Two kinds of reproduction• Asexual reproduction • (1 parent)

– Offspring inherit parent’s genes– Clones (identical copies of parent)

• Sexual reproduction • (2 parents)

– Offspring differ from parents and each another

– Different combinations of alleles – Different details of shared traits– Slightly different forms of the same

gene– Each specifies a different version of

gene product

During sexual reproduction….

• Meiosis, gamete formation, and fertilization

• All chromosomes are duplicated during interphase, before meiosis

• Two divisions, meiosis I and II, – divide parental chromosome

number by two• End result gamete is haploid

Meiosis and fertilization shuffle parental alleles– Offspring inherit new

combinations of alleles

What meiosis does?• Meiosis

– Nuclear division mechanism that precedes gamete formation in eukaryotic cells

– Halves parental chromosome number

• Fertilization– Fusion of two gamete

nuclei – Restores parental

chromosome number– Forms zygote (first cell

of new individual)

Prophase I is long • Meiosis I is a

continuous process with five distinct phases

• Chromosomes condense and align tightly with their homologues

• Each homologous pair undergoes crossing over

• Microtubules form the bipolar spindle

Prophase I (cont.)

• One pair of centrioles moves – to other side of

nucleus • Nuclear envelope

breaks up– Microtubules

growing from each spindle pole penetrate nuclear region

• Microtubules anchors homologous pair

Metaphase I • homologous

chromosomes aligns at equator region of the spindle

Anaphase I • Microtubules

separate each chromosome from its homologue, moving to opposite spindle poles

• As anaphase I ends, one set of duplicated chromosomes nears each spindle pole

Telophase I

• Two nuclei form – Typically, the

cytoplasm divides

• All chromosomes are still duplicated– Each still

consists of two sister chromatids

Meiosis II• The second nuclear

division

• Sister chromatids of each chromosome are pulled away from each other

• Each is now an individual chromosome

Prophase II Metaphase II

Anaphase II and Telophase II• anaphase II one

chromosome of each type – moves toward

opposite spindle poles

• end of telophase II,– four haploid

nuclei, each with unduplicated chromosomes Anaphase II Telophase Il

Results in haploid daughter cells

• When cytoplasm • divides,

– four haploid cells Telophase IAnaphase IMetaphase IProphase I

Meiosis I

•Two events variation in traits in sexually reproducing species– Crossing over during prophase I of meiosis

• Chromosome shuffling during metaphase I of meiosis Nonsister chromatids of homologous chromosomes undergo crossing over– They exchange segments at the same place

along their length • Each ends up with new combinations of

alleles not present in either parental chromosomeprophase I

metaphase I

Life cycles of plants and animals

• Multicelled diploid and haploid bodies• Plants

– Sporophyte: A multicelled plant body (diploid) that makes haploid spores

– Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form)

• Animals– Germ cells in the reproductive organs give rise to

sperm or eggs– Fusion of a sperm and egg at fertilization results in a

zygote

Introducing Variation in Offspring

• Three events cause new combinations of alleles in offspring: – Crossing over during

prophase I (meiosis)– Random alignment of

maternal and paternal chromosomes at metaphase I (meiosis)

– Chance meeting of gametes at fertilization

• All three contribute to variation in traits

Differences in mitosis and meiosis• Both require bipolar spindle to

move and sort duplicated chromosomes– mitosis, and may have evolved

from meiosis• Mitosis maintains parental

chromosome number– Duplicates genetic information– Occurs in body cells

• Meiosis halves chromosome number– Introduces new combinations

of alleles in offspring– Occurs only in cells for sexual

reproduction