reproduction mitosis & meiosis. cell division allows for repair & replacement of worn out...
TRANSCRIPT
Cell Division• allows for repair &
replacement of worn out cells
• basis of reproduction in every organism
• unicellular organisms– cell division
reproduces entirely new organisms
• allows organisms to develop from one fertilized egg cell into multicellular organism of 60 trillion cells
DNA• body must have way to
ensure that each time a cell divides information is maintained & directly copied
• information is found in chromosomes– can only be seen during
cell division• remainder of the time exists
as mass of very long fibers -chromatin
• each chromosome = one long DNA molecule containing thousands of genes
Chromosomes• genes are found on
chromosomes in the nucleus
• number-specific to a species
• human cells except ovum & sperm have 46 chromosomes
• dog cells have 78
Chromosomes• during cell division genetic
material makes an exact duplicate of itself
• resulting in a chromosome containing two identical copies or sister chromatids
• joined by a centromere• when cell divides
chromatids separate• one goes to one daughter
cell • other to another daughter
cell• resulttwo cells with
identical genetic material
Cell Cycle• ordered sequence
of events that begins when cell is formed & continues until cell divides
• two broad stages• interphase
– growing stage • mitotic phase
– cell division stage
Interphase• 90% of cycle • normal functions are
performed • cell prepares for cell
division• everything in
cytoplasm is doubled• cell increases in size• chromosomes
duplicate
G1 Phase• mitochondria,
cytoskeletal elements, ER, ribosomes, Golgi membranes & cytosol are made in quantities for two cells
• continues until G2 stage• centrioles begin to
replicate• may last hours, days,
weeks, or months
G2 Phase• 2-5 hours• last minute protein
synthesis• completion of
centriole replication• each chromosome
consists of 2 identical sister chromatids linked by centromere
Mitotic Phase• M phase• cell divides• produces two identical daughters cell• divided into two stages• Mitosis
– nuclear division– duplicated DNA is separated into 2
nuclei– sister chromatids separate at
centromere– one goes into each of two daughter
cells• Cytokinesis
– cytoplasm divides into two cells
Cytokinesis• cytoplasm division• animal cells-cleavage• first sign-appearance of
cleavage furrow• microfilaments surround
cell• pulled tight to divide
cytoplasm• plant cells • cell plate or cleavage
plate forms inside cell & grows outward
• eventually new piece of wall divides cell into two
Stages of Mitosis• mitosis is
continuous• divided into four
main stages• Prophase• Metaphase• Anaphase• Telophase
Prophase• begins when
chromosomes coil tightly. • become visible as
individual structures• there are 2 copies of each
chromosome• each termed a sister
chromatid• connected by centromere• as chromosomes
appearnucleoli disappear
Prometaphase• nuclear envelope
disappears• spindle fibers form
among chromosomes• kinetochore of each
chromatid attaches to spindle fiber
• centrioles begin to move to opposite poles due to spindle fibers
Anaphase• begins when centromere
of each chromosome comes apart separating sister chromatids
• kinetochores move daughter chromosomes to opposite poles of cell
• ends when complete collection of chromosomes has reached poles of cell
Telophase• nuclear membrane forms• nuclei enlarge• chromosomes uncoil• chomatin filaments form
while nucleoli reappear• mitosis is completed• cells prepare to return to
interphase• in order to make two
complete cells cytoplasm must divide
• Cytokinesis• usually takes place at
same time as telophase
Chromosomes• every nucleus in every
somatic cell carries genetic blueprint
• 46 chromosomes• each paired with a like
chromosome• 23 pairs• 23 chromosomes came
from our mothers• 23 from our fathers
Homologous Chromosomes• pairs of chromosomes
are homologous• carry same genes• genes code for a
particular trait • come in several forms
or alleles• genes may be alike
– Homozygous
• genes may be unlike– Heterozygous
Diploid & Haploid• cells containing 23 pairs of
chromosomes are diploid • abbreviated-2n• 2n = 46• all cells in human are diploid with
exception of gametes– sperm & egg cells
• have haploid number– half number in diploid cell
• 23 chromosomes• n = 23• during fertilization gametes fuse
producing diploid zygote which develops into a diploid organism
• haploid gametes keep chromosome number from doubling in each generation
• gametes are made by a special type of cell division-meiosis or reduction division
Meiosis• basis of sexual reproduction• reduction division• cells produced contain half
number of chromosomes as typical body cell
• one diploid cell4 haploid cells-4 sperm or 1 egg & 3 polar bodies
• occurs in stages• many resemble stages of
mitosis• preceded by replication of
chromosomes• followed by two successive
nuclear divisions: meiosis I (reduction) & meiosis II (division)
Phases of Meiosis I • interphase
• prophase I
• metaphase I
• anaphase I
• telophase I
• cytokinesis
Interphase• chromosomes
duplicate• end of stage
