Cell Cycle and Cell Reproduction

Cell Cycle

The order in which a cell undergoes Interphase (G1, S, and G2) and the M phase (Mitosis or Meiosis)

Interphase

The beginning of the cell cycle, G1, S, and G2

Mitosis Phases

Prophase, metaphase, anaphase, telophase (PMAT)

Cytokinesis

When the cytoplasm divides (The last step before two full cells are formed)

G1 phase

The stage in interphase where the cell doubles its organelles and accumulates material for DNA synthesis

G0 phase

This occurs only in cells which do not complete the cell cycle (Examples are nerve and muscle)

S phase

Growth and DNA replication (this results in each chromosome consisting of two sister chromatids or a dyad)

G2 phase

The cell synthesizes proteins for cell division

Chromosome

A thread-like structure within a cell's nucleus, made of tightly coiled DNA and proteins (histones), that carries an organism's genetic information, organizing genes that determine traits

Dyad/Sister Chromatid

A chromosome structure consisting of two sister chromatids joined at a single centromere

G1 checkpoint

If DNA is damaged and cannot be repaired than apoptosis will occur here. Otherwise, the cell will continue to divide.

G2 checkpoint

If DNA is damaged and cannot be repaired than apoptosis will occur here. Otherwise, mitosis will occur.

M checkpoint

Mitosis will not continue is chromosomes are not aligned properly

Apoptosis

A typical series of events which bring about cell destruction

Mitosis

The division of the nucleus that forms two daughter nuclei with the same number and kind of chromosome (identical to the parent)

Cytokinesis in Plant verse Animal cells

Plant:

- cell plate forms and separates to two new cells with cell walls

Animal:

- Cleavage furrow squeezes in and separates

Centromere

The region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.

Diploid (2n)

Cells that have two (a pair) of each type of chromosome

Haploid (1n)

Cells that have 1/2 the diploid number of chromosomes

How many chromosomes do sex cells have?

Egg and sperm cells have 23

Mitosis purpose

Growth and repair

Prophase (mitosis)

- the nuclear membrane disappears, centrosomes migrate, spindle fibers appear

- chromatin condenses and chromosomes become visible

- centromeres attach to spindle fibers

- chromosomes have no particular orientation

Metaphase (mitosis)

- spindle fully forms and consists of poles, asters and fibers

- chromosomes line up at metaphase plate

Anaphase (mitosis)

- centromeres divide, and sister chromatids are moves to opposite poles by fibers (kinetochore spindle fibers shorten, pulling daughters while polar spindle fibers push the poles apart)

Telophase (mitosis)

- nuclear membrane reappears, spindle disappears, cytokinesis occurs

- chromosomes uncoil

- nucleolus reappears in each daughter nucleus

Cell plate

a structure that forms in the middle of a dividing plant cell during cytokinesis, acting as a precursor to the new cell wall that separates the two daughter cells

What cells have meiosis

Sex cells

Synapsis

The pairing of homologous dyads into a tetrad

How many cell does meiosis result in

four

Prophase I (meiosis)

- nuclear membrane breaks down

- spindle appears, nuclear envelope fragments, and nucleolus disappears

- homologues come together to form tetrads by synapsis, line up side by side and crossing over occurs

Metaphase I (meiosis)

Homologous chromosome pairs line up at metaphase plate such that maternal or paternal member may be oriented toward either pole (independent assortment)

Independent assortment

- It occurs when these homologues separate from each other during anaphase I

- increases genetic diversity and generates cells with different combinations of maternal and paternal chromosomes

- Humans have 23 chromosomes and to calculate 2^23 leaves 8,388,608 pairs

- This genetic recombination does not include crossing over

Homologous

Used to describe chromosomes that align with corresponding DNA segments

Crossing over

the process during meiosis where homologous chromosomes exchange segments of their DNA, creating new combinations of genes (alleles) and increasing genetic diversity in offspring

Anaphase I (meiosis)

Homologous chromosomes separate, pulled to opposite poles by centromeres and spindle fibers

Telophase I (meiosis)

- May occur at end of meiosis I (species dependent)

- Nuclear envelopes re-form

- Nucleoli reappear

- cytokinesis may occur and produce two haploid daughter cells

Interkinesis

- period of time between meiosis I and meiosis II

- no replication of DNA

Where does genetic variation occur?

- Crossing over (prophase I)

- Independent assortment (metaphase I/anaphase I)

Prophase II (meiosis)

- cells have one chromosome from each homologous pair

- a spindle appears and the nuclear envelope disassembles

- each duplicated chromatid attaches to the spindle

- the nucleolus disappears

Metaphase II (meiosis)

- sister chromatids line up at the metaphase plate

Anaphase II (meiosis)

- sister chromatids separate and become daughter chromosomes that migrate toward the poles

Telophase II (meiosis)

- the spindle disappears

- the nuclear envelope re-forms

- cytokinesis occurs

Fertilization

- daughter cells of meiosis mature into gametes

- sperm and eggs fuse

- restores diploid number

Importance of meiosis

- upon fertilization, combining of chromosomes from GENETICALLY DIFFERENT GAMETES HELP ENSURE OFFSPIRING ARE NOT IDENTICAL TO PARENTS

- This GENETIC VARIABILITY is the MAIN ADVANTAGE of SEXUAL REPRODUCTION

- Long -term, GENETIC VARIATIONS INCREASES THE SURVIVAL OF A SPECIES

Meiosis compared to mitosis

- DNA replication occurs only ONCE prior to either

- Meiosis requires TWO divisions, mitosis requires ONE

- Meiosis produces FOUR HAPLOID daughter cells, mitosis produces TWO DIPLOID

- Meiosis produces GENETICALLY DIFFERENT CELLS, mitosis produces GENETICALLY IDENTICAL CELLS

Meiosis I compared to Mitosis

Meiosis II compared to Mitosis

Somatic cell

A body cell with a complete set of chromosomes (2n=46)

Autosome

A chromosome that does not determine biological sex (1-22)

Gamete

Sex cell (sperm or egg cell)

Sex chromosome

The chromosome determining the sex

Chromosome pairings

XX - biological female

XY - biological male

(female offers x while male offers both so the sex is determined by the male)

Nondisjunction

- This can occur during meiosis I, when BOTH MEMBERS OF A HOMOLOGOUS PAIR GO INTO THE SAME DAUGHTER CELL

- Or it can take place during meiosis II when the SISTER CHROMATIDS FAIL TO SEPARATE AND BOTH DAUGHTER CHROMOSOMES GO INTO THE SAME GAMETE

Aneuploidy

An abnormal number of chromosomes

Down Syndrome

- Male of female

- Trisomy 21 (specific chromosome)

- 47 chromosomes

Poly-X Syndrome

- female

- XXX or XXXX

- 47 or 48 chromosomes

Klinefelter Syndrome

- male

- XXY or XXXY

- 47 or 48 chromosomes

Jacobs Syndrome

- Male

- XYY

- 47 chromosomes

Turner Syndrome

- Female

- X

- 45 chromosomes

Chromosomal Mutations

When chromosomes break and fail to reunite properly (can occur during meiosis leading to syndromes)

Deletion (mutation)

When it is the same chromosome but just loses a piece of chromatin

Duplication (mutation)

When a piece of chromosome is just copied and added in (ex: from abcdef to abcdcdef)

Translocation (mutation)

When two non homologous chromosomes swap genetic info (the functionality depends on the mutation)

Inversion (mutation)

When a segment of chromosomes is backwards (result in poor crossing over)