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CHAPTER 13 – MEIOSIS AND LIFE CYCLES

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Introductory Vocabulary Heredity the transmission of traits from one generation to the

next Variation when offspring differ somewhat from their parents

and siblings Genetics the scientific study of heredity and hereditary

variation Genes segments of DNA that code for individual traits/proteins;

parents endow their offspring with this coded information (genes); genetic information is transmitted as specific sequences of the four nucleotides in DNA. Most genes program cells to synthesize specific enzymes and

other proteins whose cumulative action produces an organism’s inherited traits.

Gametes reproductive cells; haploid (one copy of each chromosome); egg and sperm; made during meiosis in the gonads (ovary/testes)

Fertilization fusion of egg and sperm After fertilization genes from both parents are present in the

nucleus of the fertilized egg, or zygote.

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Asexual vs. Sexual ReproductionASEXUAL (MITOSIS) →- Makes SOMATIC cells- one diploid to two

diploid cells - identical to parent- ONE parent donates

genetic information

SEXUAL (MEIOSIS) → - makes GAMETES- One diploid to four

haploid cells- All new cells are

GENETICALLY DIFFERENT from each parent and each other

- TWO parents contribute genetic information

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Haploid vs. Diploid Diploid TWO sets of each chromosome pair

Humans have a diploid number of 46; therefore each somatic cell (body cell) has 46 chromosomes

This is represented by 2n These chromosome pairs are called homologous

chromosomes (see next slide) Cells made by MITOSIS are diploid

Haploid ONE set of each chromosome pair Humans have a haploid number of 23; therefore each

gamete (sperm/egg) have 23 chromosomes This is represented by n Cells made by MEIOSIS are haploid

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Homologous Chromosomes Diploid organisms have

homologous chromosomes. These chromosomes are the same size, shape, carry genes for the same traits, and have the centromere in the same spot. However, the genes can be different! For example, the maternal

chromosome (the one you got from mom) can have a brown eye gene and the paternal chromosome can have a blue eye gene

The EXCEPTION to the homologous chromosome pattern are the sex chromosomes, X and Y Females = XX Males = XY

The other 22 pairs of chromosomes are called autosomes.

It is crucial to understand the differences among homologous chromosomes, sister chromatids, nonsister chromatids, and chromosome sets.

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Karyotypes

A karyotype is a picture of all the chromosomes of an organism. The chromosomes are usually ordered by size and can give clues about genetic disorders. They can be done on babies before they are born by amniocentesis or CVS – chorionic villi sampling.

This can tell doctors if the person has an extra chromosome or is missing one, or if they have some other type of chromosomal disorder (deletion, duplication, etc).

Down Syndrome Karyotype

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Beginning of the Life Cycle

The human life cycle begins when a haploid sperm cell fuses with a haploid ovum (egg), this joining of cells is called syngamy.

The union of these gametes, culminating in the fusion of their nuclei, is fertilization. (in other words, syngamy precedes fertilization)

The fertilized egg (zygote) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.

As a person develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body.

Gametes develop from specialized cells called germ cells in the gonads.

Gametes undergo the process of meiosis, in which the chromosome number is halved.

Fertilization restores the diploid condition by combining two haploid sets of chromosomes.

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Life Cycle Type 1: Humans and other

Animals

Gametes (HAP) → Fertilization → Zygote (DIP) → Mitosis and Development → Meiosis in Gonads → Gametes (HAP)

Adult form is Diploid…meiosis is done only to create haploid gametes

Life Cycle generation to generation sequence of stages in the reproductive history of an organism

Fertilization and meiosis alternate in all sexual life cycles, in plants, fungi, protists, and animals.

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Life Cycle Type 2: Plants and some algae (protists)

- This life cycle includes two multicellular stages, one haploid and one diploid.

- The multicellular diploid stage is called the sporophyte.

- Meiosis in the sporophyte produces haploid spores.

