Meiosis and Sexual Reproduction

Ch. 7

Biology

Ms. Haut

Introduction to Heredity

• Offspring acquire genes from parents by inheriting chromosomes

• Inheritance is possible because:

– Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg

Actual transmission of genes depends on the behavior of chromosomes

•Chromosomes-organizational unit of hereditary material in the nucleus of eukaryotic organisms•Contain hundreds of thousands of genes, each of

which is a specific region of the DNA molecule, or locus

Meiosis

• Reduces chromosome number from diploid to haploid• Increases genetic variation among offspring• Steps resemble steps in mitosis• Single replication of DNA is followed by 2

consecutive cell divisions – Meiosis I– Meiosis II

• Produces 4 different daughter cells which have half the number of chromosomes as the original cell

Interphase I

• Chromosomes replicate (still as chromatin)• Duplicated chromosomes consist of 2 identical sister chromatids attached by centromere• Centriole pairs replicate

Meiosis I

• This cell division separates the 2 chromosomes of each homologous pair and reduce the chromosome number by one-half

Prophase I• Chromosomes condense• Synapsis occurs

(homologues pair)• Chromosomes seen as

distinct structures; each chromosome has 2 chromatids, so each synapsis forms a tetrad

Prophase I

• Sister chromatids held together by centromeres; non-sister chromatids held together by chiasmata where crossing-over occurs (exchange of DNA)

Late Prophase I

• Centriole pairs move apart and spindle fibers form

• Nuclear envelope disappears and nucleoli disperse

Prophase I

Metaphase I

• Homologous chromosomes line up along metaphase plate

Metaphase I

Anaphase I

• Homologous chromosomes separate, independently from others

Anaphase I

Telophase I and Cytokinesis

• Each pole now has a haploid set of chromosomes (each with 2 sister chromatids)

• Usually, cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells (cleavage furrow forms in animals; cell plate forms in plants)

Telophase I

Meiosis II

• This cell division separates the 2 sister chromatids of each chromosome

Prophase II

• Spindle apparatus forms and chromosomes move toward metaphase II plate

Prophase II

Metaphase II

• Chromosomes align singly on the metaphase plate

Metaphase II

Anaphase II

• Sister chromatids of each pair (now individual chromosomes) separate and move toward opposite poles of the cell

Anaphase II

Anaphase II

Telophase II and Cytokinesis

• Nuclei form at opposite poles of the cell

• Cytokinesis occurs producing 4 haploid daughter cells (each genetically different)

Telophase II

Telophase II

Key Differences Between Mitosis and Meiosis

• Meiosis is a reduction division

– Mitotic cells produce clones (same xsome #)

– Meiosis produces haploid cells

• Meiosis creates genetic variation

– Mitosis produces 2 identical daughter cells

– Meiosis produces 4 genetically different daughter cells

• Meiosis is 2 successive nuclear divisions

– Mitosis has one division

Mechanisms of Genetic Variation

1. Independent assortment—each pair of homologous chromosomes separate independently

– Results in gametes with different gene combinations

2. Crossing-over—exchange of genetic material between non-sister chromatids

– Results in genetic recombination

3. Random fertilization—random joining of two gametes

Importance of Genetic variation

• Essential to evolution (change over time)

• Variation can cause changes that leads to different traits– Some favorable– Some unfavorable

Spermatogenesis

• Process of sperm production

• Results in 4 viable sperm

Oogenesis

• Process of egg (ova) production

• Results in 1 viable egg and 3 polar bodies that will not survive

• Polar bodies result from an uneven division of cytoplasm

Asexual Reproduction

• Prokaryotes—binary fission

• Eukaryotes—2 mechanisms– Fission—separation of a

parent into two or more individuals of identical size

– Budding—new individuals split off from existing ones

Asexual Reproduction

Sexual Reproduction

Advantages

•Produce offspring in short time•Uses no energy

•Genetic diversity•Allows adaptation to changing environment

Disadvantages

•No genetic diversity•Harder to adapt to changing environment

•Uses energy

Eukaryotic Sexual Life Cycles

• Life cycle—entire span in the life of an organism from one generation to the next

• All sexually reproducing organisms follow a basic pattern– Alteration between diploid and haploid

chromosome numbers

Haploid Life Cycles

n

n

nFresh water green algae

Diploid Life Cycles

Alternation of Generations Life Cycles

Parthenogenesis

• New individual develops from an unfertilized egg

• Offspring is a clone of the mother

• Occurs in long absences of male companionship