Chapter 13 - Meiosis and Sexual Life Cycles Flashcards
What is heredity?
The transmission of trains from one generation to the next; inheritance
What is genetics?
The scientific study of heredity and hereditary variation
What is variation?
Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
What are genes?
Units of hereditary code; coded information in the form of hereditary units made up of DNA; (genes) program the specific traits that emerge as we develop from fertilized eggs into adults
What are gametes?
Reproductive cells that act as vehicles to transmit genes from one generation to the next; sperm and eggs
How is DNA packaged?
In chromosomes
How many chromosomes do humans have?
46 in their somatic cells- 2 pairs of 23, one from mom and one from dad
What are somatic cells?
Non-reproductive cells; all cells except gametes and their precursors
What is the locus?
A genes specific location along the length of a chromosome
What is asexual reproduction?
A single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes; i.e. single celled eukaryotic organisms
DNA is copied and allocated equally to two daughter cells and the genomes of the offspring are virtually exact copies of the parent’s genome
*Sometimes genetic differences arise in asexually reproducing organisms as a result of mutations
What is a clone?
A group of genetically identical individuals; an individual that reproduces asexually gives rise to a clone
What is sexual reproduction?
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents; vary genetically
What is a life cycle?
The generation to generation sequence of stages in the reproductive history of an organism
What is a karyotype?
An ordered display of the pairs of chromosomes in a cell
What are homologous chromosomes?
The two chromosomes in each pair; the pair have the same length, centromere position and staining pattern; also called homologs
Homologous pairs are the same length and shape and carry genes controlling the same inherited characteristics
- Each pair of homologous chromosomes include one chromosome from each parent
What are sex chromosomes?
Determine the sex of an individual; x and y chromosomes
Females - xx
Males - xy
What are autosomes?
The remaining 22 pairs of chromosomes
What is a diploid cell?
2n; a cell with two sets of chromosomes
For humans, the diploid number is 46 (2n = 46)
What is a haploid cell?
Gametes contain a single set of chromosomes; each haploid cell has a haploid number of chromosomes (n)
For humans, the haploid number is 23 (n = 23)
The set of 23 consists of 22 autosomes plus a single sex chromosome (an unfertilized egg contains an x chromosome but a sperm may contain an x or a y chromosome)
What is fertilization?
The union of gametes (sperm and egg) culmination from the fusion of their nuclei
What is a zygote?
A fertilized egg; has one set of chromosomes from each parent
(diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines)
*The zygote produces somatic cells by mitosis and develops into an adult
When are haploid gametes produced?
At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produces by meiosis
What is a key to genetic variation that is common to all organisms that reproduce sexually?
The alternation of meiosis and gertilization
How do the three types of sexual life cycles differ?
The timing of meiosis and fertilization in the life cycle varies depending on the species; these variations can be grouped into three main types of life cycles
What is alternation of generations?
Plants and some species of algae exhibit this alternation of generations
- Includes both diploid and haploid stages that are multicellular; the multicellular diploid stage is called the sporophyte
- The diploid organism, called the sporophyte, makes haploid spores by meiosis
- Each spore grows by mitosis into a haploid organism called a gametophyte; a gametophyte makes haploid gametes by mitosis
- Fertilization of gametes results in a diploid sporophyte
What is the sexual life cycle in animals?
Gametes are the only haploid cells; meiosis occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization; after fertilization, the diploid zygote divides by mitosis, producing a multicellular organism that is diploid
What is the third type of sexual life cycle that takes place in most fungi and some other protists?
The only diploid stage is the single celled zygote; there is no multicellular diploid stage
The zygote produces haploid cells by meiosis; each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis
What type of cells can divide by mitosis?
Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle
- However, only diploid cells can undergo meiosis
- In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring
What does meiosis reduce?
Meiosis reduces the number of chromosome sets from diploid to haploid
What is meiosis?
Preceded by the duplication of chromosomes (S phase); this single duplication takes place in two consecutive cell divisions meiosis I and meiosis II
These two divisions result in four daughter cells, each with only half as many chromosomes as he parent cell - one set rather than two
What are the basic stages of meiosis?
- Chromosomes duplicate during interphase
- The resulting sister chromatids are closely associated along their lengths; sister chromatid cohesion
- The chromatids are sorted into four haploid daughter cells
What happens in meiosis I?
Homologous chromosomes separate
What happens in meiosis II?
Sister chromatids separate
What are the four stages of meiosis I and what happens at each stage?
Prophase I (duplicated homologous chromosomes pair and exchange segments)
- Centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis; chromosomes condense progressively throughout this phase
- During early prophase I, each chromosome pairs with its homolog, aligned gene by gene, and crossing over (sharing of genetic information) occurs; the DNA molecules of non sister chromatids are broken by proteins and are rejoined to each other
- Each homologous pair has one or more x-shaped regions called chiasmata, where cross overs have occurred
- Later in prophase I, microtubules from one pole or the other will attach to the two kinetochores, one at the centromere of each homolog; the homologous pairs will then move toward the metaphase plate
Metaphase I (chromosomes line up by homologous pairs)
- Pairs of homologous chromosomes are now arranged at the metaphase plate, with one chromosome in each pair facing each pole
- Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole
Anaphase I (each pair of homologous chromosomes separate)
- Breakdown of proteins that are responsible for sister chromatid cohesion along chromatid arms allows homologs to separate
- The homologs move towards opposite poles, guided by the spindle apparatus
- Sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit towards the same pole
Telophase I and Cytokinesis (two haploid cells form; each chromosome still consists of two sister chromatids)
- When telophase I begins, each half of the cell has a complete haploid set of duplicated chromosomes; each chromosome is composed of two sister chromatids; one of both chromatids include regions of non-sister chromatid DNA
- Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells
- In animal cells like these, a cleavage furrow forms (plants = cell wall)
*No chromosome duplication occurs between meiosis I and meiosis II
What are the four stages of meiosis II and what happens at each stage?
The second round of cell division where the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes
Prophase II
- A spindle apparatus forms
- In late prophase II, chromosomes, each still composed of two chromatids associated at the centromere, move toward the metaphase II plate
Metaphase II
- The chromosomes are positioned at the metaphase place as in mitosis
- Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical
- The kinetochores of sister chromatids are attached to microtubules extending from opposite poles
Anaphase II
- Breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate
- The chromatids move toward opposite poles as individual chromosomes
Telophase II and Cytokinesis
- Nuclei form, the chromosomes being decondensing, and cytokinesis occurs
- The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated chromosomes)
- The four daughter cells are genetically distinct from one another and from the parent cell
