Chapter 13: Meiosis and Sexual Life

Cycles

Essential Knowledge

3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (13.1-13.3).3.c.2 – Biological systems have multiple processes that increase genetic variation (13.4).

HeredityThe transmission of traits from parents to offspringComment - Humans have been aware of heredity for thousands of yearsAlso known as inheritance

GeneticsThe scientific study of heredityComment - Genetics is only about 150 years old NEW/MODERN field of science DNA was only discovered in 1950s

Only 60 years ago!

Chromosome ReviewChromosomes (23 pairs=46) Inherit one chromosome of each

pair from each parent Total of 46 (23 maternal, 23

paternal)1 pair/2 = Sex chromosomes 22 pairs/44 = Autosomes

Genes

The DNA for a traitLocus - the physical location of a gene in a chromosomeGenes program traits How? Program cells (through protein

synthesis) to make specific proteins These proteins become “visible” to

us through outside traits

ReproductionA method of copying genes to pass them on to offspringFertilization: male/female gametes unite during reproduction Results in zygote (fertilized egg)Two main types: Asexual reproduction Sexual reproduction

Asexual ReproductionParent passes all of its genes to its offspring Offspring are exact copies of parents Also known as cloning (clones)Uses mitosisOften called buddingComment - many organisms reproduce this way Ex: jellyfish, hydra, sponges

Asexual Bud

Advantages of AsexualOnly need 1 parentOffspring are identical to the parentGood genetic traits are conserved and reproducedUsually requires less energy Why?

Disadvantages of AsexualNo new DNA combinations for evolution to work on Only genetic differences come from

DNA mutations (during DNA replication, etc)

Clones may become extinct if attacked by a disease or pest

Sexual ReproductionTwo parents contribute DNA to an offspringComment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others

Sex ChromosomesHomologous? Mostly no! Only a SMALL part of

these 2 are homologousMales: One X chrom, One Y chrom Y chromosomes are smaller (with

more genetic disorders)Females: Two X chromosomes X chromosomes are bigger (usually

contain more genes)

Egg vs. SpermEggs contain ONLY X chromosomesSperm contain EITHER X or Y chromosomes Why?

Because boys (when they are made during fertilization) get an X chromo from mom and a Y chromo from dad – making them a MALE!If the cell were to get two X chromos, the fertilized cell would become a FEMALE!

Who determines sex of baby?

Males determine what sex the offspring will be Why?

Their sperm have the “option” of having X or Y

X from egg and Y from sperm = XY (male)

X from egg and X from sperm = XX (female)

Advantages of SexualOffspring has a unique combination of DNA which may be an improvement over both parentsNew combination of DNA for evolution to work with

Disadvantages of Sexual Need two parents (requires time and energy to look/find the right mate)Good gene combinations can be lostOffspring may not be an improvement over the parents

Question ?Do parents give their whole DNA copy to each offspring?What would happen to the chromosome number if they did?

Chromosome NumberIs usually constant for a speciesTwo of these are usually sex chromosomes XY or XX in humans

Examples: Humans – 46 (pair #23 is sex) Corn - 20 Onions - 16 Dogs - 72

Two options for life cycle…1) Mitotic cell division continues life2) Meiosis cell division continues life

WHICH way does life reproduce?

Result of Life cycle: if Mitosis

46 in egg + 46 in sperm=92 total chromosomesChromosome number would double each generation.Need a method to reduce the chromosome number.

Result of life cycle: if Meiosis

Egg 23 + sperm 23 = 46 total chromosomesChromosome number will remain the same with each sexual reproduction event.Meiosis is used to produce the gametes or sex cells. 46 reduced to 23 through meiosis

YES!!

Meiosis - PurposeTo reduce the number of chromosomes by half 46 to 23 (in humans)Prevents doubling of chromosome numbers during sexual reproduction

Sexual Life CycleHas alternation of meiosis and fertilization to keep the chromosome numbers constant for a species Fertilization: 23 + 23 = 46 total Mitosis: takes original cell made through

fertilization and multiplies it to form organs, etc.

