MB Chapter 12

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What are gametes?

Gametes, such as eggs and sperm, are haploid reproductive cells that fuse during fertilization in sexual life cycles.

Rank the size of these DNA containing structures.
genome
chromosome
gene

Largest -genome, chromosome, gene

DNA when complexed with protein is called?

chromatin

Chromatin in its condensed form is called?

chromosomes

Chromosomes are made of?

DNA

Chromosomes together constitute the cells nuclear?

genome

Chromosomes are passed on to the next generation in?

gametes

Chromosomes are located in?

nucleus

Genes are made of?

DNA

Genes are organized on?

chromosomes

Genes each have?

locus

A locus specifies a genes position on a?

chromosome

Genes encode heritable traits called?

traits

Chromosome pairs of the same length and centromere location are called?

homologous chromosomes

Replicated forms of a chromosome are called?

sister chromatids

Crossing over occurs between the _________ of homologous chromosomes early in meiosis I.

non sister chromatids

Chromosomes that differ in length and centromere location are called?

non homologous chromosomes

What is the term for the original chromosome and its exact copy?

sister chromatids

Which statement correctly describes a key difference between cells entering prophase of mitosis versus prophase I of meiosis?

For cells entering mitosis, homologous chromosomes function independently; for cells entering meiosis, homologous chromosomes pair up.

If a parent cell is diploid, the daughter cells produced by mitosis are _____.

diploid

What effect do meiosis and fertilization have on the ploidy level of cells produced by each process?

Meiosis decreases the ploidy level from 2n to n; fertilization increases the ploidy level from n to 2n.

Describe life cycle of an organism in terms of n.

Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the next generation.
In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from generation to generation.

Which statement is correct concerning the relationship between chromosomes and chromatids?

A replicated chromosome contains a single chromatid.
An unreplicated chromosome contains a single chromatid.
An unreplicated chromosome contains two sister chromatids.
A replicated chromosome contains two sister chromatids.

A replicated chromosome contains two sister chromatids.

A replicated chromosome contains two identical DNA double helices that correspond to each sister chromatid.

Which event makes meiosis a reduction division and why?

separation of homologs in meiosis I because it produces 2 haploid (n) daughter cells from a single diploid (2n) parent cell. Separation of homologs results in a reduction in chromosome number.

Homologous chromosomes _____.

carry the same genes

In an organism in which 2n = 6, how many chromosomes will be in each gamete?

3

What is asexual reproduction?

A single individual is reproducing.

Human gametes are produced by _____.

meiosis. Meiosis produces haploid gametes from a diploid parental cell.

Normal human gametes carry _____ chromosomes.

23. This is the number of chromosomes in a single set of human chromosomes.

Once meiosis I is completed, cells are _____. (ploidy?)

haploid

A diploid organism whose somatic (nonsex) cells each contain 32 chromosomes produces gametes containing _____ chromosomes.

16. 16 is half of 32.

Pairs of homologous chromosomes align in the center of the cell

metaphase I

The separated chromatids elongate and usually cytokinesis occurs, forming four genetically distinct haploid daughter cells.

telophase II

Homologous chromosomes separate by moving with the spindle microtubules toward the poles.

anaphase I

A spindle apparatus and individual chromosomes, each composed of sister chromatids, begin to move toward the center of the cell.

prophase II

Chiasmata form and crossing over occurs.

prophase I

Sister chromatids separate and move toward the poles.

anaphase II

The separated chromosomes cluster at the poles and cytokinesis curs, forming two daughter cells, each with a haploid set of replicated chromosomes.

telophase I

Individual chromosomes, each composed of sister chromatids, align in the center of the cell.

metaphase II

In terms of the behavior of the chromosomes, how does meiosis I compare to meiosis II?

Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.

What are the requirements and consequences of crossing over?

1. For crossing over to occur, homologous chromosomes must align precisely early in prophase I so that nonsister chromatids can exchange corresponding segments of DNA.
2. Crossing over occurs at the ends of chromosomes, rather than near the centromeres, because segments of DNA near the centromeres cannot break and rejoin easily.
3. As a result of crossing over, sister chromatids are no longer identical to each other.
4. Crossing over prevents homologous chromosomes from separating during meiosis I.

1, 2, and 3

Fertilization ______ ploidy.

increases

Meiosis ______ ploidy.

decreases

DNA replication, mitosis and meiosis II _______ ploidy.

does not change

Which statement correctly describes how cellular DNA content and ploidy levels change during meiosis I and meiosis II?

DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. During anaphase of both meiosis I and meiosis II, the DNA content (number of copies of chromosomes) in a cell is halved. However, the ploidy level changes only when the number of unique chromosome sets in the cell changes. This occurs only in meiosis I (where separation of homologous chromosomes decreases the ploidy level from 2n to n and produces daughter cells with a single chromosome set).

What is the ploidy of organisms that have two sets of chromosomes?

2n. An organism that has two sets of chromosomes has a ploidy of 2n.

What information can not be obtained from an individual's karyotype?

The sequence of bases of a particular gene.Karyotypes only give information about the chromosomes in an individual.

True or false? A haploid organism has one pair of homologous chromosomes.

False. A haploid organism only has one set of chromosomes, so it has no homologous chromosomes.

If a cell has 24 chromosomes, how many chromosomes would each of its four daughter cells have after meiosis?

12. Meiosis is a reduction division that reduces the number of chromosomes passed on to daughter cells by one-half.

Which of the following statements about meiosis and mitosis is true?

Meiosis II is similar to mitosis in that sister chromatids of each chromosome separate.

Both meiosis II and mitosis are cell divisions involving the separation of sister chromatids.

Which structure is directly correlated with the production of genetic variability in the daughter cells produced during meiosis?

Chiasma. The chiasma is an X-shaped structure formed from the crossing over of homologous chromosomes, which can then exchange segments at the crossover sites.

What is the composition of a tetrad at the beginning of prophase I?

One pair of homologous chromosomes. One chromosome in a tetrad is paternally inherited, while the other is maternally inherited. Each chromosome consists of two chromatids.

How would genetic variation be affected if during meiosis I maternal chromosomes always lined up together on one side of the metaphase plate and paternal chromosomes always lined up on the other side (ignoring the effects of crossing over)?

Gametes would contain either maternal chromosomes only or paternal chromosomes only. All maternal chromosomes would go to one daughter cell and all paternal chromosomes to the other, so different combinations would not be produced in gametes.

Which statement is FALSE concerning gametes produced by meiosis?
Gametes contain a random assortment of maternal and paternal alleles.
Gametes contain a random assortment of maternal and paternal chromosomes.
Gametes contain half as many chromosomes as the parent cell.
Gametes contain nonrandom combinations of maternal and paternal chromosomes.

Gametes contain nonrandom combinations of maternal and paternal chromosomes.

This statement is false. Gametes receive a random assortment of maternal and paternal chromosomes when homologs separate in meiosis I.

In a haploid organism, how many different alleles can there be for each gene?

one

Crossing over occurs _____.

during synapsis of homologous chromosomes

Which of the following creates new combinations of alleles along one chromosome?

crossing over

Which of the following creates different combinations of maternal and paternal chromosomes in gametes?

independent assortment

An advantage of sexual reproduction is hypothesized to be _____.

avoiding the transmission of harmful alleles to offspring

For species that alternate between sexual and asexual reproduction, which statement is most accurate?

Sexual reproduction is common when environmental conditions are changing rapidly. In a rapidly changing environment, sexual reproduction is advantageous because the genetic variation among offspring makes it more likely that some will be well adapted to the new conditions.

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