IBHL Y1 Biology Unit 8

What is DNA packaging?

DNA is packaged together in order to fit all of the DNA within the nucleus of the cell.

8 histone proteins: structural core proteins are wrapped together by a nucleosome: a stretch of DNA, and held together by H1: a 9th histone protein.

Linker DNA connects multiple neighboring nucleosomes.

DNA is further packaged into chromatin: loosely packaged DNA found in non-dividing cells, and when the cell divides, the chromatin condenses into a chromosome: a tightly wound DNA in dividing cells.

When the DNA packs into chromosomes, it creates an X shaped structure of a duplicated chromosome made of 2 sister chromatids tethered together at the centromere.

Tight packaging of chromosomes make the gene inaccessible, and there is no transcription or gene expression. Looser packaging=more gene expression.

What is cell division?

Unicellular organisms: used for asexual reproduction of a new organism where the new cells are genetically identical to the original cell and each other.

Multicellular organisms: used for cell proliferation to ensure continuity of genetic information across all cells within an organism, and it ensures a high SA:V ratio to allow for specialization and exchange of nutrients.

1. Growth of a cell to increase the size and complexity (EX: embryonic development)

2. Cell replacement to maintain healthy tissues and replace dead cells that are lost (EX: skin cells)

3. Tissue repair to repair damaged tissues. (EX: wound cuts)

What is interphase of the cell cycle?

1. Gap1: cell doubles in size, undergoes normal metabolic functions such as protein synthesis, and mitochondria divide using binary fission.

2. Synthesis: DNA made of chromatin is replicated semi-conservatively within the nucleus

3. Gap2: cell continues to grow and prepares microtubuals: proteins for cell division.

Then, the mitotic phase occurs with mitosis and cytokinesis.

What is the mitotic phase?

One round of division where DNA is found as chromosomes, and Mitosis divides the nucleus while Cytokinesis divides the cytoplasm, creating two genetically identical daughter cells.

This does not create genetic diversity.

What is prophase of mitosis?

1. Chromatin condenses into (duplicated) chromosomes.

2. The nuclear membrane breaks down.

Plant cells use microtubual organizing centers/MTOCs to organize the spindle fibers/mitotic spindle: made of microtubuals while animal cells use 2 centrioles called centrosomes.

MTOC’s migrate to the poles of the cell.

What is metaphase of mitosis?

1. The duplicated chromosomes/sister chromatids line up along the metaphase plate at the equator of the cell

2. Spindle fibers attach to the kinetochore: motor proteins that are located in the centromere region facilitated by the mitotic spindle.

What is anaphase of mitosis?

Spindle fibers split the centromere, separating the sister chromatids as they move away from each other towards the poles of the cell, becoming unduplicated chromosomes.

What is telophase of mitosis?

1. The chromosomes decondense into chromatin

2. The nuclear membrane reforms around the two new nuclei

3. Spindle fibers disassemble

What happens during cytokinesis?

The division of the cytoplasm occurs differently in animal and plant cells because of the absence of a cell wall in animal cells.

Animal cells: actin and myosin proteins form a contractile ring at the center of the cell, which pinches the cell membrane and forms a cleavage furrow that deepens and splits the cell into to 2.

Plant cells: vesicles carrying cell wall material assemble into the cell plate which grows outwards and forms the cell wall between the 2 daughter cells.

How do prokaryotes divide?

Prokaryotes create 2 daughter cells and divide by binary fission

1. DNA replication

2. Cell elongation

3. Cytoplasm divides

How is the cell cycle regulated?

The cell cycle is regulated in order to maintain healthy tissue.

G1: determines if the cell will continue onto the rest of the cell cycle

• G0 will cause the cell to exit the cycle and not prepare to divide. Some cells can re-enter the cycle, others cannot.

G2: determines how much the cell has grown, if DNA has replicated fully, and if the cell has produced enough organelles, proteins, and energy in preparation for cell division.

