Meiosis & genetic variability

From the point of view of meiosis, genetic variation and evolution occurs when...

- Phenotypic traits are genetically encoded (we're dealing with chromosomes and genes) - Genes can vary

2. Crossing over

- Produces recombinant chromosomes, which combine genes inherited from each parent. - Begins early in prophase I. - Homologous portions of two non-sister chromatids trade places; - Contributes to genetic variation by combining DNA from two parents into single chromosome.

Inheritance

- cf is recessive disease - You need two copies of the mutant allele (homozygous for cf) before getting disease. - Heterozygotes are normal (+/cf) - If two parents that are normal heterozygotes (+/cf) reproduce they can produce both AFFECTED (cf/cf) and NON-AFFECTED (+/cf) or (+/+) offspring

Advantages of sexual reproduction

- generation of genetic diversity - ability to combat pathogens (Red Queen hypothesis) - repair of double-stranded DNA damage (use copy as template) - stabilization of homologous chromosome pairs by chiasmata

Disadvantages of sexual reproduction

- investment of time and energy (finding mates) - slower than asexual reproduction - random mixing of genes may = suboptimal results - conflicts between offspring / mother / father - STIs

how does homologous recombination work?

- paternal and maternal chromosomes line up - chiasmata form at regions of homology - segments of DNA are exchanged

The selfish gene theory

- the unit of evolution is the gene not the individual - individuals will die, genes are passed on (survive) - sexual reproduction is good for gene survival

The behaviour of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation. *The three mechanisms that contribute to genetic variation are*:

1. *Independent assortment of chromosomes* 2. *Crossing over* 3. *Random fertilisation*

A human cell has 46 total or 23 pairs of chromosomes. Following mitosis, the daughter cells would each have a total of ______ chromosomes. After meiosis I, the two daughter cells would have _____chromosomes, and after meiosis II ______ chromosomes.

46, 23, 23 The chromosome number is reduced to one half during the first meiotic division. Recall that the number of chromosomes is determined by the number of centromeres, and not the number of chromatids.

The stage of meiosis where cells become haploid.

Anaphase I Separation of homologous pairs occurs at anaphase I. Even though each chromosome has two chromatids, the total number of chromosomes is 23 for humans after anaphase I of meiosis I. Recall that the number of chromosomes is determined by the number of centromeres.

Somatic cells

Any cell of the body except sperm and egg cells. Somatic cells are diploid, meaning that they contain two sets of chromosomes, one inherited from each parent.

Coral in the ocean grows by budding, where the new organism grows out of the old one by mitosis. This form of replication is an example of:

Asexual reproduction; can produce large numbers of organisms but without genetic diversity.

What precedes mitosis and meiosis?

Chromsomal replication, happens before both mitosis and meiosis

sister chromatids

Either of the two identical copies formed by the replication of a single chromosome, with both copies joined together by a common centromere.

What are some differences between the production of reproductive cells in men and women.

Females: Meiosis begins before birth, stops at menopause; produces one ovum per month Males: Meiosis begins at puberty, doesn't stop, continues until death; produces many / hundreds millions of sperm per day

Genetic variability is really important from the point of evolution. Why?

Genetic diversity is essential for a species to evolve. With very little gene variation within the species, healthy reproduction becomes increasingly difficult.

What are the effects of mutations?

Genetic variation. Mutations are: - the changes in an organism's DNA - the original source of genetic diversity - create different versions of genes called alleles - reshuffling of alleles during sexual reproduction produces genetic variation

Which of the following is unique to mitosis and not a part of meiosis?

Homologous chromosomes behave independently. Pairing of homologous chromosomes allowing independent segregation and crossing over is unique to meiosis. In mitosis, homologous chromosomes behave independently.

The Thompson seedless grape is triploid, with three copies of each chromosome. Which phase of the cell cycle would you expect triploid cells to be unable to complete.

Meiosis ; homologous chromosomes must pair in meiosis I. Triploid cells would be unable to complete meiosis I.

What are the stages of meiosis II (equation division)?

Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. Prophase II Metaphase II Anaphase II Telophase II Cytokinesis

What is the difference between mitosis and meiosis?

Mitosis conserves the number of chromosome sets, producing two daughter cells genetically identical to the parent cells (46 chromosomes) Meiosis reduces the number of chromosome sets, from two to one (diploid to haploid), producing four daughter cells genetically different (non-identical) to each other and the parent cell.

chromatids

One of two identical halves of a replicated chromosome, joined to the other copy by a single centromere; one of the strands in the X shape.

One of the earliest events that distinguishes meiosis occurs in prophase I and involves:

Pairing of homologous chromosomes is unique to meiosis, and allows crossing over.

The process of meiosis produces four cells with nonidentical chromosomes. This diversification occurs during:

Prophase 1; includes recombination of genetic material and the random arrangement of the homologous pairs, producing diversification of the resulting haploid cells.

