CHAPTER 10ASexual Reproduction

In the dvlpt of most multicellular

organisms, a single cell (fertilized egg) gives rise to many different types of cells, each w/ a different structure & corresponding function

The fertilized egg gives rise to a large # of cells through cell division

As the cells inc. in # through this division, they also become specialized in structure & function through differentiation The various types of cells then arrange into

tissues which are organized into organs, & finally organ systems

Nearly all of the cells of a multicellular organism have exactly the same chromosomes & DNA During differentiation, only a specific segment

of the DNA is activated The segment activated deter. the function &

structure of the cell So all cells initially have the potential to

become any type of cell Once a cell differentiates, the process can’t be

reversed

Stem Cells are unspecialized cells that continually reproduce &, under the appropriate conditions, can differentiate into 1 or more types of specialized cells Embryonic Stem Cells – have this ability Adult Stem Cells are only found in certain

areas such as bone marrow**Scientists have discovered under the right

lab culture conditions, embryonic & adult, can differentiate

Chromosomes: It is a structure in the nucleus of a cell

consisting of 1 long thread of DNA that is tightly coiled

Types of Reproduction:1. Asexual reproduction – when a cell just divides

to form 2 identical daughter cells – only 1 parent In eukaryotic cells, it requires mitosis

2. Sexual reproduction – the union of a sperm and egg to form an organism that has a combination of DNA from both parents

Since the offspring produced have cells that are diploid (containing 2 sets of chromosomes – 1 from each parent), the gametes (sex cells) must be haploid (containing only 1 of each type of chromosome)

Requires meiosis

Meiosis: Purpose – to reduce the # of

chromosomes Occurs in 2 steps:

Meiosis I – Chromosome pairs replicate Results in 2 haploid daughter cells w/

duplicated chromosomes diff. from the sets in the original diploid cell

Meiosis II – The haploid daughter cells from Meiosis I divide Results in 4 haploid daughter cells called

gametes (sex cells - egg & sperm) w/ undoubled chromosomes

Meiosis I: Begins after Interphase in which cells (1)

inc. in size, (2) produce RNA, (3) synthesize proteins, & (4) replicate DNA

Meiosis I cont… Prophase I –

Nuclear membrane breaks down Centrioles separate from each other & take

up positions on the opposite sides of the nucleus

Spindle fibers are produced Chromosomes pair up & become visible as

a cluster of 4 chromatids called a tetrad

Meiosis I cont… A homologous chromosome pair consists

of 2 chromosomes containing the same type of genes One from the male & one from the female

Each chromosome consists of 2 sister chromatids attached @ the centromere

Because the homologous chromosome pairs are close to each other, crossing over can occur (the exchange of chromosome genetic material btw pairs)

Meiosis I cont… Metaphase I:

The chromosomes are attached to the spindle fiber at the centromere& are pulled into the midline (or equator) of the cell in pairs

Meiosis I cont… Anaphase I:

The chromosome pairs separate, one chromosome to each side of the cell Each daughter cell will receive only 1

chromosome from each homologous chromosome pair

Sister chromatids remain attached to each other

Meiosis I cont… Telophase I & Cytokinesis:

Chromosomes gather @ the poles, nuclear membrane may form & the cytoplasm divides

***Each of the 2 daughter cells from meiosis I contains only 1 chromsome (consisting of 2 chromatids) from each parental pair

***Each daughter cell from meiosis I proceeds to meiosis II

Meiosis II Prophase II:

Spindle fibers form in each of the daughter cells from meiosis I & attaches to the centromeres of the sister chromatids

The chromosomes progress towards the midline of each cell

The nuclear membrane breaks down

Meiosis II cont… Metaphase II:

Chromosomes, made up of 2 sister chromatids, line up across the center of the cell

Spindle fibers from opposite poles of the cell attach to 1 of each pair of chromatids

Meiosis II cont… Anaphase II:

The chromosomes separate so that 1 chromatid from each chromosome goes to each pole

Meiosis II cont… Telophase II & Cytokinesis:

Nuclear membrane forms around each set of chromosomes

The resulting daughter cells are haploid, containing 1 single chromosome from each pair of chromatids Either from father or mother contributor