5- The Mitotic Cell Cycle

Hayflick limit

max number of times a population of cells can divide before telomeres are lost completely

chromatin

combination of DNA and associated proteins (mostly histone)

histone proteins

positively charged globular proteins

organise and condense the DNA tightly so tat it fits into the nucleus

sister chromatids

2 identical lengths of DNA that make up chromosome following DNA replication

revert back to being chromosomes at anaphase

autosomes

any chromosomes that aren’t sex chromosomes

homologous chromosomes

carry the same genes at the same locus (position), maybe with different alleles

one chromosome in each pair from each parent

X/Y pair is not homologous

importance of mitosis in the production of genetically identical daughter cells

growth of multicellular organisms

replacement of damaged or dead cells

repair of tissues by cell replacement

asexual reproduction

1st stage of interphase

G1

cell contents replicated (excl. nucleus contents and centrosomes)

cells make RNA, enzymes and other proteins required for growth

2nd stage of interphase

S- synthesis

DNA is replicated (short phase)

centrosome replicated

triggered by signal recieved via cyclins telling cell to divide again

3rd stage of interphase

G2

cell continues to grow

new replicated DNA is checked and errors repaired

other preparations made for cell division (e.g. tubulin production for microtubules)

(Early) Prophase

chromosomes start condensing (chromatin coils up)

Prometaphase (Late prophase)

nuclear envelope breaks down into small vesicles

nucleolus breaks down (forms parts of several chromosomes)

spindle forms as centrosomes begin to move to opposite ends of the nucleus

chromosomes are seen to consist of 2 identical chromatids

metaphase

centrosomes have reached opposite poles

chromosomes line up across equator of cell, attached to spindle via kinetochores attached to centromeres

each chromosome splits at the centromere

anaphase

chromatids move to opposite poles, centromeres first, pulled by microtubules

Telophase

chromatids have reached poles of spindle, and begin to decondense

nuclear envelope and nucleolus reform

spindle breaks down

cytokinesis

cytoplasm and cell surface membrane are divided by the contraction of a contractile ring

made from actin filaments and myosin (a motor protein)

a cleavage furrow is formed between the nuclei, followed by the final separation- abcission

cytokinesis in plant cells

vesicles appear along equator of cell, growing to form new cell membrane and wall

euchromatin

chromatin found in coding DNA, loosely packed, has acetyl groups

heterochromatin

chromatin found in telomeres and centromere, non-coding so more tightly packed

centromere

central part of a chromosome made from heterochromatin, joining 2 sister chromatids

centrosome

2 centrioles at right angles to each other, surrounded by pericentriolar matrix (PCM)

it is a microtubule organising centre (MTOC)

centriole

cylindrical tubular (made out of microtubules) organelle that composes part of the centrosome

which monomer is used to build microtubules

tubulin

kinetochore

protein structure attached to the centromere

bind to kinetochore microtubules in spindle

collar-like structure allows polymerisation and depolymerisation of tubulin so microtubules can grow and break down while remaining attached

significance of microtubule polarity

+ ends of microtubules more dynamic, de/polymerisation occurs more rapidly

-ends at the pole (centrosome) less active, de/polymerisation less rapid and frequent

types of microtubules in spindle

astral- attaches to cell membrane, positioning centrosomes at poles

interpolar- attach to other microtubules from opposite pole, positioning centrosomes, can extend to push centrosomes apart during mitosis

kinetochore- attaches to kinetochore of chromosomes, pulls chromatids to opposite poles of cell

telomeres def

sections of non-coding DNA at the ends of chromosomes that prevent degradation following DNA replication and loss of coding genes.

made up of short repeating sequences rich in guanine and cytosine.

why are telomeres needed

in order to bind and replicate the lagging strand, DNA polymerase requires a 3’ OH group, found on the end of every DNA and RNA nucleotide, so cannot replicate the final nucleotides.

without telomeres coding DNA would be lost with every DNA replication

how do telomeres prevent the degradation of DNA?

enzyme telomerase is associated with an RNA strand that is complimentary to unreplicated 3’ overhang

associated RNA is used to synthesize more of the leading strand, a (non-coding) repeating sequence rich in guanine

telomerase shifts and repeats this process to extend overhang

DNA polymerase can now bind to non-coding leading strand sections, synthesizing the lagging strand.

stem cell

undifferentiated cell that can divide an unlimited number of times, with the ability to become a specialised cell

totipotent

can differentiate into any cell in the body (i.e. zygote and first few embryonic cells)

pluripotent

can differentiate into all cells excluding extra-embryonic tissues (umbillical chord and placenta)

multipotent

can differentiate into a select few cells (e.g. blood cells)

role of stem cells

stem cells are found in embryos and are necessary for the development of all tissues and organs in the body

some adult stem cells remain and are multipotent, used to produce new cells for the essential processes of growth, repair and replacement.

e.g. stem cells in adult bone marrow can only differentiate into blood cells. This is necessary because many blood cells e.g. erythrocytes cannot self-replicate, so need constant replacement

cancer

a group of diseases caused by uncontrollable cell division, due to a breakdown of the mechanisms that regulate cell division

how does a tumour form and spread

1. oncogenes (mutated gene) are transformed by carcinogens

2. cancerous cell doesn’t respond to mitosis regulating signals, continues to divide by mitosis

3. cancerous cells not removed by the immune system, rapid mitosis

4. tumour enlarges, cells change characteristics (observable change under microscope)

5. tumour supplied with blood and lymph vessels, tumour cells spread in blood and lymph to other parts of body

6. metastasis- tumour cells invade other tissues, secondary cancers form