What are the stages of meiosis I (redcution division)?

Prophase I (crossing over also happens) Metaphase I Anaphase I Telophase I Cytokinesis

What happens in each stage of meiosis I?

Prophase I: replicated homologous chromosomes pair and exchange segments; chiasmata (points where crossing over has occured) Metaphase I: chromosomes line up by homologous pairs on metaphase plate, chromatids attached to kinetochore Anaphase I: each pair of homologous chromsomes separates Telophase I: two haploid cells form; each chromosome still consists of two sister chromatids Cytokinesis: homologous chromosomes separate into two daughter cells, 23 chromosomes each

What happens in each stage of meiosis II?

Prophase II: spindle apparatus forms, sister chromatids move toward metaphase plate Metaphase II: chromosomes line up on metaphase plate Anaphase II: chromatids separate, move towards opposite poles as individual chromosomes Telophase II: nuclei forms, chromosomes decondense Cytokinesis - separates the cytoplasm

Red Queen hypothesis

Proposes organisms must constantly adapt and evolve to survive and gain reproductive advantage whilst pitted against ever-evolving opposing organisms in an ever-changing environment.

1. Independent assortment of chromosomes

Randomly assorting chromosomes into cells. Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs; for humans there are 8 million possible combinations of chromosomes

Mutations provide a source of variability but most mutations are damaging so why is recombination and meiosis not damaging?

Recombination and meiosis take the mutations that have made it through into surviving, reproducing individuals and recombine them to make the most out of the variability that we have in the population.

Some organisms are capable of asexual or sexual reproduction. Under favorable conditions, reproduction proceeds asexually. When conditions become more stressful reproduction switches to a sexual mode. Why?

Sexual reproduction produces individuals with new combinations of recombined chromosomes increasing diversity. This is the reason for sexual reproduction and death. Our species survives if we increase variation by meiosis.

chromosomes

Structure made of protein and a single molecule of deoxyribonucleic acid (DNA), located in the nucleus of animal and plant cells, contains genetic material passed from parents to offspring.

During anaphase, a free kinetochore formed by disrupting an attached spindle fiber results in an immediate block to the process. This result shows that:

The cells have mechanisms to insure that errors are not made during anaphase. Scientists are now working on the mechanisms by which detachment is detected, a process probably involving phosphorylation.

Karyotype

The number and appearance of chromosomes in the nucleus of a eukaryotic cell; complete set of chromosomes in a species, or an individual organism.

True of false. Meiosis I is more complicated than meiosis II and meiosis II is very similar to mitosis?

True.

How many cell cycles in meiosis?

Two: meiosis I (reduction) and meiosis II (equation) (mitosis has one cell division, equational)

How does a mutant allele vary from a normal allele?

Varies by deletion of just one amino acid. Example is cystic fibrosis (CF), involves mutant allele of a gene CFTR on chromosome 7

crossing over

also called recombination; parents' DNA is shuffled; each gamete gets a different set of genes; approx 50% from each

Meiosis

cell division in which the four daughter cells are haploid (23 chromosomes (sticks) in each of the four cells, half as many chromosomes as the original parent cell)

3. Random fertilisation

contributes to genetic variation because any sperm can fuse with any ovum (unfertilised egg); the fusion of two gametes produces a zygote with any 70 trillion diploid combinations.

Gametes

egg and sperm cells with haploid (n) number of chromosomes; one set of chromosomes; union of two haploid cells restore the diploid number (unite at fertilization to create embryo)

What is the role of mitosis in the animal body?

enables multicellular zygoet to become an adult cell growth, repair and asexual reproduction (in some species)

At the end of meiosis II there are how many daughter cells?

four, genetically distinct, non-identical daughter cells

Aneuploid

having a different number of chromosomes than is usual (more or fewer)

Euploid

having an equal number of all the chromosomes of the haploid set

Polyploid

having more than two complete sets of chromosomes

Haploid

having one set of chromosomes

Diploid

having two sets of chromosomes

prophase I specific stages

leptotene zygotene pachytene Diplotene Diakinesis (then to metaphase I then anaphase I then telophase I (two daughter cells)

_________________ most closely resembles events of mitosis except that the cells are ___________.

meiosis II, haploid Meiosis II events are similar to mitosis with haploid cells

What is the role of meiosis in the animal body?

produces gametes, reduces number of chromosomes by half, introduces genetic variability among gametes.

homology

similarities in DNA sequences due to common ancestral source; does NOT mean identical

Aneuploidy occurs when...

there is an error in meiosis, most commonly trisomy 21, 18 and 13

At the end of meiosis I there are how many daughter cells?

two daughter cells

homologous chromosomes are

two sets of sister chromatids (two X's together, 4 strands)