Parent cell provides
- genetic info. DNA
- mitochondria, ribosomes and other components.
Cell division is
a form asexual reproduction in some organisms.
Prokaryotic cell cycle: binary fission
- 1. bacterial have a circular chromosome, attached at one point to plasma membrane.
- 2. DNA is replicated during growth phase.
- 3. the points of attachment move apart after the DNA is replicated.
- 4. the cell becomes constricted, eventually dividing.
- 5. Prokaryotic cells can divide as often as every 20 minutes.
Eukaryotic Cell Cycle:
Interphase: most of the cell's time is spent here.
- G1: growth, synthesis of cellular materials.
- S: synthesis of DNA
- G2: complete growth
some cells never divide. THese cells usually differentiate into specialized cell types such as muscle, many kinds of nervous tissue & immune cells.
2 kinds of cell division in eukaryotes:
Mitotic cell division
Meiotic cell division
Mitotic cell division:
in which there is a nuclear division (mitosis) normally followed by cytoplasmic division (cytokinesis)
Meiotic cell division
required for sexual reproduction. Meiosis reduces the chromosome number and produces gametes (eggs and sperm)
genes, telomeres and centromeres
sequence of DNA hundreds to thousands of nucleotides long
long stretches of repeated necleotide sequences found at the tips of chromosomes. They protect from damaging the ends of chromosomes.
- 1. 2 DNA helices remain attached after replication
- 2. Centromere: point of attachment
- 3. Sister chromatid: the 2 copies of DNA that remain attached. Sisters will seperate during division
Members of a given homologous pair share certain characteristics:
shape, banding pattern and size. ANd they carry the same genes in the same order.
cells with paired choromosomes.
How many pairs of chromosomes are in humans.
- 22- autosomes; and one pair of sex chromosomes: x and y; which differ in size and banding pattern but behae like homologues during division.
sex cells or gametes ( sperm and eggs)
have half the number of chromosomes: one of each pair and are called haploid.
a cell division occurs in 2 parts
Mitosis: division of the nucleus; divided into 4 phases.
Cytokinesis: division of the cytoplasm (cell division)
the 4 phases of mitosis:
- 1. duplicated chromosomes condense.
- 2. nucleulus disappears.
- 3. nuclear envelope disintegrates
- 4. spindle microtubules form.
- 5. chromosomes become attached to the spindle.
- *attach to the chromosome at the kinetochore, near centromere
- *The kinetochore on each sister chromatid attaches to microtubule originating from opposite pole.
- 1. chromosomes become aligned at the equator
- 2. 2 kinetochores of each pair are attached to opposite poles via the spindle microtubules, one kinetochore facing each pole.
- 1. centromeres split.
- 2. sister chromatids seperate and travel towar opposite poles, each now becomeing a seperate daughter chromosome.
- 3. spindle microtubules lengthen to elongate cell.
- One set of chromosomes reaches each pole and relazed into extended state: nuclear envelopes start to form around each set; spindle microtubules begin to diappear.
- Usually cytokinesis occurs.
cytokinesis (animal cells)
- microfilaments around equator contract.
- constrict cell's equator
- Eventually punch cell in two.
Cytokinesis (plant cells)
- In plants, the presence of stiff cell walls mean a different mechanism is required.
- 1. carb-filled vesicles accumulate at equator.
- 2. Fuse to form cell plate
- 3. cell plate grows at edges, eventually partitioning cell.
different forms of genes that exist.
- 1. new alleles arise through mutation of DNA.
- 2. Most mutations are harmful and eliminated
- 3. a few are helpful; raw material for evolution.
- 4l sexual reproduction allows mixing up of alleles.
How does Meiotic cell division produce haploid cells?
Meiosis: division of the nucleus that reduces chromosome number in half, usually followed by cytokinesis.
forms 2 daughter cells
Each daughter cell is haploid (n). During subsequent fertilization, diploid number (2n) is restored in offspring.
Meiosis consists of two divisions:
Meiosis I: seperates pairs of homologues, one to each daughter cell.
Meiocsis II: seperates sister chromatids, one into each daughter cell.
stages of meiosis I:
Homologous chromosomes pair up and synapse. Maternal and paternal homo are "zipped" together by proteins. The homo may exchange segments of DNA, called crossingover. This is a type of genetic recombination. Chiasmata form at sites of crossing over.
- Othe events are similiar to mitosis:
- 1. nuclear envelope dissolves.
- 2.chromosomes condense.
- 3. spindle forms.
Metaphase I of Meoisis:
Paired homo line up at the equator. One member of pair is attached to microtubule from on pole, othe other member is attached to other pole. At this time, sister chromatids will remain together.
The arrangement of paternal and maternal homo is random.
homo seperates from each other but sister chromatids remain attached and move together.
chromosomes cluster at each pole.
cytokinesis usually occurs
there is no replication of DNA between meiosis I and meiosis II
The stages of meiosis II are almost identical to those of mitosis:
- Prophase II: spindle re-forms and chromosomes attach as in mitosis
- Metaphase II: individual chromosomes line up at equator.
- Anaphase II: centromeres split & daughter chromosomes move to opposite poles
- Telophase II: nuclear envelopes reform; cytokensis occurs.