• “Go” signal sends the cell into mitosis

M: occurs during the metaphase of mitosis, and checks to make sure all chromosomes have attached to spindle fibers and and lined up at the metaphase plate.

• “Go” Signal sends the cell into anaphase.

What are signals that regulate the cycle?

External regulators are responses to events outside of the cell, directing cells to speed up or slow down the cycle.

Growth factors: stimulate the growth and division of cells (EX: embryo development and wound healing)

Anchorage dependence: a concept that animal cells need in which they must be attached to substratum in order to divide.

Density dependent inhibition: crowded cells stop dividing, which prevents excess cell division.

What other signals control the cell cycle?

Internal regulators are a response to an event occuring inside a cell and allows the cycle to proceed when certain events have occured.

Cyclins: proteins that bind to cyclin dependent kinases: enzymes that phosphorylate proteins to progress the cell cycle, to create a cyclin-cdk complex.

• Different cyclins accumulate at different stages, thus activating different CDKs at different times and these fluctuations control the cycle, but the cyclin much reach a critical concentration in order for the cell to progress to the next stage

• Cyclin E triggers DNA before the S phase

• Cyclin B triggers mitosis

What causes uncontrolled cell division?

Proto-oncogenes are genes that code for proteins to promote cell growth and division, but mutations in proto-oncogenes leads to a breakdown in the cell control cycle and over-expression, becoming an oncogene leading to uncontrolled cell division.

Tumor suppressor genes are genes that code for proteins that slow down or prevent the cell cycle possibly triggering apoptosis: programmed cell death. Mutations in this gene leads to the absence of the protective function of this protein, malfunctioning it, and causing uncontrolled cell division.

What is the result of uncontrolled cell division?

Uncontrolled cell division leads to the accumulation of an abnormal mass of cells called a tumor.

Benign tumors: non-cancerous mass that grows slowly, may or may not cause issues.

malignant tumors: cancerous mass that grows rapidly, causing issues by metastasizing: spreading from the primary tumor to other parts of the body, becoming a secondary tumor.

Can be treated by surgery, radiation, or chemotherapy.

Chromosome terminology

Diploid cells: Somatic: body cells that contain 2 copies of each chromosome: pairs of homologous chromosomes: same length, same location, and same centromere location.

Haploid cells: Gametes: sex cells that contain 1 copy of each chromosome.

(A duplicated chromosome is 1 chromosome made of 2 sister chromatids, DNA replication increases number of chromatids not chromosomes.)

• Autosomes: homologous chromosomes 1-22 in humans

What is meiosis?

Meiosis is a reduction cell divison that takes 1 diploid cell, separates the homologous chromosomes, and creates 4 genetically unique haploid cells that must occur after DNA replication, and must have a parent diploid cell with duplicated chromosomes.

What is prophase I of meiosis I?

1. The nuclear membrane breaks down

2. Spindle fibers begin to form

3. Microtubual organizing centers/MTOCs move away from each other towards opposite poles of the cell

4. Chromatin condense into chromosomes, and homologous chromosomes pair up during synapsis creating bivalents/tetrads

5. Crossing over occurs by non-sister chromatids exchanging equivalent segments of DNA at the chiasmata: the x shaped region, creating recombinant chromatids that are genetically different.

What happens during metaphase 1 of meiosis 1?

1. Bivalents line up along the metaphase plate

2. Each bivalent pair lines up independently of the other pairs, showing random orientation and independent assortment towards the poles.

3. Spindle fibers attach to the kinetochores on either side of the homologous pairs.

What happens during anaphase I of meiosis I?

1. Spindle fibers move the homologous chromosomes away from each other, but sister chromatids remain connected to each other at the centromere

What happens during telophase and cytokinesis I?

1. The homologous chromosomes have reached the poles of the cell, and they decondense into chromatin.

2. The nuclear membrane re-forms around the haploid nuclei

3. Spindle fibers break down.

4. Cytokinesis divides the cytoplasm producing two daughter cells that are haploid, duplicated chromosomes, and genetically unique.

Interkinesis occurs, where NO DNA replication occurs.