Lesson 15: Life cycle

Question 1

What is the human life cycle?

Answer 1

Starts with two haploid gametes (n=23), the fusion of the two nuclei of the gametes is called fertilization, now there is two sets of chromosomes (2n=46), and a fertilized egg is created called a zygote (diploid), through repeated mitosis and development a single cell becomes a mature multicellular adult

Question 2

What is fertilization?

Answer 2

The fusion of the two nuclei of the gametes

Question 3

What is a zygote and what kind of cell is it?

Answer 3

A fertilized egg and its a diploid cell

Question 4

Why does the human cycle alternate between haploid and diploid states?

Answer 4

Fertilization with two diploid cells would double the chromosome number

Question 5

What does meiosis do for the human life cycle?

Answer 5

Meiosis counterbalances the doubling of chromosomes that occurs with fertilization

Question 6

What is meiosis?

Answer 6

Meiosis is a modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division, but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell.

Question 7

What is the overview of meiosis?

Answer 7

Similar to mitosis, starts with diploid cell, DNA is duplicated during S phase of cell cycle, unlike mitosis there are two cell divisions that produce four daughter cells, each daughter cell is haploid and unique, two cell divisions are called meiosis I and meiosis II, the DNA is not replicated between the two cell divisions (only prior to meiosis I)

Question 8

What happens prior to meiosis?

Answer 8

Before meiosis a copy of every chromosome must be made (just like in mitosis)

Question 9

What is an easy way to remember meiosis?

Answer 9

Meiosis: diploid cell turns into haploid cells REDUCTION DIVISION

Question 10

What are homologous chromosomes?

Answer 10

Same size, same shape, and same genes

Question 11

What happens in prophase I?

Answer 11

Same as mitosis, chromosomes condense, nuclear envelope breaks down, centrosomes separate, spindle forms, and microtubules attach to kinetochores. However, homologous chromosomes pair up (synapsis) to form tetrads, then crossing over occurs

Question 12

Why is crossing over important?

Answer 12

To cause genetic variation

Question 13

What happens in prophase I during synapsis?

Answer 13

Homologous chromosomes physically connect to each other along their lengths, held together by a zipper-like protein structure called the synaptonemal complex

Question 14

What happens during prophase I crossing over?

Answer 14

Crossing over which is a genetic swap between homologous, non sister chromatids, occurs 1-3 times in each human tetrad, point where a cross over occurs is a chiasma which forms an X shape between the two chromosomes

Question 15

What happens during metaphase I?

Answer 15

Pairs of homologous chromosomes (tetrads) line up at the metaphase plate, chromosomes are joined at the chiasmata, and the kinetochores on both chromatids in one chromosome (X) are attached to microtubules from the same spindle pole

Question 16

What happens in anaphase I?

Answer 16

Homologous chromosomes separate and move to opposite poles and sister chromatids remain attached, maternal goes in one direction and paternal goes the other direction

Question 17

What happens during telophase I- cytokinesis in meiosis?

Answer 17

Nuclear envelope forms around each set of chromosomes at each spindle pole, cytokinesis separates the two daughter cells, each daughter cell is now haploid. It has one set of duplicated chromosomes (X), only maternal OR paternal derived for each chromosome, both new cells enter interkinesis (like interphase but no DNA replication), and both cells move from interkinesis to meiosis II

Question 18

What happens in meiosis II?

Answer 18

Happens in both cells simultaneously and resembles mitosis

Question 19

What happens in prophase II?

Answer 19

Nucleus disappears, chromosomes condense, centrosomes migrate, spindle forms, and microtubules from opposite poles attach to the kinetochores of sister chromatids

Question 20

What happens in metaphase II?

Answer 20

Chromosomes align at the metaphase plate (single file) and because crossing over occurred, sister chromatids may not be genetically identical

Question 21

What happens in anaphase II?

Answer 21

Sister chromatids separate and move toward opposite spindle poles

Question 22

What happens in telophase II- cytokinesis of meiosis?

Answer 22

One diploid parent cell produces four haploid daughter cells, each daughter cell is genetically unique from each other and the parent cell

Question 23

What is the difference between meiosis I and meiosis II?

Answer 23

Meiosis I= reduction division and meiosis II= separates sister chromatids (like mitosis)

Question 24

What is the difference between mitosis and meiosis?

Answer 24

Mitosis:
DNA replication occurs during interphase before mitosis begins

Number of divisions is one including prophase, metaphase, anaphase, and telophase

Synapsis of homologous chromosomes does not occur

Number of daughter cells and genetic composition it two, each diploid (2n) and genetically identical to the parent cell

Role in the animal body is it enables multicellular adult to arise from zygote; produces cells for growth, repair, and in some species , asexual reproduction

Meiosis:
DNA replication occurs during interphase before meiosis I begins

Number of divisions are two, each including prophase, metaphase, anaphase, and telophase

Synapsis of homologous chromosomes occurs during prophase I along with crossing over between non sister chromatids; resulting chiasmata hold pairs together due to sister chromatids cohesion

Number of daughter cells and genetic composition is four which are each haploid, containing half as many chromosomes as the parent cell, genetically different from the parent cell and from each other

Role in the animal body is to produce gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Question 25

What is nondisjunction?

Answer 25

Improper segregation of chromosomes, spindle fibers don’t properly attach to kinetochores

Question 26

What is euploidy?

Answer 26

Correct number of chromosomes for a species

Question 27

What is aneuploidy?

Answer 27

Change in chromosome number resulting from nondisjunction

Question 28

How does down syndrome occur?

Answer 28

Results from a third copy of chromosome 21- trisomy 21

Question 29

When does nondisjunction occur?

Answer 29

Occurs in anaphase I of meiosis or anaphase II of meiosis