chapter 11 Flashcards
4 main event in all organisms
cell division
DNA replication
DNA segregation
Cytokinesis
cell division is prokaryotes
Cell division (binary fission) results in reproduction of the entire single-celled organism
Cell division signals are usually external factors such as nutrient concentration and environmental conditions
replication - prokaryotes
- One chromosome
- a single molecules of DNA
- often circular, but folded
2 important regions: - ori - where replicaiton starts (origion)
- ter - where replication ends (terminus)
- in a replication fork
DNA segregation
- replication is complete
- ori regions move to the opposite ends of the cell
- segregation of the daughter chromosomes
cytokinesis
- cell membrane pinches in
- protein fibers form a ring
- new cell wall materials are synthesized
- separation of the two cells
DNA replication (eukaryotes)
- eukaryotes have more than one chromosome
- replication starts are many origins on the chromosome
DNA segregation
Mitosis
- separates chromosomes into 2 new nuclei
- ends up in each daughter cell
cytokinesis
splitting of cells into 2 cells
cell cycle
phases a cell passes through to produce daughter cells by cell division
interphase
nucleus is visible and cell functions occur
- duration of interphase is highly variable
- interphase has 3 subphases: G1(Chromosomes are single (unreplicated)
- duration is variable, from mins to yrs, some enter resting phase (G0)), S (DNA replicates, sister chromatids remain together until mitosis)
, G2 (cell prepares for mitosis, e.g. by synthesizing the structures that move the chromatids; sister chromatids are held tg during G2 by proteins called cohesins)
M phase includes mitosis and cytokinesis
Cell cycle is controlled by
- cyclin-dependent kinases (CDKs)
- Protein kinases catalyze transfer of a phosphate group from ATP to a protein (phosphorylation)
- changes the shape and function of the protein
- growth factors activate signal transduction
chromatin
DNA molecules are bound to proteins to chromatin
Mitosis
all cells except sex cells
production of 2 nuclei that are genetically identical
subdivided into:
prophase
prometaphase
metaphase
anaphase
telophase
at mitosis cohesin is removed, except at the centromere. condensins coat the DNA molecules and make them more compact
during mitosis chromosomes are so compact, they are inaccessible to replication and transcription factors
asexual reproduction is based on mitosis
a single-celled organism reproduces itself with each cell cycle; some multicellular organisms also reproduce asexually
sexual reproduction
- offspring are not identical to the parents
- deals with gametes
gametes
Gametes (eggs or sperm cells) are created by meiosis; each parent contributes one gamete to an offspring
- Gametes and offspring differ genetically from each other and from the parents.
- gametes contain only one set of chromosomes—one homolog of each pair
Meiosis
Meiosis generates genetic diversity that is the raw material of evolution
somatic cells
- body cells other than sex cells, not specialized for reproduction
- each somatic cell has homologous pairs of chromosomes
haploid
chromosome number haploid
fertilization
2 haploid gametes fuse to form a diploid zygote
meiosis
2 nuclear divisions, but DNA is replicated only once
- reduces chromosomes # from diploid (2n) to haploid (n)
- ensures each haploid product has a complete set of chromosomes
- generates genetic diversity among the products
- can occur only in diploid cells
meiosis I
- preceded by DNA replication in S phase
- homologous chromosome pairs separate but individual chromosomes (sister chromatids) stay together
- 2 haploid nuclei result, each with half of the original chromosomes (one member of each homologous pair)
prophase I (of meiosis I)
Homologous chromosomes pair by adhering along their lengths (synapsis)
the 4 chromatids of each homologous pair form a bivalent
chiasmata
regions of attachment that form between nonsister chromatids
crossing over
exchange of genetic material between nonsister chromatids at the chiasmata
results in recombinant chromatids and increases genetic variability of products
one reason for genetic diversity in meiosis I products
independent assortment
also allows for chance combinations
matter of chance how they line up in anaphase I (meeting in middle)
Meiosis II
- not preceded by DNA replication
- sister chromatids are separated
- chance assortment of chromatids contributes to genetic diversity
- final products are 4 haploid daughter cells (n)
errors in meiosis
Nondisjunction
- homologous pairs may fail to separate at anaphase I
- in meiosis II, sister chromatids may fail to separate
- results in aneuploidy—chromosomes are either lacking or present in excess
aneuploidy
- may be caused by lack of cohesins that hold the homologous pairs tg
- without cohesins both homologos may go to the same pole
- resulting gametes will have 2 of the same chromosome or none
down syndrome
results from a gamete with 2 copies of chromocome 21. after fertilization there are three copies (trisomic)
a fertilized egg that did not receive a copy of chromosome 21 will be monosomic, which is lethal
translocation
a piece of chromosome may break away and attach to another chromosome
an individual with a translocated piece of chromosome 21 plus 2 normal copies will have down syndrome
karyotype
display of chromosomes-number, shape, and sizes of all chromosomes of a cell
can be used to diagnose abnormalities such as trisomies
- branch of medicine called cytogenetics
polyploid
- organisms w complete extra sets of chromosomes are called polypoid
- triploid (3n, sterile and cannot undergo meiosis)
- tetraploid (4n) and higher levels are possible
if there is nondisjunction of all chromosomes during meiosis I, diploid gametes will form, leading to autopolyploidy after fertilization
cell death
occurs in 2 ways
Necrosis: cell is damaged or starved of oxygen or nutrients
- the cell swells and bursts, cell contents are released to the extracellular environment and can cause inflammation
Apoptosis: programmed cell death
- cells may no longer be needed (e.g. fetus finger tissue)
- old cells are prone to genetic damage that can lead to cancer
Events of apoptosis
- cell detaches from neighbors
- chromatic is digested by enzymes that cut DNA between nucleosomes
- the cell forms membranous lobes called “blebs” that break into fragments
- surrounding living cells ingest remains of the dead cell and recycle the contents
cancer cells
lose control over cell division and can migrate to other parts of the body, divide continuously, forming tumors (large masses of cells)
- benign tumors grow slowly, resemble the tissue they grow from, and remain localized
- malignant tumors do not resemble the parent tissue
metastasis: cancer cells invade surrounding tissue and travel through bloodstream or lymph system to lodge and continue dividing to form new tumors
surgical removal of tumors in the optimal treatment, but sometimes not all cancer cells can be removed
other treatments target the cell cycle:
- drugs that prevent cell division
5-fluorouracil blocks synthesis of thymine
paclitaxel prevents functioning of microtubules in the mitotic spindle
radiation treatment causes DNA damage in tumor cells
cell regulation
in normal cells, the cell cycle is regulated
- positive regulators such a growth factors stimulate cell division
- negative regulators such as retinoblastoma protein (RB) inhibit the cell cycle
the 2 regulator systems ensure that cells divide only when needed
