• James Rickards High School

• BIOLOGY MISC

MITOSIS: INTERPHASE: Stages: G1-growth, SDNA Rep, G2-prep for cell division; Checkpoints:

 

G1-cell size, nutrients, growth factors, DNA

 

damage, G2-DNA Rep, M-chromosomal

 

attachment to spindle; Well-defined nucleus and

 

nucleolus; DNA not visible (chromatin state –

 

DNA-Protein complex) PROPHASE: Chromatin

 

condense to change to chromosome state (visible

 

DNA); Nuclear envelope and nucleolus disappear;

 

spindle forms; centrioles move to poles

 

PROMETAPHASE: spindle fibers attach to

 

kinetochores of centromeres METAPHASE:

 

chromosomes align in metaphase plate (spindle

 

fibers for movement) ANAPHASE: sister

 

chromatids separate at kinetochores; polar

 

microtubules lengthen TELOPHASE: microtubules

 

at opp. Poles; cytokinesis: daughter nuclei form

 

(animals- actomyosin contractile ring; plantsvesicles line up in middle and fuse=> cell plate);

 

chromosomes uncoil; spindle fiber breaks down

 

LEPTOTENE: chromosomes begin to condense. Each chromosome is in a haploid state and

 

consists of two sister chromatids; chromatin of the sister chromatids is not yet condensed

 

enough to be resolvable in microscopy. Homologous regions within homologous

 

chromosome pairs begin to associate with each other. ZYGOTENE: all maternally and

 

paternally derived chromosomes have found their homologous partner. The homologous

 

pairs then undergo synapsis, (synaptonemal complex (a proteinaceous structure) aligns

 

corresponding regions of genetic information on maternally and paternally derived nonsister chromatids of homologous chromosome pairs. The paired homologous chromosome

 

bound by the synaptonemal complex are referred to as bivalents or tetrads. Sex (X and Y)

 

chromosomes do not fully synapse because only a small region of the chromosomes are

 

homologous. PACHYTENE: Chromatin has condensed enough that chromosomes can now

 

be resolved in microscopy. Structures called recombination nodules form on the

 

synaptonemal complex of bivalents. These recombination nodules facilitate genetic

 

exchange between the non-sister chromatids of the synaptonemal complex in an event

 

known as crossing-over or genetic recombination. Multiple recombination events can occur

 

on each bivalent. In humans, an average of 2-3 events occur on each chromosome.

 

DIPLOTENE: crossing-over is completed. Homologous chromosomes retain a full set of

 

genetic information; homologous chromosomes are now of mixed maternal and paternal

 

descent. Chiasmata hold the homologous chromosomes together at locations where

 

recombination occurred as the synaptonemal complex dissolves. Stage where meiotic arrest

 

occurs DIAKINESIS: full chromatin condensation has occurred and all four sister chromatids

 

can be seen in bivalents with microscopy. The rest of the phase resemble the early stages of

 

mitotic prometaphase, as the meiotic prophase ends with the spindle apparatus beginning

 

to form, and the nuclear membrane beginning to break down.

 

MEIOSIS I: PROPHASE: homologous

 

chromosomes pair up to form tetrad

 

(attached at synaptonemal complex);

 

crossing over METAPHASE: tetrads line up at

 

metaphase plate ANAPHASE: tetrads

 

separate TELOPHASE: each pole has nonidentical sister chromatids; 2 cells formed

 

INTERKINESIS: interphase between Meiosis

 

I and II; NO replication; single spindle of first

 

meiotic division disassembles and

 

microtubules reassemble into 2 new

 

spindles for 2nd division

 

Telomeres: Hayflick Limit- a normal human cell can only replicate and divide forty to sixty

 

times before it cannot divide anymore, and will break down by programmed cell death or

 

apoptosis Too much telomerase: cancer Sequence: TTAGGG P53: the following will be

 

affected by a cell containing a nonfunctional copy of the protein: Apoptic Pathways, DNA

 

Repair Pathways, Ability to arrest the Cell Cycle Cdk Inhibitor: stops progression of cell cycle.

 

Telomeres are repetitive nucleotide sequences located at the termini of linear chromosomes

 

of most eukaryotic organisms. For vertebrates, the sequence of nucleotides in telomeres is

 

TTAGGG. Most prokaryotes, having circular chromosomes rather than linear, do not have

 

telomeres. Telomeres compensate for incomplete semi-conservative DNA replication at

 

chromosomal ends. A protein complex known as shelterin serves to protect the ends of

 

telomeres from being recognised as double-strand breaks by in

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