Lesson 15: Life cycle Flashcards
What is the human life cycle?
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
What is fertilization?
The fusion of the two nuclei of the gametes
What is a zygote and what kind of cell is it?
A fertilized egg and its a diploid cell
Why does the human cycle alternate between haploid and diploid states?
Fertilization with two diploid cells would double the chromosome number
What does meiosis do for the human life cycle?
Meiosis counterbalances the doubling of chromosomes that occurs with fertilization
What is meiosis?
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.
What is the overview of meiosis?
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)
What happens prior to meiosis?
Before meiosis a copy of every chromosome must be made (just like in mitosis)
What is an easy way to remember meiosis?
Meiosis: diploid cell turns into haploid cells REDUCTION DIVISION
What are homologous chromosomes?
Same size, same shape, and same genes
What happens in prophase I?
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
Why is crossing over important?
To cause genetic variation
What happens in prophase I during synapsis?
Homologous chromosomes physically connect to each other along their lengths, held together by a zipper-like protein structure called the synaptonemal complex
What happens during prophase I crossing over?
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
What happens during metaphase I?
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
What happens in anaphase I?
Homologous chromosomes separate and move to opposite poles and sister chromatids remain attached, maternal goes in one direction and paternal goes the other direction
What happens during telophase I- cytokinesis in meiosis?
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
What happens in meiosis II?
Happens in both cells simultaneously and resembles mitosis
What happens in prophase II?
Nucleus disappears, chromosomes condense, centrosomes migrate, spindle forms, and microtubules from opposite poles attach to the kinetochores of sister chromatids
What happens in metaphase II?
Chromosomes align at the metaphase plate (single file) and because crossing over occurred, sister chromatids may not be genetically identical
What happens in anaphase II?
Sister chromatids separate and move toward opposite spindle poles
What happens in telophase II- cytokinesis of meiosis?
One diploid parent cell produces four haploid daughter cells, each daughter cell is genetically unique from each other and the parent cell
What is the difference between meiosis I and meiosis II?
Meiosis I= reduction division and meiosis II= separates sister chromatids (like mitosis)
What is the difference between mitosis and meiosis?
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