- Unlike a gamete, a haploid spore doesn’t fuse with another cell but rather divides by mitosis to form a multicellular haploid gametophyte stage.

- Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the sporophyte by mitosis.

- In this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation.

Alternation of Generations

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Life Cycle Type 3:Most

fungi and Protists

“Alternation of Generations”

Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result: genetic variation among offspring

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Meiosis Many steps of meiosis resemble steps in mitosis. Both meiosis and mitosis are preceded by the duplication of

chromosomes. (during interphase) In meiosis, there are two consecutive cell divisions, meiosis

I and meiosis II, resulting in four daughter cells. The first division, meiosis I, separates homologous

chromosomes. The second division, meiosis II, separates sister chromatids. The four daughter cells at the end of meiosis have only HALF

as many chromosomes as the original parent cell.

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Meiosis I is preceded by interphase, in which the chromosomes are duplicated to form sister chromatids.

Two genetically identical sister chromatids make up one duplicated chromosome.

The sister chromatids are closely associated all along their length. This association is called sister chromatid cohesion. (Recall this from Chapter 12!)

In contrast, the two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents.

Homologous chromosomes appear to be alike, but they may have different versions of genes, each called an allele, at corresponding loci.

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Synapsis = Formation of Tetrads When the homologous chromosomes come together

they are held together by the synaptonemal complex; this process is called synapsis

Crossing over also happens at this stage Occurs during Prophase I

Prophase 1 – Synapsis

Ignore the poor English…

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Prophase 1 – Crossing over

Crossing over occurs during Prophase I (only Meiosis I – not meiosis II because tetrads are not present in Meiosis II) of meiosis. The homologous chromosomes of the tetrads can switch positions and the genes can be exchanged. This is one of the processes that leads to genetic variation.

Crossing over occurs between NON-SISTER CHROMATIDS. The location where crossing over happens is called the chiasmata. Crossing over can happen 2-3 times per chromosome.

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Meiosis I

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Meiosis II

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Meiosis I → homologous chromosomes are separated

Meiosis II → sister chromatids are separated

Meiosis = Two divisions; creates 4 new haploid cells that are genetically different from the parent cell.

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BOTH preceded by Interphase Meiosis HALVES the chromosome number Mitosis CONSERVES the chromosome

number Meiosis makes gametes (egg/sperm) Mitosis makes somatic cells (body cells) Meiosis yields daughter cells that are all

genetically different Mitosis yields daughter cells that are all

identical

Comparing Mitosis to Meiosis

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Processes that are UNIQUE to MEIOSIS

1. Synapsis and Crossing Over1. Making the tetrads (synapsis) and crossing

over between non-sister chromatids; occurs during Prophase I

2. Tetrads (homologs) on the metaphase plate

1. In mitosis this step is just the individual chromosomes on the metaphase plate, not the tetrads

3. Separation of the tetrads (homologs)1. In Anaphase I, the tetrads separate; in mitosis

the sister chromatids separate during anaphase

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Although MUTATIONS are the ORIGINAL SOURCE of genetic diversity, there are three factors that contribute to increasing genetic variation during meiosis: - Independent Assortment - Crossing Over - Random Fertilization

Genetic Variation

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Independent AssortmentIndependent assortment means that the maternal chromosomes and the paternal chromosomes can line up any way possible. This contributes to genetic variation.

To determine how many possibilities there are, use the following equation:

N = haploid number

2N = number of combinations

If n = 3, there are 23 = 8 possible combinations.For humans with n = 23, there are 223, or about 8.4 million possible combinations of chromosomes.

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Crossing Over

Produces RECOMBINANT CHROMOSOMES. This process combines genes inherited from each parent. Recall: the location of crossing over is called chiasmata

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Evolutionary Adaptation depends on a population’s

genetic variation Charles Darwin recognized the

importance of genetic variation in evolution.

As the environment changes, the population may survive if some members can cope effectively with the new conditions.

Mutations are the original source of different alleles, which are then mixed and matched during meiosis.