Lecture 11 - Meiosis and Sexual Life Cycles

Meiosis

Only occurs in sexually reproducing eukaryotic organisms. Occurs in the GONADS. Produces four cells called gametes, each with a single chromosome

Gonads

Ovaries and Testes

Haploid

Only contains a single chromosome set (if parent had 46, haploid would only have 23) Originate from a diploid parent cell. Ex: reproductive cells = gametes (sperm and egg cells)

Diploid

Contains 2 complete “sets” of chromosomes. Parent cell.

Heredity or inheritance

The transmission of traits to the next generation

Genetic Variation

Offspring differ from what their parents or siblings resemble.

Genes

ARE SEGMENTS OF DNA that hold instructions for building specific proteins that in turn govern the traits and characteristics, and behaviors that emerge in organisms as they develop. Cells convert this info into synthesis of proteins.

Genome

the sum total of all the genes of a given species.

Most genes program cells to synthesize specific enzymes and other proteins that result in the production of an organism’s inherited traits.

It is the number, type, timing, and interaction of these proteins that give an organism its physical traits & biochemical characteristics.

Gene locus

the specific location of a gene — each chromosome has hundreds or thousands of genes along the length of chromosome, each gene located at specific spots

Asexual reproduction

Produces a clone of the parent cell - mutations may occur that result in any genetic variation.

Single-celled eukaryotes can reproduce asexually by mitotic cell division.

Sexual Reproduction

when two parents produce offspring that have unique combinations of genes inherited from the two parents.

Karyotype

Image of all chromosomes arranged in pairs produces a ____ display.

Have the same gene loci, length, centromere, position, and staining pattern

Homologous chromosomes (two chromosomes in a pair) have the same,,,,

Homologous

Carries identical genes, control the same inherited traits. However, they may have different versions of the gene called alleles.

Alleles

different version of the same gene. gene locus remains the same between homologous chromosomes, but have different types of that gene.

Sex chromosomes

X and Y chromosomes are called ______. Make up one chromosome pair. they are not homologous but are paired together.

Autosomes

Not the sex chromosomes, but the other 22 pairs of chromosomes are called…

We inherit one chromosome of each homologous pair from each parent. The 46 chromosomes in each somatic cell are two sets of 23 (maternal set and paternal)

Inheritance of chromosomes…

Diploid

Any cell with two full sets of chromosomes (2n)

Sperm cells contain 22 autosomes and an X or Y chromosome, Ovum has 22 autosomes and an X chromosome.

Sperm and Ova sets of chromosomes

Meiosis I

separates homologous chromosomes… A Reductional division that halves the number of chromosome sets from two to one.

Meiosis II

Separates sister chromatids - equational division (2 haploid cells divide to yield 4 haploid cells)

1. Synapsis - during prophase I… homologs pair up and form synaptonemal complex, holding them together in synapsis

2. Crossing over - sister chromatid cohesion along the arms resulting in formation of chiasma indicating the location of crossing over

Unique events occuring during Meiosis I Prophase (do not happen in mitosis)

Chiasma

The x-shape indicating the location of crossing over

Homologs on the metaphase plate - unlike mitosis pairs of homologs rather than individual chromosomes line up at the metaphase plate

Unique events occuring during Meiosis I, metaphase

Separation of homologs - duplicated chromosomes of each homologous pair separate and move towards opposite poles, while sister chromatids of each duplicated chromosome remain attached

Unique to Meiosis I during Anaphase I

Life Cycle

The generation to generation sequences of stages in the reproductive history of an organism

• ____ starts at the conception of the organism and continues until the organism produces its own offspring.

Both haploid and diploid cells can undergo mitosis, but only diploid can undergo meiosis.

Haploid vs diploid replication

Gametes do not divide but fuse (fertilize) to form a diploid zygote that divides during mitosis to produce a multicellular diploid organism

First type of life cycle in which gametes are the only haploid cell

Alternation of Generations - plants and some algae

Type of Life cycle where it includes 2 multicellular stages… sporophyte generation produces gametophyte as its offspring and the gametophyte generation produces the next sporophyte generation

Sporophyte — meiosis in sporophytes produces haploid spores

Multicellular diploid stage found in plants

Unlike a gamete, haploid spore do not fuse together, they replicate through mitosis to form a multicellular haploid gametophyte stage.

Haploid spore

Gametophyte

Multicellular haploid found in plants

Gametes fuse to form a zygote (only diploid phase), haploid adult produces gametes by mitosis

Life cycle of most fungi and some protists

Independent Assortment of Chromosomes

This adds to the genetic variability due to the random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I.

-fifty fifty chance that a particular daughter cell from meiosis I will get the maternal chromosome or the paternal.

• Each homologous pair separate independently of each other, not related.

• Number of combinations possible when chromosomes separate independently of each other = 2ⁿ

Recombinant Chromosomes

Crossing over results in ____. which combine genes inherited from each parent. During crossing over each gene is alined with its corresponding gene found on the other homolog

Random Fertilization (adds to genetic variation)

Any sperm can fuse with any egg… crossing over adds even more variation to this.

 Mutations are the original source of different alleles, which are then mixed and matched during meiosis (increasing allele combinations and in so doing, genetic variation).

 New and different combinations of alleles may work better than previous ones.

 Charles Darwin recognized the importance of genetic variation in evolution.