Week 7 BIO1011

Cell Division

Cell division is a process by which a single cell produces 2 daughter cells.

• In order to divide successfully, it must be large enough and contribute sufficient nuclear and cytoplasmic components to each daughter cell.

• Before cell division, key cellular components such as DNA are duplicated.

Binary Fission 


Prokaryotes produce daughter cells by binary fission.

• In this form, cell replicates DNA-> increases in size -> divides into 2 daughter cells.

• Each daughter cell receives 1 copy of the replicated parental DNA.

• This process is similar to archaea, chloroplasts, mitochondria, organelles within plants/fungi and animal cells that evolved from free-living prokaryotic cells.

Example- intestinal bacterium Escherichia-Coli:

• Circular genome is attached by proteins to the inside of the cell membrane.

• Replication is initiated at a specific location on the circular DNA called the "Origin of Replication" which proceeds in the opposite direction around the circle.

• Results- 2 DNA molecules, each attached to the cell-membrane at a different site.

• Initially close together -> cell elongates -> 2 DNA attachment sites move apart.

• When cell is 2 times its original size and DNA molecules are separated which the constriction forms midpoint in cell (inward pinching) where new membrane and cell wall are synthesised and divides into 2 cells.

FtsZ protein 


• Several genes play key roles in binary fission such as FtsZ.

• FtsZ encodes a protein that forms a ring at the site of constriction where the new cell wall forms b/w 2 daughter cells.

• Present in genomes of diverse bacteria and archaea and its evolutionarily related to the protein tublin.

• Tublin makes up micro-tubes found in Eukaryotic cells-important in intracellular transport, cell movement and division.

Mitotic cell division 


• They first divide the nucleus by mitosis and then divide the cytoplasm into 2 daughter cells by cytokinesis- mitotic cell division.

• The genetic material is packaged into chromosomes, consisting of a single DNA molecule and associated proteins.

• Compared to prokaryotic cells, Eukaryotic is typically much larger and organised into 1 or more linear chromosomes.

• Eukaryotic cells have their DNA located in the nucleus so its cell division requires the breakdown and reformation of the nuclear envelope as well as other mechanisms to separate replicated DNA.

Mitosis evolved from Binary Fission

• However, some unicellular eukaryotes can have characteristics of binary fission such as dinoflagellates.

• While it has a nucleus and linear chromosomes, the nuclear envelope does not break down and stays in tact during cell division.

• The replicated DNA attaches to the nuclear envelope and grows/divides in a manner reminiscent of binary fission. ted DNA attaches to the nuclear envelope and grows/divides in a manner reminiscent of binary fission.

Cell Cycle proceeds in phases 


The cell cycle consists of 2 phases: M-Phase and Interphase

• During interphase, cell grows, replicates its chromosomes, and prepares for cell division

• During M-Phase, the parent cell divides into 2 daughter cells.

• 2 events happen in the M-Phase: Mitosis, separation of chromosomes into 2 nuclei and cytokinesis, the division of the cell itself into 2 separate cells.

Semiconservative DNA Replication

• Basic steps are universal, suggesting they evolved in a common ancestor of all life.

• This means that when DNA replicates (copies itself), each new DNA molecule has one original (parental) strand and one new (daughter) strand.

Why Replication Must Start in Multiple Places?

• Eukaryotic DNA replication is slow (~50 nucleotides/sec).

• At this rate, it would take 2 months to replicate the largest human chromosome.

• In reality: it takes only a few hours due to multiple origins of replication.

Replication Bubbles

• Each origin of replication forms a replication bubble with two replication forks.

• Each fork has a leading and lagging strand.

• Helicase, topoisomerase II, and single-strand binding proteins assist.

• As forks move in opposite directions, the bubble expands.

• When bubbles fuse, DNA ligase joins the meeting strands.

Circular DNA Replication

• Seen in bacteria, mitochondria, and chloroplasts.

• Circular DNA has one origin of replication.

• Replication forks move in opposite directions until they meet on the other side.

Problem at the End of Lagging Strand

At the very end of a linear chromosome:

• The last RNA primer is placed about ~100 base pairs (bp) away from the end.

• Normally, DNA polymerase removes RNA primers and replaces them with DNA.

• But after the last primer is removed, there's no upstream 3' OH group for DNA polymerase to extend from.

Result:

• That last ~100 bp cannot be filled in.

• The new daughter strand ends up shorter than the parental strand.

• This leads to the gradual shortening of chromosomes with each round of replication.

Solution: Telomerase

• Telomeres are repeating sequences (e.g., TTAGGG in humans) at ends of chromosomes.

• Telomerase has an RNA template to restore lost sequences.

• It extends the template strand, allowing synthesis of the complementary strand.

• No genes in telomeres, so slight shortening has no harmful effect.

Telomerase Activity in Cells

• High in germ cells (sperm/egg producers) and stem cells (self-renewing & differentiating).

• Low or inactive in most adult somatic cells.

• In adults: telomeres shorten ~100 bp per division, limiting cells to ~50 divisions.

Telomeres and Aging

Short telomeres may explain:

• Aging of tissues,

• Slower healing,

• Reduced cell division.

• Known as the telomere hypothesis of aging (still debated).

• In cancer cells, telomerase is often reactivated, enabling uncontrolled growth.

Purpose of Mitotic Cell Division

• Mitosis (division of the nucleus) + cytokinesis (division of the cytoplasm) = mitotic cell division.

  It allows:

• Asexual reproduction in unicellular eukaryotes.

• Growth, repair, and maintenance in multicellular organisms.

• Ensures that each daughter cell gets an identical set of chromosomes.

Chromosomes and DNA packaging

• DNA is very long (up to 2 m!) and must be highly condensed to fit into the nucleus and separate correctly during division.

• Each chromosome contains one long DNA molecule.

• When visible under a microscope during mitosis, chromosomes can be recognized by their distinctive shapes and sizes, forming a karyotype.

Human Chromosomes

• Humans have 46 chromosomes (23 pairs):

• 22 autosome pairs (numbered by size).

• 1 pair of sex chromosomes (XX for females, XY for males).

• Each pair contains homologous chromosomes—one from each parent.

• Ploidy refers to the number of complete chromosome sets:

• Diploid (2n) = 2 sets (e.g., humans).

• Haploid (n) = 1 set (e.g., gametes).

• Polyploid = 3 or more sets (common in plants).

S Phase and Sister Chromatids

• During S phase, all chromosomes are duplicated, forming sister chromatids (identical copies) joined at the centromere.

• The number of chromosomes = the number of centromeres.

Stages of Mitosis (M phase)

Prophase – Chromosomes Condense

• Chromosomes become visible and compact.

• Centrosomes (in animals) move to opposite poles and start forming the mitotic spindle (microtubules that help move chromosomes).