Biology Chapter 2 - The Cell Cycle

Amount of chromosomes in human cells

46 chromosomes - 23 homologous pairs. n represents amount of DNA in a haploid set of chromosomes (only in gametes). Mostly human cells are 2n, where the cell has two chromosomes per set, one from mother and one from father, but they go up to 4n during cell division

Chromosome structure

Two strands of DNA nucleotides coiled into a double helix, wrapped around histone proteins and coiled/stacked into chromosomes

Nucleotide

pentose sugar, phosphate group, nitrogenous base

Complementary base pairing rule

Adenine - Thymine

Guanine - Cytosine

Universal for all organisms

Cell division occurs to...

create a new generation of single cells, single celled organisms or multicellular organisms, or to grow or repair multicellular organisms

Cells with long vs short lifespans

Short - skin, inner cheek, growing plant cells

Long - neurons (rarely if ever are replaced)

Why must genetic instructions be copied for cell division?

So that both daughters receive all the DNA they need for normal cell functioning to occur

How is DNA copied?

Chromosomes replicate when the 'double stranded' DNA molecule 'unzips' and makes a copy of each side of the DNA molecule, creating two identical molecules, each called a chromatid. They stay together until later in mitosis

DNA replication diagram

Chromosomes either consist of a single chromatid, or two chromatids once duplicated. Chromosomes that consist of one chromatid are one complete chromosome as well

The cell cycle

The series of events that take place from one cell division to another. Includes phases Interphase, Mitosis and Cytokinesis.

Interphase

The stage between nuclear divisions; a period of active growth and normal cell functions. Includes G1, S and G2 phases

G1 phase

Growth and normal cell functions, cellular contents excluding chromosomes (proteins and organelles) are duplicated

S phase

DNA replication, all 46 chromosomes duplicated

G2 phase

Growth and preparation for mitosis

Cell cycle picture

G0 phase

When cells are not actively dividing and instead perform normal cell functions. Can re-enter cell cycle under certain circumstances

How often do different cells divide?

Specialised/differentiated cells - Rarely

Skin, root tips - Every 24 hours

Prokaryotes/bacteria - Every 20 minutes

Mitosis

Division of the nucleus in somatic eukaryotic cells. Made of 4 phases: Prophase, Metaphase, Anaphase, Telophase

Prophase

- Chromosomes condense to be visible

- Chromatids held together by centromere

- Spindle forms

- Nucleolus and nuclear envelope dissapear

Metaphase

Chromosomes line up in cell centre (metaphase plate), attached to spindle by centromere

Anaphase

Centromere divides and chromatids are pulled towards separate poles of the spindle. Separated they are now called chromosomes.

Telophase

- Spindle disassembles

- New nuclear membranes form around each chromosome group

- Chromosomes de-condense

Cytokinesis

Follows mitosis; cytoplasm divides and new nuclear membranes form to enclose new cells. Often occurs concurrently with telophase

Cytokinesis in plants vs animals

Plants - New cell wall called cell plate forms in between two cells

Animals - cytoplasm divides by cleavage, where a cleavage furrow gets deeper and deeper until it meets in the middle and the cell splits.

Binary fission

Prokaryotic bacteria cells replicate their single circular chromosome (nucleoid). A wall forms across the cell to divide it.

Apoptosis

Programmed cell death using dozens of genes in an orderly, active and vital process for normal development and the removal of unneeded or diseased cells

Process of apoptosis

1. DNA shreds and nucleus condenses

2. Bleb formation occurs as cell membrane separates from cytoskeleton

3. Apoptotic bodies break away and are engulfed and destroyed by macrophages

Failure of apoptosis

Mutations in apoptosis genes can lead to cells not dying and instead replicating uncontrollably, leading to cancerous tumours

When are the 3 checkpoints of the cell cycle?

G1 - In G1 phase, prior to S phase

G2 - At the end of G2 phase

Mitosis - In the metaphase/anaphase transition

Gatekeepers of the cell cycle

Cyclins and cyclin-dependent kinases (CDKs) regulate the cell cycle. Cyclins and some CDKs promote cell progression, while other CDKs inhibit and stop the cycle

G1 checkpoint

Checks for cell size, nutrients, growth factors and DNA damage. Errors result in CDK inhibitors stopping cycle. DNA damage is repaired or apoptosis initiated.

G2 checkpoint

DNA is screened for damage or errors in replication and cell size is checked. If any errors are found, CDKs can pause for DNA to be repaired, or start the apoptosis process.

Mitosis checkpoint

Checks for defects in chromosome attachment to spindle and alignment. If errors can't be solved apoptosis is triggered. Errors in this checkpoint may lead to daughter cells having fewer or extra chromosomes.

Proto-oncogenes

Accelerate cell division. Code for proteins that stimulate cell division stop cell differentiation and regulate apoptosis. Mutations produce oncogenes

Oncogenes

Permanently switched on proto-oncogenes that cause uncontrolled cell division by coding for proteins that stimulate excessive cell division and an inhibition for apoptosis.

Tumour-suppressor genes

Inhibit cell division by coding for proteins to slow it down, repair DNA and promote apoptosis. Typically they prevent the cell from dividing too quickly to form cancerous tumours. Are mutated/silenced in cancer cells

p53 gene

A type of tumour-suppressor gene that is activated when DNA damage or cell injury occurs. If damage occurs early in cycle, p53 initiates repair by enzymes. If late in cycle, p53 initiates apoptosis. Mutation leads to cancer

Genetic predispositions to cancer

Caused by harmful inherited mutations

Mutagens

Chemical, physical or biological agents that can induce or increase the frequency of mutation in DNA (e.g. can mutate proto-onco into oncogenes)

Mutagen types (with examples)

Chemical - Chemicals that can interfere with DNA, e.g. tobacco contains carcinogenic (cancer-causing) mutagens

Physical - UV, X- and nuclear radiation

Biological - Viruses insert their DNA into proto-oncogenes or tumour-suppressor genes to mutate them, leading to virus proteins being produced and cell cycle disrupted. E.g. papillomavirus

Stem cells

Undifferentiated (not yet specialised) cells with the potential to be many different types of cell

Potency of stem cells

Totipotent - total potential

Pluripotent - many cell types

Multipotent - a few similar cell types

Morula

Occurs 4 days after fertilisation. Totipotent cells formed after first few divisions of zygote. Have the potential to become any type of cell including embryo, placenta and amniotic sac

Blastocyst

Occurs after 5 days from fertilisation. Pluripotent ball of cells that can become most cells in the foetus (only can't be placenta), surrounded by an outer layer of cells that will become the placenta.

Specialisation

Pluripotent/totipotent cells specialise into multipotent cells, which give rise to cells with a particular function. For example, multipotent blood stem cells in the bone marrow can become RBCs, WBCs or platelets

How are germ layers developed?

When the the blastocyst implants in the uterus, the inner mass forms 3 layers of cells. Happens around day 12

3 germ layers

ectoderm, mesoderm, endoderm

Germ layer image

Endoderm

Will develop into organs like lung, thyroid and digestive cells

Mesoderm

Will develop into muscle cells, RBCs and tubule kidney cells

Ectoderm

Will develop into skin cells, neurons, and pigment cells

Ways stem cells can be used for treating damaged tissues

- Umbilical cord and placenta multipotent stem cells (makes blood cells)

- Pluripotent embryonic stem cells (makes any bodily cell types)

- Adult multipotent stem cells (useful as body doesn't reject its own stem cells)