Meiosis and Crossing Over

How are Chromosomes matched?

In sexually reproducing organisms, chromosomes exist in pairs called homologous chromosomes. These pairs are crucial for inheritance and genetic diversity.

Somatic cells

body cells) that contain a species-specific number of chromosomes. For example, humans have 46 chromosomes, organized as 23 pairs of homologous chromosomes.

Locus

homologous pair carries genes for the same traits at the same location, known as a?

Alleles

Homologous chromosomes may carry different versions (blank) of the same gene.

Gametes

(egg and sperm), which are specialized cells with half the chromosome number of somatic cells.

How many sets of chromosomes does gametes have?

Gametes are haploid (n), containing only one set of chromosomes.

How many sets of chromosomes do cells have?

Cells with two sets of homologous chromosomes are diploid 

Sexual life cycles 

alternate between haploid and diploid stages

What produces haploid gametes?

Meiosis

What restores diploid chromosome number in the zygote?

Fertilization

What enables the growth and development of the multicellular diploid organism?

Mitosis

Which type of cell division reduces the chromosome number by half, producing four genetically unique haploid cells from one diploid cell?

Meiosis

Before the two rounds of cell division of meiosis can begin what has to happen?

chromosome duplication during interphase.

Mow many consecutive divisions does meiosis consist of?

Two divisions: Meiosis I and Meiosis II

Meiosis I

• Homologous chromosomes pair up and exchange segments in a process called crossing over.

• Homologous chromosomes are separated, resulting in two haploid cells with duplicated chromosomes.

Meiosis II

• Sister chromatids of each chromosome are separated.

• Results in four haploid cells, each genetically distinct from the parent cell and from each other.

Key differences between mitosis and meiosis 

• Mitosis produces two genetically identical diploid somatic cells.

• Meiosis produces four genetically unique haploid gametes.

What leads to many different combinations of chromosomes in eggs and sperm

Random arrangements of chromosome pairs at metaphase I of meiosis

What leads to varied offspring?

Random fertilization of eggs by sperm greatly increases this variation

What chromosome can bear different versions genes?

Homologous Chromosomes

What is the exchange of corresponding segments between nonsister chromatids of homologous chromosomes

Crossing over 

An abnormal chromosome count is the result of?

Nondisjunction

what can nondisjunction result from?

the failure of pair of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II 

What is a Karyotype?

A photographic inventory of an individuals chromosomes

How to prepare a karyotype?

White blood cells are isolated, stimulated to grow, arrested at metaphase, and photographed under a microscope

The chromosome number abnormality that results in a condition called down syndrome is?

trisomy 21

Abnormal numbers of sex chromosome do not usually affect survival, why?

In some cases (such as X X Y), this leads to syndromes that can affect the health of the individual. In other cases (such as XXX), the body is normal