Unit 3a Sexual Reproduction and Meiosis

sexual reproduction

The combining of two sex cells to produce an offspring. It leads to genetic variation and organisms with 2 sets of chromosomes.

homologous chromosomes

Pair of chromosomes that are the same size, same shape, same centromere position, and same gene locations but the nucleotide sequence can be different.

Alleles

Different forms of a gene

Homozygous

having two identical alleles for a trait

Heterozygous

having two different alleles for a trait

Diploid (2n)

Somatic cell that contains homologous pairs and have both sets

Haploid (n)

Sex cell that does not contain homologous pairs and have one set

Gametes

sperm and egg cells

Zygote

fertilized egg

Karyotype

A picture that shows the complete diploid set of chromosomes grouped in pairs and arranged in size and order

Mitosis

Diploid cells divide into two and create genetically identical cells

Meiosis

A specialized type of cell division that creates four genetically unique haploid cells. It occurs prior to sexual reproduction, only occurs in sexually reproducing organisms, and the number of chromosomes is reduced by half.

Why is it important for sex cells to have only have a half set of
chromosomes?

the haploids come from the egg and sperm cells to make a diploid zygote

Meiosis I

A diploid cell divides to form two haploid cells

Prophase I

DNA condenses into chromosomes, centrioles move to poles, spindle fibers begin to form, nuclear membrane disappears, homologous pairs come together, and crossing over occurs (DAC)

crossing over

Homologous chromosomes swap genes when they are paired up in a tetrad during prophase I

Tetrad

structure containing 4 chromatids that forms during meiosis

Metaphase I

Tetrads line up in the middle of the cell and spindle fibers attach to centromeres

independent assortment

the random lining up of pairs of genes on different chromosomes that lead to different combinations (DAC)

Anaphase I

tetrads are pulled to opposite ends of the cells

Telophase I

furrow starts to form, nuclear membrane reforms, DNA uncoils to chromatin, and spindle fibers disappear (DAC)

Cytokinesis I

cytoplasm divides

Meiosis II

two haploid cells containing DAC divide into four haploid cells with SAC. no DNA replication

Prophase II

nuclear membrane brakes down, DNA condenses into DAC, centrioles move to the poles, and spindle fibers begin to form (DAC)

Metaphase Ii

double arm chromosome line up in the middle of the cell and spindle fibers attach to the centromere

Anaphase II

sister chromatids separate (SAC)

Telophase II and Cytokinesis

nucleus reappears, chromosomes uncoil into chromatin, and cytoplasm divides

Spermatogenesis

Formation of sperm in testes

Oogenesis

Formation of eggs in ovaries that creates 1 viable egg and 3 polar bodies.

random combination of sperm and egg

it is entirely random which sperm will fertilize the random egg

XX

female sex chromosomes

XY

male sex chromosomes

Trisomy

extra chromosome

Monosomy

missing a chromosome

Nondisjunction

Error in meiosis in which homologous chromosomes fail to separate