Reproduction

Mitosis & Meiosis

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

Substages of Interphase•G1 phase

•S phase

•G2 phase

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

S Phase• 6-8 hours• chromosomes duplicate–DNA replicates

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

Interphase• mitosis begin after

interphase

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

Metaphase• mitotic spindle

fully formed• chromosomes

line up on metaphase plate

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

Identify the Stages of Mitosis

Meiosis

Reduction Division

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

Meiosis II• essentially same as mitosis

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