Biology Pop-quiz Review 1. mRNA is formed during… 2. A waste product of photosynthesis 3. Proteins are made during… 4. Gas produced during respiration 5.The function of photosynthesis is to produce… 6. Organic compound that makes up the cell membrane A) transcription B) CO2 C) translation D) lipid E) oxygen F) sugar

4-2 Reproduction Asexual reproduction (binary fission): ex. Bacteria, each new cell is identical to the parent cell. (Mitosis is a form of asexual reproduction). Sexual reproduction: When gametes (sex cells), each containing half the # of chromosomes, are produced (meiosis) and come together.

Meiosis Produces gametes containing half the number of chromosomes as the parent cell Cells that undergo meiosis are germ cells Reduction of chromosome number by half is important. Without meiosis, the chromosome number of a species would double with each generation. Male gametes = sperm Female gametes = egg Sperm + egg = zygote  Full chromosome number is restored!

Meiosis… Cells produced by meiosis have unpaired chromosomes because the number has been reduced by half…they are haploid (n) as opposed to cells produced by mitosis (2n = diploid) Meiosis is a 2 stage process! / \ Meiosis I (PMAT) Meiosis II (PMAT 2) *homologous chromosomes *Each nucleus divides again Separate from their partners producing 4 haploid nuclei

Meiosis I Interphase Prophase I Chromosomes replicate There are now double the normal # of chromosomes Chromosomes are uncoiled Prophase I Chromosomes condense Each chromosome is double, consisting of 2 chromatids Homologous chromosomes pair up – This is called Synapsis  Each synapsing group consists of 4 chromatids (2 chromatids/chromosomes) called a tetrad. Crossing over occurs here!

Meiosis I continued… Metaphase I Anaphase I Tetrads move along spindle fibers to the equator of the cell. Anaphase I Homologous chromosomes are pulled apart Pairs of chromatids move toward opposite poles of the cell

Meiosis I continued… Telophase I The cell divides into 2 smaller cells Each cell contains 1 homologous chromosome from the original pair The new cells are NOT identical because homologous chromosomes do not contain identical genetic information.

Back to “Crossing Over”… Crossing Over occurs in Prophase I when homologous chromosomes pair up (synapsis) Segments of chromatids may be exchanged which rearranges genes. This causes genetic variety in the resulting gametes.

Meiosis II All the same, just now with 2 cells dividing Prophase II Metaphase II Anaphase II Telophase II RESULT: 4 haploid, genetically different sex cells!

Results of Meiosis Each new cell contains a new combination of genetic material. This makes the offspring different from the parent. Meiosis, aka “reduction division” 2n  n diploid to haploid

Terms Spermatogenesis: formation of sperm cells 1 germ cell  4 functional sperm cells Oogenesis: formation of egg cells 1 germ cell  1 functional egg cell + polar bodies Nondisjuction: failure of homologous chromosomes to separate in meiosis I. Ex: Trisomy 21 (Down’s Syndrome), Monosomy X (Turner’s)

46 46 2 copies of 46 2 copies of 46 46 46 46 46 23 23 23 23

Back to nondisjunction… Homologous chromosomes fail to separate in (both chromosomes move to the same pole of the cell). Trisomy – a gamete w/ an extra chromosome is fertilized by a normal gamete = zygote w/ an extra chromosome Ex. Down Syndrome if this happens with the 21st chromosome in humans Organisms with extra chromosomes tend to survive where those lacking one usually do not. One non-lethal monosomy = Turner’s Syndrome, which is a female with only 1 X chromosome.

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