Meiosis and Sexual Life Cycles Chapter 13 Meiosis and Sexual Life Cycles

Hereditary Similarity and Variation Living organisms Are distinguished by their ability to reproduce their own kind Only oaks can produce oaks and elephants can make more elephants

Genetics Is the scientific study of heredity and hereditary variation Is the transmission of traits from one generation to the next Variation Shows that offspring differ somewhat in appearance from parents and siblings

Offspring acquire genes from parents by inheriting chromosomes Parents endow their offspring with coded information in the genes The tens of thousands of genes we inherit from parents Genes are segments of DNA Inherited information is passed on in the form of gene`s specific sequence (letters) Similar with reading “apple” and imaginig it Cells translate gentic “sentences” into visible characteristics

Transmission of hereditary traits has molecular basis in the replication of DNA Produces the copies which are passed from parents to offspring Gametes- in animals and plants reproductive cells; vehicles that transmit genes from one generation to the next

DNA of eukaryotic cells is in nucleus Exception: small amounts in mitochondria and chloroplasts All living species have characteristic number of chromosomes 46 in human somatic cells

Each gene in an organism’s DNA Has a specific locus on a certain chromosome We inherit One set of chromosomes from our mother and one set from our father

Comparison of Asexual and Sexual Reproduction In asexual reproduction One parent produces genetically identical offspring by mitosis Exact copy of themselves Individual that reproduces asexually gives rise to a clone (group of genetically identical individuals) Figure 13.2 Parent Bud 0.5 mm

In sexual reproduction Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Not exact replicas, but variations

Fertilization and meiosis alternate in sexual life cycles A life cycle Is the generation-to-generation sequence of stages in the reproductive history of an organism

Sets of Chromosomes in Human Cells In humans Each somatic cell has 46 chromosomes, made up of two sets (23 each) One set of chromosomes comes from each parent Upon condensation they can be examinated by microscopy

A karyotype Is an ordered, visual representation of the chromosomes in a cell 5 µm Pair of homologous chromosomes Centromere Sister chromatids Figure 13.3

Homologous chromosomes Are the two chromosomes composing a pair Have the same characteristics May also be called autosomes Same length, centromere position and staining pattern

Sex chromosomes Are distinct from each other in their characteristics Are represented as X and Y Determine the sex of the individual, XX being female, XY being male Only small parts of X and Y are homologous They determine an individual sex

We inherit one chromosome pair from each parent Maternal set (from mother)-one set of 23 chromosomes Parental set (from father)-one set of 23 chromosomes We represent the number of chromosomes in a single set as n

A diploid cell Has two sets of each of its chromosomes In a human has 46 chromosomes (2n = 46) Number of chromosomes in somatic cells

Unlike somatic cells Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes For humans haploid number is 23 Number of chromosomes found in gamete The set of 23 consists of 22 autosomes and one sex chromosome

Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity The ovaries and testes produce haploid gametes by meiosis Human life cycle begins when those two haploid cells fuse Fusion is called fertilization Result- fertilized egg (zygote) is diploid Contains two haploid sets of chromosomes

Meiosis Takes place in two sets of divisions, meiosis I and meiosis II These division result in four daugther cells Each with only half as many chromosomes as parent cell

Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

Interphase Chromosomes replicate during S phase but remain uncondensed Each replicated chromosome consists of 2 genetically identical sister chromatides

Prophase I Chromosomes begin to condense Homologous chromosomes loosely pair along their lengths Kinetochores attach to microtubules from one pole to the other (late prophase I)

Metaphase I Pairs of homologous chromosomes are arranged on the metaphase plate One chromosome of each pair facing each pole Both chromatids of homologue are attached to kinetochore microtubules from one pole

Anaphase I Chromosomes move toward the poles guided by spindle apparatus Sister chromatides remain attached at the centromere Homologous chromosomes each composed of two sister chromatids move toward opposite pole

Telophase I and cytokinesis Each half of the cell has a complete haploid set of chromosomes But each chromosome is still composed of two sister chromatids Cytokinesis occurs simultaneously with telophase I forming two haploid daughter cells Formation of cleavage furrow

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II Since chromosomes are already replicated

Prophase II Spindle apparatus forms Chromosomes move toward the metaphase II plate (late prophase II, not shown on the figure)

Metaphase II The chromosomes are positioned on the metaphase plate as in mitosis Sister chromatides are not genetically identical Kinetochores of sister chromatides are attached to microtubules extending from opposite poles

Anaphase II Centromers separate, sister chromatids come apart Sister chromatids move as two individual chromosomes toward opposite poles

Telophase II and cytokinesis Nuclei form, chromosomes begin to decondensing and cytokinesis occurs Meiotic division of one parent cell produces four daughter cells Each with haploid set of (unreplicated) chromosomes Each daughter cell is genetically distinct from another one and from parent cell

A Comparison of Mitosis and Meiosis Meiosis can be distinguished from mitosis by three events in Meiosis l: Synapsis and crossing over- during prophase I (synapsis-homologous chromosomes become physically connected; crossing over- rearrangment between non-sister chromatids) Tetrads on metaphase plate- at metaphase I paired homologous chromosomes (tetrads) are so positioned Separation of homologues-at anaphase I

A Comparison of Mitosis and Meiosis

Evolutionary Significance of Genetic Variation Within Populations Is the raw material for evolution by natural selection Natural selection results in the accumulation of those genetic variations favored by environment

Mutations Sexual reproduction Are the original source of genetic variation Sexual reproduction Produces new combinations of variant genes, adding more genetic diversity

Summary Chapter 13 Heredity/variation Genes Asexual and sexual reproduction Life cycle Meiosis Difference between mitosis and meiosis