Unit 4: Lecture 18

Define ploidy.

The \# of sets of chromosomes in a cell. Each set \= 'n'

Define haploid (n).

Has one set of chromosomes (e.g. gametes)

What does n mean?

Each set

Define diploid (2n).

Two sets, homologous chromosomes (e.g. somatic cells-\> one set from the mother, other from the father)

Define homologous chromosomes.

Similar but not identical

Define karyotype.

Number, appearance, and size of chromosomes for a given organism

Define genome.

All of the DNA within a cell

How many pairs of chromosomes do humans have? How many total?

23 pairs (22 are autosomes and one is the sex chromosomes); 46 total chromosomes

What are homologs?

Homologous chromosomes that are the same length and carry genes controlling the same inherited characters at the same location (loci)

What are humans?

Diploid, two (slightly different copies of each chromosome)

The two parental copies are considered \_________________.

Homologous

What happens in asexual reproduction?

A single individual passes all of its gene to its offspring; Offspring are clones

What happens in sexual reproduction?

2 parents produce unique offspring through the nuclear division of meiosis

How many chromatids are created in the duplication process of S phase?

92 chromatids (still 46 chromosomes)

What are sister chromatids?

Identical copies of each chromosomes

Are the cells still diploid after replication?

Yes

What is gametogenesis?

The production of gametes- reproductive cells, sperm and egg

What are somatic cells?

Cells not specialized for reproduction

What do germ cells undergo?

Meiosis to produce the gametes

What ploidy are gametes?

Haploid

What are examples of germ cells (2n)?

Ovaries and testes

In Oogenesis, how is the secondary oocyte formed?

Once per month (a period), primary oocyte undergoes meiosis, the cell splits, and the first polar body is removed (in order to get rid of 1 set of chromosomes); left with Oocyte II/ secondary oocyte

In spermatogensis, how is the secondary spermatocyte formed?

Primary spermatocyte undergo Meiosis 1 to form 2 haploid cells (secondary spermatocyte); the 2 haploid cells undergo Meiosis II to form a total of 4 haploid cells (spermatid) that become sperm

Define fertilization.

The union of haploid gametes to produce a diploid zygote (2n\=46).

How do sexual life cycles differ in animals, plants (and some algae), and most fungi/some protists?

\-Animal (Diploid mulitcellular organism): 2n-> meiosis-> 1n-> fertilization (1n+1n)-> 2n (zygote)-> mitosis

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\-Plants and some algae (haploid multi-cellular organism \[gametophyte\]/diploid mulitcellular organism \[sporophyte\]): 2n-> meiosis-> 1n-> mitosis-> n (spores)-> mitosis-> fertilziation (1n+1n)-> 2n (zygote); Plants use alternation of generations

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\-Most fungi/some protists (haploid unicellular or multicellular organisms): 2n-> meiosis-> n-> mitosis-> n-> mitosis-> fertilizaiton (1n+1n)-> 2n (zygote); No multicellular diploid stage

Meiosis only occurs in...

The reproductive cells of the testes and ovaries by halving the genetic material in the cell

Comparison between mitosis and meiosis:

\-no difference in interphase stage

\-M-phase differs; 2 rounds of division (interphase is not done twice)

What is the purpose of meiosis?

1. Reduce the chromosome number from diploid to haploid
2. Ensure each haploid product has a complete set of chromosomes (i.e. accuracy)
3. To generate genetic diversity

What happens in interphase of meiosis?

\-chromosomes duplicate

\-chromatin begins condensing

\-centrosomes divide

What happens in prophase I of meiosis?

\-Nuclear membrane/ nucleolus goes away

\-DNA condenses into chromosomes

\-occupies more than 90% of the time required for meiosis

\-synapsis occurs -crossing over occurs

Define synapsis.

Alignment of the tetrad

Define tetrad.

The two homologous chromosomes and sister chromatids align gene by gene

Define crossing over.

Exchange of genetic material between nonsister chromatids; allows for recombination (to help mix up DNA-\>diversity)

Define chiasmata.

The X-shaped region where crossing over has occurred between homologous nonsister chromatids.

What happens in metaphase I of meiosis?

1. Homologous chromosomes line up in parallel in mitosis, but in series in meiosis I
2. Microtubule attachment differs

What ploidy does meiosis I result in?

Haploid cells

What occurs in anaphase I of meiosis?

Pairs of homoglous chromosomes separate (sister chromatids still attached at centromere)

What occurs in meiosis II?

Similar process of mitosis where in anaphase II, sister chromatid separate; the final cell: 1 sister, 1 homolog, 4 total haploid cells (each genetically different)

What is nondisjunciton?

Process where the spindle forms incorrectly

What is aneuploidy?

Extra set of chroomomes, due to replication or spindle

What are the consequences of nondisjunction?

Fetuses that are incompatible with life or trisomy 21 (Down's syndrome)

What are the three sources of variation in meiosis?

1. Independent assortment- homologous chromosomes alignment is random; an avg of 1-3 crossover events occurs per chromosome
2. Recombination at synapsis (i.e. crossing over)
3. Random Fertilization

How do prokaryotes evolve?

1. Bacterial Conjugation:


1. horizontal gene transfer
2. recombination


1. Vertical gene transfer
2. Transformation (from the environment)

Does meiosis and mitosis occur in prokaryotes?

No