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Biology Chapter 13 2


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the scientific study of the transmission of traits from parents to offspring (heredity) and the varitions between and within generations

The scientific study of heredity and hereditary variation
Living Organisms
Are distinguished by their ability to reproduce
The transmission of traits from one generation to the next
Shows that offspring differ somewhat in appearance from parents nad siblings
Asexual Reproduction
Mitosis produces genetically identical cells

May result in clones of one parent
Sexual Reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from parents
reduces the number of chromosome sets from diploid to haploid
Haploid Cell
contains only one set of chromosomes

Generally in humans 23 chromosomes (n=23)

Haploid cells produce Diploid cells with fertilization (fusing two gametes)
Diploid Cell
has 2 sets of each of its chromosomes

One set of chromosomes from mother and one set from the father

Generally humans, 26 chromosomes (2n=46)

Diploid produce Haploid daughter cells with Meiosis (halving the chromosome number)
Types of haploid and diploid cells in animals
Gamete (germ cells) (sperm and egg cells) are haploid cells

Somatic cells (body cells) are diploid cells

many exceptions
Sexual Maturity
the ovaries and testes produce haploid gametes by meiosis

During fertilization these gametes, sperm and ovaum, fuse, forming a diploid zygote
The Zygote
Develops into an adult organism by mitosis
Chromosomes in Human Cells
A Kayotype is an ordered, visual representation of the chromosomes in a cell
Homologous Chromosomes
occur in pairs

have the same characteristics: length, centromere position, gene Loci

Both homologous chromosomes carry genes controlling the same inherited characters but usually pairs are not identical
Sex Chromosomes
Determines the sex of the individual

In Humans
are represented by X and Y

XX being female

XY being male

are non-homologous
Autosome chromosomes
Do not dtermine sex (body genes)
Behavior of Chromosomes in Life Cycle
In cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids
Meiosis I
called reductional division because it reduces the chromosome sets from two (diploid) to one (haploid). The sister chromatids of each homologue do not separate until meiosis II
Meiosis II
Produces 4 haploid daughters

Looks like a regular mitotic division in which chromosomes line up individually on the metaphase plate and sister chromatids separate and move apart in anaphase II. At the end of telophase II there are four haploid daughter cells.
Interphase I
Interphase I (each chromosome replicates, producing 2 genetically identical sister chromatids that remain attached to their centromeres. During prophase I homologous chromosomes synapse and crossovers may occur forming chiasmata
Stages of Meiosis
Stage 1 - Interphase I

Stage 2 - Metaphase I

Stage 3 - Anaphase I

Stage 4 - Telophase I

Stage 5 - Cytokenesis

Meiosis II

Stage 1 - Anaphase II

Stage 2 - Telophase II
Differences between Mitosis and Meiosis
2 division in Meiosis

1 division in Mitosis

By events in Meiosis I
Tetrad Information

synapsis with crossing over at chiasmata

Homologous pairs separate

Meiosis makes non-identical daughter cells (haploid cells from diploid cells)

Mitosis makes identical daughter cells
Prophase I of Meiosis
Tetrads form

Tetrads are paired, homologoug, duplicated chromosomes

Synapsis and crossing over occur in tetrads. Homologous chromosomes physically connect and exchange genetic information
Separation of chromosomes
Anaphase I of meiosis, tetrads separte into homologous pairs that move toward opposite poles of the cell

In anaphase II of meiosis, the sister chromatids separate
Comparison of Mitosis and Meiosis

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