Chapter 8-Part 2

Explain the life cycle of a sexually reproducing organism

Alternates between haploid and diploid stages: Adults are diploid (2n); Meiosis produces haploid (n) gametes; Fertilization joins gametes → zygote (2n); Zygote grows by mitosis → adult organism

Does meiosis produce gametes that are genetically identical to the parent cell?

No. Gametes are genetically unique due to crossing over and independent assortment

Do all chromosomes in the offspring come from one parent or is there an equal mix of genetic information from each parent?

Equal mix. Each offspring gets half chromosomes from mother and half from father

Define gamete

A haploid reproductive cell with half the chromosomes of a body cell

Male gamete

Sperm

Male gametes are made in

Testes

Female gamete

Egg (ovum)

Female gametes are made in

Ovaries

Gametes are formed by

Meiosis

A gamete has

Half (n) the number of chromosomes of a parent cell

Define fertilization

Fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote

Define zygote

A diploid cell formed immediately after fertilization, containing genetic material from both parents

Define haploid

A cell with one complete set of chromosomes (n), typical of gametes

Define diploid

A cell with two complete sets of chromosomes (2n), typical of body cells

Which cells are the results of meiosis?

Gametes (sperm and egg)

When do we use mitosis?

For growth, repair, and asexual reproduction

When in the life cycle are cells haploid and when are cells diploid?

Haploid → gametes; Diploid → zygote and all body cells

What happens at fertilization?

Two haploid gametes fuse → diploid zygote forms

What is a zygote?

The first diploid cell of a new organism after fertilization

If a haploid gamete has 8 chromosomes, how many are in the diploid cell?

16 chromosomes (2 × 8)

If a 2n cell has 46 chromosomes, how many are in the haploid cell?

23 chromosomes (46 ÷ 2)

Define homologous chromosome

A pair of chromosomes, one from each parent, with the same genes in the same order, but possibly different alleles; contain genes for the same traits but are not identical

Define allele

Different forms of the same gene (e.g., blue eye allele vs brown eye allele)

Define sister chromatids

Two identical copies of a single chromosome connected by a centromere, formed after DNA replication; copies of each other, they contain exactly the same genetic information

When is DNA copied for meiosis?

During interphase, before meiosis I

When does crossing over take place?

During prophase I of meiosis

When do homologous chromosomes separate?

During meiosis I

When do sister chromatids separate?

During meiosis II

Describe Prophase I of meiosis

Homologous chromosomes pair up and exchange segments (synapsis)

-condense

-crossing over (exchange of DNA)

Describe Metaphase I of meiosis

Pairs of homologous chromosomes line up

Describe Anaphase I of meiosis

Pairs of homologous Chromosomes split up and move to opposite poles

-sister chromatids stay together

Describe Telophase I of meiosis

two haploid cells form

-each still made of two sister chromatids

Describe Prophase II of meiosis

Chromosomes condense again

-Nuclear membrane breaks down

-No homologous pairing this time

Describe Metaphase II of meiosis

Chromosomes line up individually along the metaphase plate

Describe Anaphase II of meiosis

Sister chromatids separate and move to opposite poles

Describe Telophase II of meiosis

-cells divide four haploid cells total

-each cell has single (unduplicated) chromosomes

Describe spermatogenesis

Occurs in testes; produces 4 functional sperm per meiosis

Describe oogenesis

Occurs in ovaries; produces 1 functional egg + 3 polar bodies; polar bodies degenerate

What are the sources of genetic variation

1. Crossing over → DNA exchange between homologous chromosomes (prophase I); 2. Independent assortment → random alignment of homologous chromosomes (metaphase I); 3. Random fertilization → any sperm can fertilize any egg

What is a chiasma

The site of crossing over between homologous chromosomes

Define monosomy

Missing a chromosome;

- missing autosome → usually fatal(serious like deadly) ; missing sex chromosome → sometimes survives

Define trisomy

Extra chromosome; not all trisomies survive; e.g., trisomy 21 (Down syndrome) is viable

Define non-disjunction

Chromosomes fail to separate during meiosis; Meiosis I → homologous chromosomes fail; Meiosis II → sister chromatids fail

Describe chromosomal abnormalities on a karyotype

Look for extra or missing chromosomes; example: Down syndrome = 3 copies of chromosome 21

Describe Down syndrome

Trisomy; extra chromosome 21; seen as 3 copies of chromosome 21 on a karyotype

Mitosis vs Meiosis Chart

Mitosis vs Meiosis:

Rounds of DNA replication: 1 | 1

Rounds of cell division: 1 | 2

Homologous chromosomes separate? No | Yes (Meiosis I)

Sister chromatids separate? Yes | Yes (Meiosis II)

Cells produced: 2 | 4

Haploid or diploid: Diploid | Haploid

Function: Growth, repair, asexual reproduction | Produce gametes for sexual reproduction