Meiosis: 46 reduced to 23 (in sex organs)

Life Cycle Variations

Diploid2 sets of chromosomesReferred to as 2nMost common number in body or somatic cells Humans 2n = 46 Corn 2n = 20 Fruit Flies 2n = 8

Haploid1 set of chromosomesNumber in the gametes or sex cells Humans n = 23 Corn n = 10 Fruit Flies n = 4 Half of diploid #!!!!!!!

Polyploids

Multiple sets of chromosomesExamples 3N = triploid 4N = tetraploidCommon in plants, but usually fatal in animals Ex: Down Syndrome Ex: Seedless watermelons

Triploid Daffodil = more stem produced per bulb

Triploid watermelon = no gametes produced (no seeds)

Meiosis/Mitosis Preview of differences

LOOK at pg. 256Meiosis Two cell divisions, not one Four cells produced, not two Synapsis and chiasmata will be

observed

Meiosis/Mitosis Preview of differences

Meiosis, cont. 1st division separates PAIRS of

chromosomes, not duplicate chromatids (of chromosome)

Known as homologous pairs of chromosomes

Interkinesis is present.Mitosis vs. Meiosis Animation

Meiosis

Has two cell divisions (I and II) Steps follow the names for mitosis A “I” or “II” will be

added to label

Replicated chromatids

(dupl. during Inter)

Prophase IBasic steps same as in prophase of Mitosis.Synapsis occurs as the chromosomes condense. Synapsis - homologous chromosomes

form bivalents or tetrads Synaptonemal complex - proteins which

hold chromosomes together

Tetrad

Prophase I, cont.Chiasmata observed Spot where chromatids cross-over Held together by this until AnaphaseLongest phase of division Cell spends 90% of meiosis in this stageChromatin condenses, nucleolus disappears, nuclear envelope dissolves

Metaphase ITetrads or bivalents align on the metaphase plate Attached to kinetochores of opposite

polesCentromeres of homologous pairs point toward opposite poles

Anaphase IHomologous PAIRS separate Uses spindle apparatusDuplicate chromosomes are still attached at the centromeresMaternal and paternal chromosomes are now separated randomly

Telophase ISimilar to MitosisChromosomes may or may not unwind to chromatinCytokinesis separates cytoplasm and 2 cells are formedNOW: Each cell has a haploid set of chromosomes

Each chromosome is STILL comprised of 2 identical sister chromatids!!!

InterkinesisNo DNA synthesis/copying occursMay last for years, or the cell may go immediately into Meiosis IICan appear similar to Interphase of Mitosis

Meiosis IISteps are the same as in Mitosis Prophase II = Prophase Metaphase II = Metaphase Anaphase II = Anaphase Telophase II = Telophase

Meiosis - Results4 cells produced Each cell has ONE sister chromatid Remember: Started with homologous

chromosome PAIRChromosome number reducedGametes (sex cells) madeGenetic variation increased

Meiosis animation

Sexual Sources of Genetic Variation

1. Independent Assortment of Chromosomes during Meiosis2. Random Fertilization3. Crossing Over

Independent AssortmentThere are 23 pairs of chromosomes in humansThe chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2 Random arrangement of homologous

pairs during metaphase I This arrangement is totally random!

Gamete PossibilitiesWith 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are:

223 or 8,388,608(w/ each offspring!)

Random FertilizationChoice of which sperm fuses with which egg (random choice)With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the possible combos of offspring is over 64 million (for EACH set of parents)

Crossing-OverThe exchange of sister chromatid material during synapsisOccurs ONLY in Prophase IProduces recombinant chromosomes Chromosomes with DNA from both

parentsChiasmata: The point of contact where two

chromosomes are crossing-over

Crossing over

ImportanceBreaks old linkage groupsCreates new linkage groups increases genetic variationVery common during meiosisFrequency can be used to map the position of genes on chromosomes

CommentsOffspring can never be 100% like a parent if sexual reproduction is usedMultiple cross-overs are common, especially on large chromosomesGenes near the centromere do not cross-over very often

SummaryRecognize the general relationships between genes, DNA, and chromosomes.Identify characteristics, advantages, and limitations of asexual and sexual reproduction.Recognize several sexual life cycles differing in the timing of meiosis and fertilization.Recognize the stages and characteristics of the meiosis cell division process.Contrast and compare meiosis to mitosis. Identify sexual sources of genetic variation.Use Chapter 46 to see differences in male/female meiosis (pgs. 1006-1007)