Chapter 10 - Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction

Loosely coiled - euchromatic

• active chromatin

• can be transcribed by RNA polymerase and transcription factors to make proteins

Tightly coiled - heterochromatin

• inactive chromatin

• genes hardly ever transcribed

• compact chromosomes more easily moved than extended chromatin

Most chromosomes have both compaction levels

Interphase

• divided into 3 phases: G1 = 1st Gap

• cell doing its “everyday job”

• S = DNA Synthesis

• copies chromosomes

• G2 = 2nd Gap

• prepares for division 

• cell grows (more)

• produces organelles, proteins, membranes

Cell Cycle

• cell have a “life cycle”

1. Cell is formed from miotic division

2. Cell grows and matures to divide again 

3. G1, S, G2, M

4. Epithelial cells, blood cells, stem cells

1. Some cells grow, mature, and divide to never divide again

2. G1 -> G0

3. Brain/nerve cells, muscle cells

Liver cell can come out of G0 and go through phases, then rest again

Copying/Replicating DNA

• S - Synthesis phase of Interphase

• dividing cell replicates DNA

• must separate DNA copies correctly to 2 daughter cells

• human cell duplicates ~3 meters DNA

• each daughter cell gets complete identical DNA

• error rate = ~1 per 100 million bases

• 3 billion base pairs in mammalian genome

Mitosis- produce cells with same information, identical daughter cells

• Chromosomes are replicated in S phase of interphase then halved prior to fertilization

• Parents are diploid (2n)

• Meiosis produces haploid (n) gametes

• Haploid cells contain a single set of chromosomes

• If there were no reduction of chromosomes in meiosis the number of chromosomes would double each generation

• Gametes fuse in fertilization to form a diploid (2n) zygote

• The zygote becomes the next diploid (2n) generation

• If meiosis goes wrong, gametes contain the wrong number of chromosomes

Meisos = reduction division

• special cell division in sexually reprudocing organisms

• reduce 2n -> 1n

• diploid -> haploid (half)

• makes gametes (sperm, eggs)

Meiosis 1: diploid cell enters prophase 1, homologous chromosomes separate leaving Haploid cells with pairs of sister chromatids (n)

Meiosis 2: sister chromatids separate leaving 4 haploid cells with individual chromosomes with no homologous pairing (n)

Double division of meiosis:

1st division separates homologous pairs

2nd division separates sister chromatids

1st step of Meiosis

• meiosis evolved after mitosis

• convenient to use “machinery” of mitosis

DNA replicated in S phase of meiosis (just like mitosis)

Asexual reproduction (Mitosis, binary fission, etc): produces genetically identical clones, advantageous when the environment is stable

Sexual reproduction

1. Mutations can generate variation in offspring

2. Random fertilization: union of male and female gametes

3. Meiosis brings about genetic variation in two key ways: Crossing over and Independent assortment

Crossing Over

• During prophase 1

• homologous pairs swap pieces of chromosome

• sister chromatids intertwine

• “synapsis”

1. Exchange of genetic material between non sister chromatids

2. May occur several times in each chromosome

3. Holds homologous together aligns DNA of non-sister chromatids, allows crossing over to occur

4. Homologues separate and are distributed to different cells

Meiosis 1: 1st division of meiosis separates homologous pairs (2n -> 1n)

• interphase 1

• prophase 1

• metaphase 1

• anaphase 1

• telophase 1

Meiosis 2: 2nd division of meiosis separates sister chromatids (1n -> 1n)

• prophase 2

• metaphase 2

• anaphase 2

• telophase 2

Meiosis and crossing over introduce great genetic variation to population and drives evolution

• Random assortment in humans produces 2^23 (8,388,608) different combinations 

• Any 2 parents will produce a zygote with over 70 trillion (2^23 x 2^23) diploid combinations

Prophase I: Crossing over

Anaphase I: Homologs separate

Anaphase II: Sisters separate

Changes in Chromosome Number and Structure

Euploidy is the correct number of chomrosomes in a species for multiples of that set

Aneuploidy is a change in the chromosome number resulting from nondisjunction  

Nondisjunction (Failure of separation):

• homologos chromosomes do not separate normally during anaphase I

• sister chomarids did not separate normally during anaphase II

• result in gain or loss of chromosomes 

- one gamete receives two of the same type of chromosome, and another gamete receives no copy

Aneuploidy results from fertilization involving gametes in which nondisjunction occurred.

Offspring with this condition have an abnormal number of a particular chromosome.

• Monosomic zygote only has one copy of a particular chromosome (Almost all result in miscarriage, except X0)

• Trisomic zygote has three copies of a particular chromosome (Down syndrome) 

Tuner syndrome (X0) - monomsy

• female with a single X chromosome

• Shoer, broad chest and widely spaced nipples

• Can be of normal intelligence and function with hormone therapy 

Klinfelter syndrome (XXY) - trisomy

• male with underdeveloped testes and prostate, some breast development

• long arms and legs, large lands

• near-normal intelligence unless XXXY, XXXXY, etc

• no matter how many X chromosomes are present, the presence of a chromosome Y renders in the male 

• Environmental agents like radiation, organic chemicals, or certain viruses can cause chromosome breakage

• If broken ends of chromosomes don’t rejoin, mutations can occur

Human Syndromes

• changes in chromosomes can cause various syndromes

Williams syndrome: loss of end of chromosome 7

Alagille syndrome: translocation between chromosome 2 and 20

Chronic myelogenous leukemia: blood cancer caused by translocation between chromosomes 22 and 9

• Plants and haploid multicellular “individuals” alternate with diploid multicellular “individuals”

• Haploid individual is the gametophyte

• Diploid individual os the sporophyte   

Meiosis only occurs during gametogenesis 

• production of sperm = spermatogenesis: all 4 cells become sperm

• production of eggs = oogenesis

• one of the four nuclei recives the majority of the cytoplasm

• becomes the egg or ovum

• others wither away as polar bodies