for mitosis

M Phase

• The nucleus divides and is followed by cytokinesis, which splits the cytoplasm

Cell Under Tight Control checkpoints.

• A series of chemical checkpoints regulate the cell cycle
• Checkpoints ensure all the DNA is replicated correctly, that the DNA is not damaged, and that chromosomes line up and separate properly
• M stands for Metaphase and G stands for Gap
• The Phases include: G1, S, G2, and M

Cancer

• Cancer cells divide out of control and is uncontrolled.

Mitosis Generations

• Generates exact cell copies

Mitosis Phases

• Prophase
• Metaphase
• Anaphase
• Telophase

Mitosis Particulars

• chromosome condense and become visible
• The Nucleolus vanishes and the nuclear envelope breaks, and spindle fibers attach to chromosomes.
• Division of Cytoplasm from G2-E1
• Chromsome lines midline
• Sister chromatids form
• cells can resume normally

Cytoplasm

• Is splitting also known as Cytokinesis

Cancer Arises

• Cancer arises when genes are mutated
• Mutations alter the function of key cell-cycle control genes.

Proto-oncogene

• a normal gene that controls cell growth and division
• Mutated oncogenes are a mutated form of that gene that can cause cancer where it may cause cells to divide rapidly. Proto-oncogenes tell the cells when, how and how fast to grow.

Cancer Development

• Cancer may arise from overactive proto-oncogenes or underactive tumor suppressor genes that suppress irregular growth.

Cancer Cells

• Continue to divide when they do not need to and override the checkpoints
• Cancerous cells divide out of control and form a mass called a tumor, where tumors can be benign (contained) or malignant (spreadable).

Cancer Treatments

• Surgically remove tumors and include Anticancer drugs (chemo) that slow or block cell division as well as Radiation that kills targeted tumor cells

Mutations to avoid

• Surgery may remove tumors, but cells with DNA mutations may already be spreading through the body
• Immunotherapy and gene therapy are the newest approaches that promise to be the most selective

Cancer Cells are Abnormal.

• Removing tumors is not enough
• Compared to regular body cells, cancer cells have distinct features:
• They lose specialization
• They are "immortal" in that they can continue to divide endlessly.
• They can regenerate the ends of their chromosomes (Telomeres).

Adaptive Therapy

• Is a new approach

Apoptosis

• limit their growth and replication of cells, that keep homeostasis and balance by getting rid of cancer and old cells
• process that can carve out distinctive structures during development

Apoptosis is programmed cell death as a coordinated set

• A Death receptor on the cell membrane receives a signal to die
• Executioner proteins and enzymes begin to break down the components of the cell
• The immune system cell eliminates excess cells, carving out structures

Reducing Cancer is possible by reducing DNA Damage

Risk Factors and Ways to Reduce Risk

• Unhealthy Diet: Eat less saturated fat; eat more fruits and vegetables
• Obesity: Maintain healthy body weight; get regular, vigorous exercise
• Tobacco Use: Quit smoking or chewing tobacco, or never start.
• Environmental Toxins: Minimize exposure to harmful chemicals at home and at work
• UV Radiation: Avoid UV radiation from sunlight and tanning beds
• Cancer-causing Viruses: Use condoms to avoid sexually transmitted infections
• Detection Time:Use self-tests and medical exams for early detection
• Age: Cannot be avoided.