chromosomes composed of two attached, identical sister chromatids
• centrosomes have duplicated
Prophase I• chromatin coils up so
individual chromosomes become visible
• homologous chromosomes-each composed of two chromatids pair up
• form tetrad• composed of 2 chromatids
forming thick, 4-strand structure
• spindle starts to form between them
Crossing Over• during prophase I
synapsis forms (chiasmata)
• crossing over • chromatids break• become reattached to
different homologous chromosomes– rearranges genetic
information
• important to producing variability
Metaphase I• tetrads line up on
metaphase plate• sister chromatids still
attached by centromeres
• spindle fibers are attached to kinetochores at centromere region of each homologous chromosome pair
Anaphase I• tetrads separate• drawn to opposite
poles by spindle fibers• centromeres remain
intact so each pole has two chromosomes attached to centromere
• only tetrad has separated
Telophase I• chromosomes arrive
at poles of cell
• each in duplicate form
• cytokinesis usually takes place at same time
Prophase II & Metaphase II • Prophase II• nuclear envelope (if
formed) dissolves• spindle fibers form
moving chromosomes to middle of cell
• Metaphase II• spindles move
chromosomes to metaphase plate with kinetochores of sister chromatids of each chromosome pointing to opposite poles
Anaphase II & Telophase II • anaphase II• centromeres of sister
chromatids separate• move toward opposite
poles of cell• telophase II• nuclear envelopes form
at the poles• cytokinesis• occurs at same time
Genetic Variation• like begets like• truer of asexual than sexual
reproduction• in sexually reproducing species
like does not exactly beget like• none of you look exactly like
your parents• none of your siblings look exactly
like you– unless you are an identical twin
• each offspring inherits a unique combination of genes from parents producing unique combinations of traits
• genetic variability is due to two factors
Genetic Variation• half your chromosomes came from your father• half came from your mother• giving you 46• when you produce sperm or egg cells with a haploid number
of chromosomes some gametes got your mom’s chromosome & some your dad’s
• metaphase I-each homologous pair of chromosomes aligns on metaphase plate
• orientation of homologous pair to poles is random• there are 4 possible gametes that could form• this is true if an organism has only two pair of chromosomes• humans have 23 pairs• an independent orientation at metaphase 1• for humans with 23 pairs of chromosomes this works out to
8 million possible chromosome combinations
Genetic Variation• also due to crossing over• during prophase I-synapsis
of chromosomes occurs• genetic information is
exchanged between pairs of homologous chromosomes
• results in new genetic combinations
• offspring inherit gene combinations totally different from those inherited from previous generations
Genetic Variation• Fertilization• contributes to genetic
variability• any egg may be fertilized
by any sperm• one egg represents one of
eight million possibilities being fertilized
• sperm represents one of eight million possibilities
• resulting zygote has any one of 64 trillion possible combinations
Chromosomal Abnormalities
• mistakes can occur in meiosis
• detected using diagnostic tool
• karyotype
Aneuploidy• aberrant number of chromosomes• usual cause-non disjunction• Monosomy
– one chromosome lags behind another– left out of newly formed cell nucleus resulting in one
daughter cell with a normal chromosome number and one with a deficiency
– not compatible with life• polysomy
– too many chromosomes• autosomal polysomay may result in viable fetus but is
nearly always associated with severely disability• Advanced maternal age, radiation, viruses & chemicals• implicated in chromosomal abnormalities
Trisomy 21-Down Syndrome• extra 21st chromosome• most common chromosomal
disorder• leading cause of mental challenges• occurs in 1/700 live births• first described-1866• mental retardation• protruding tongues• low set ears• poor muscle tone• short stature• epicanthal folds• flat face• often congenital heart deformities• increased susceptibility to
respiratory infections and leukemia• ¾ of fetuses with syndrome are still
born or miscarried
Sex Chromosome Disorders• unusual number of
sex chromosomes
• typically has less debilitating symptoms than extra autosomal chromosomes
• may be because Y chromosome carries few genes
Klinefelter Syndrome• extra X chromosome-XXY• abnormal sexual
development• not diagnosed until puberty• secondary sex
characteristics do not develop
• boy lacks testosterone leading to infertility
• child is tall with long arms & legs
• female hair distribution, breast enlargement and high pitched voice
• testosterone therapy reduces feminine characteristics
Turner Syndrome• only one X chromosome• monosomy disorder
– only one not fatal in humans
• females have characteristic appearance-often short
• web of skin between neck & shoulders
• ovaries do not develop- sterility
• may be poor development of secondary sex characteristics
• estrogen can alleviate symptoms