Modified from: Kim Foglia, Chapter 13. Meiosis & Sexual Reproduction Modified from: Kim Foglia, Explore Biology

Human female karyotype

Human male karyotype

How do we make sperm & eggs? reduce 46 chromosomes  23 chromosomes half the number of chromosomes 23 46 23 46 egg meiosis 23 46 23 fertilization sperm

Meiosis: production of gametes Alternating processes, alternating stages chromosome number must be reduced diploid  haploid 2n  n humans: 46  23 meiosis reduces chromosome number fertilization restores chromosome number haploid  diploid n  2n

Homologous chromosomes Paired chromosomes both chromosomes of a pair carry genes control same inherited characters homologous = same information diploid 2n homologous chromosomes double stranded homologous chromosomes

Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids

Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n  1n) 2nd division of meiosis separates sister chromatids (1n  1n) * just like mitosis *

Crossing over During Prophase 1 homologous pairs swap pieces of chromosome sister chromatids intertwine crossing over tetrad synapsis

What are the advantages of sexual reproduction? Crossing over 3 steps cross over breakage of DNA re-fusing of DNA New combinations of traits

Genetic variation Meiosis & crossing over introduce great genetic variation to population drives evolution Consider the greater variation with 23 pairs of chromosomes = mixing and matching

The value of meiosis Meiosis introduces genetic variation gametes of offspring do not have same genes as gametes from parents genetic recombination random assortment in humans produces 223 (8,388,608) different combinations new gametes made by offspring from Mom from Dad

And more variation… Crossing over creates completely new combinations of traits in next generation

Random fertilization Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

Sources of genetic variability Genetic variability in sexual reproduction independent assortment homologous chromosomes in Meiosis 1 crossing over between homologous chromosomes in prophase 1 random fertilization random ovum fertilized by a random sperm metaphase1

Mitosis Asexual Reproduction Fusion Meiosis Cell A Cell B Cell C Cell A has 4 chromosomes and Cell D has 2 chromosomes. What process created Cell D? Mitosis Asexual Reproduction Fusion Meiosis Cell A Cell B Cell C Cell D 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

Cell A is considered _______ while Cell D is considered ________. Haploid; Diploid Tetrad; Diploid Diploid; Haploid Homologous; Haploid Cell A Cell B Cell C Cell D 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Cell A is considered _______ while Cell D is considered ________. somatic cell; gamete gamete; somatic cell stem cell; somatic cell stem cell; gamete 30

How can we best describe the diagram below? Two sister chromatids Homologous chromosomes One replicated chromosome Haploid chromsomes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

What process is best demonstrated by the diagram below? Anaphase I Segregation of alleles Crossing over Independent assortment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

Which of the following leads to genetic variation during meiosis? Crossing Over DNA replication Independent assortment Mitotic spindle formation All of the above (A, B, C, D) A & B A & D A, B, & D A & C 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 10

Mitosis vs. Meiosis Mitosis Meiosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n  2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n  1n produces gametes crossing over

Mitosis vs. Meiosis

Changes in Chromosomes Number Euploidy = correct # of chromosomes Aneuploidy = a change in chromosomes number due to non-disjunction during meiosis Monosomy- only 1 copy of an individual chromosome Trisomy- 3 copies of an individual chromosome

Primary nondisjuction = Meiosis I Secondary nondisjuction = Meiosis II

Trisomy 21 : Down Syndrome Delayed mental and social skills Decreased muscle tone at birth Asymmetrical or odd-shaped skull Small skull Small mouth with protruding tongue Broad short hands Increased risk of developing Leukemia and Alzheimer’s later in life

Trisomy 18 : Edward’s Syndrome Most children die in the first year of life, some have lived 10 years Growth deficiency Feeding difficulties Breathing difficulties Developmental delays Mental Retardation Overlapped, flexed fingers Webbing of the second and third toes Clubfeet Structural heart defects at birth

Trisomy 13 : Patau Syndrome Mental retardation, severe Seizures Small head Scalp defects Cleft lip and/or palate Eyes close set (hypotelorism) –may fuse Extra digits (polydactyl) Hernias Undescended testicle Children die in the first year of life

Karyotype A visual display of the chromosomes arranged by size, shape, and banding pattern Used to identify aneuploid conditions

Procedure: Amniocentesis and Karyotyping

Karyotyping

Changes in Sex Chromosome # Turners Syndrome (XO) – missing Barr Body Kleinfelter’s Syndrome (XXY) Swyer Syndrome (XY female) La Chapelle Syndrome (XX male) Poly-X Females (XXX, XXXX) Jacob’s Syndrome (XYY males) -SRY gene (located on short arm of Y chromosome) -hormone= testis-determining factor -Barr Body – Inactive X chromosome (XX)

Chromosomal Mutations

Deletion Syndromes Williams Syndrome (deletion of a piece of chromosome 7) Cri du chat (cat’s cry) (deletion of a piece of chromosome 5)

Translocation Syndromes Alagille syndrome – Chromosomes 2 and 20 exchange segments Cancers Chronic myelogenous leukemia (2 and 9) Burkitt lymphoma (8 and 14)