Chapter 8 - DNA Replication, Binary Fission, Mitosis

All cells divide

Unicellular organisms divide in order to reproduce.

Cell division produces a continuous supply of replacement cells in multicellular organisms.

Sexual life cycles

Eukaryotic cells divide by mitosis

A fertilized egg, or $zygote$, divides by $mitosis$.

The fetus grows and develops into a mature adult consisting of countless cells.

Eukaryotic cells divide by meiosis

Each mature individual produces sex cells by another form of division called $meiosis$

$Meiosis occurs only during reproduction$

Gametes fuse during fetilization

Sex cells produced by $meiosis$ contain half the DNA as body cells.

As fertilization, the zygote inherits DNA from both gametes

Mitosis has many roles

$Mitotic cell divisions allow an organism to grow and develop, repair tissues, and regenerate lost body parts.$

Some organisms reproduce asexually by mitosis

Mitosis produces body cells

Many rounds of rapid mitosis convert one cell into trillions

Cell death is part of life

$Apoptosis, or cell death,$ is a process that can carve out distinctive structures during development

DNA replication precedes cell divides

For each of the daughter cells from this division to have identical DNA, the cell must first replicate its $genome, all of the cell’s genetic material.$

DNA replication proceeds in multiple steps

This overview shows that DNA unwinds, each strand binds to complementary nucleotides.

DNA replication builds two new strands

After replication, each DNA molecule has one parental strand and one daughter strand

The DNA helix is unwound

Enzymes called $helicases$ unwind the DNA.

$Single - strand binding proteins hold the strands apart.$

New strands are synthesized

$DNA polymerase$ is the enzyme that makes new DNA strands, starting at the RNA primers

Nucleotides are added to the 3 - prime end

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

Leading strand synthesis is continuous

On one strand, DNA polymerase follows helicase as it unzips the DNA. This is the leading strand.

Lagging strand synthesis is discontinuous

On the other strand, DNA polymerase adds nucleotides in the opposite direction from helicase movement. This is the lagging strand.

DNA replication has many points of origin

DNA replication begins simultaneously at multiple spots along the chromosomes. These spots are called origin of replication.

Prokaryotes divide by binary fission

In prokaryotes, reproduction occurs by $binary fission$ an asexual process that replicates DNA and distributes it to two daughter cells

Prokaryotes have one chromosome

$Prokaryotes have a single circular chromosome$

Prokaryotes replicate their DNA

As the cell prepares to divide, DNA is replicated and attached to the inside of the cell membrane.

Prokaryotes separate two copies of DNA

The cell membrane extends and grows between the two DNA molecules, partitioning them off

Prokaryotes divide into two new cells

The cell divides in half, forming two daughter cells, each identical to the original

Eukaryotes divide by mitosis

Eukaryotic cells do not divide by binary fission. Instead, they divide by $mitosis$, which is similar but more complex

DNA packaging condenses DNA

Naked DNA wraps $histone proteins,$ forming bead like structures called $nucleosomes$

$Nucleosomes cluster together into chromatin. Scaffold proteins within chromatin keep it tightly packed.$

Chromosomes are extra condensed

After DNA is replicated, discrete chromosomes form.

The highly folded chromosomes take up less space and are easier to move than unwound chromatin

The cell cycle is a repeated series of events

Many different events occur in one round of cell division

During interphase, the cell is not dividing

During $interphase,$ the cell replicates its DNA and carries out many functions unrelated to cell division

Interphase includes…

G1 phase

In G1 phase, the cell grow and functions normally. Protein synthesis occurs

G0 phase

During $G0 phase$, the cell continues to function, but it does replicate its DNA or divide. Most of your cells are currently in G0

S phase

In S phase, the cell replicates its entire genome - every chromosome. Now it is committed to divide

G2 Phase

During G2 phase, the cell produce proteins that are necessary for mitosis.

Mitosis is cell division

Cells leaving G2 phase enter $mitosis$ the division of the nucleus, which is following by $cytokinesis$ the division of the cell itself.

Mitosis is a series of steps

Mitosis is divided into five phases, summarized here

Prophase: Chromosomes and spindle form

Prometaphase: Chromosomes attach to spindle

Metaphase: Chromosomes line up

Anaphase: Chromatids separate

Telophase: Two new nuclei form

Cytoplasm splits in cytokinesis

In animal cells, the $cleavage furrow$ first appears in telophase as an indentation of the cell membrane.

It deepens as a ring of proteins beneath the cell membrane contracts.

Cancer cells are abnormal

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 end of their chromosomes. which are celled $telomeres$

The cell cycle is under tight control

A series of chemical checkpoints regulate the cell cycle

Checkpoints ensure that

• all the DNA has replicated
• DNA is not damaged
• Chromosomes line up and separate properly

Cancer cells divide out of control

But what if the body loses control of division?

Cells continue to divide when they do not need to, and override the checkpoints.

A tumor an abnormal mass of tissue

Cancerous cells divide out control and form a mass called a $tumor$

$Tumors are either benign (contained) or malignant (spreadable)$

Cancer arises when genes are mutated

$Mutations alter the function of key cell - cycle control genes.$

Cancer may arise from overactive $proto - oncogenes /un or from underactive tumor suppressor genes$

Cancer risk can vary by ethnicity

Gene mutations such as BRCA1 are inherited, making them more common in particular ethnic groups.

BRCA1 increases the risk of breast cancer

Reducing DNA damage reduces cancer risk

Inherited mutation and factors in the environment that damage DNA can increase cancer risk

Removing tumors is not enough

Surgery may remove rumors, but cells with DNA mutations may already be spreading through the body.

$Drugs (chemotherapy) and radiation can selectively kill cancer calls, which are physiologically different from normal cells.$

$Immunotherapy and gene therapy are the newest approaches that promise to be the most selective.$

Apoptosis is programmed cell death

Apoptosis is a set of coordinated events that dismantle a cell from the inside out.

Apoptosis is closely regulated

A receptor on the cell membrane receives a signal to die

Executioner proteins and enzymes begin to break down the components of the cell

Apoptosis has two functions

$Eliminates excess cells, carving out structures and weeds out aging or defective cells.$