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MCAT Bio

Mitosis

Normal cell division used for growth and replacement of cells; a parent cell is copied in order to produce two identical daughter cells

Meiosis

Sexual reproduction; genetically diverse gametes must be created

Homologous pairs

Chromosomes occur in these pairs; for each pair of chromosomes found in an individual, one member of the pair came from the maternal parent and the other member of the pair came from the paternal parent

Diploid

2N; the total number of chromosomes found in an individual; in humans this is 46

Haploid

N; when individuals reproduce, the total number of chromosomes must be cut in half, which produces haploid egg and sperm cells; in humans this is 23

The process of mitosis begins with ___ and ends with two ____.

Begins with a diploid cell and ends with two identical diploid cells

In the process of meiosis, a ___ begins the process and produces ___.

A diploid cell begins the process and produces four haploid gametes

Chromatid

When a cell is not dividing, each chromosome exists in a single copy called a chromatid

Sister chromatids

When the cell is preparing to divide, each chromosome must be replicated so that it contains two chromatids, sometimes called sister chromatids

Centromere

The compressed region of each chromosome

When the chromosome replicate, what happens to the sister chromatids?

The sister chromatids stay attached to each other at the centromere

What does a replicated chromosome consist of?

Two sister chromatids attached to each other at the centromere

Process of mitosis

It begins with a single parent cell that replicates all components within the cell, divides the components into two piles, and then splits to form two genetically identical daughter cells; the most critical components for replication and division are the chromosomes

The cell cycle

It is used to regulate the process of cell division in each individual cell

What are the phases of the cell cycle?

1. Interphase: consists of the G1, S, and G2 phases (preparation for cell division)
2. M phase

G1

This is the first gap phase of the cell cycle. In this stage, the parent cell is growing larger, adding additional cytoplasm, and replicating organelles

S

During this phase of DNA synthesis, the chromosomes are all being replicated. Once this stage is complete, each chromosome consists of two sister chromatids connected at the centromere

G2

This is the second gap phase. The cell continues to grow in size and make final preparations for cell division

M

During the M phase, mitosis actually occurs. The replicated chromosomes and other cellular components are divided to ensure that each daughter cell receives equal distributions.

Cytokinesis

The division of the cytoplasm at the end of the M phase

G0 phase

Cells that lose the ability to progress through the cell cycle and are thus unable to divide

What are the 4 stages of the M phase?

Prophase, metaphase, anaphase, and telophase

What is the primary concern in these stages?

Alignment and splitting of sister chromatids to ensure that each daughter cell receives and equal contribution of chromosomes from the parent cell

Nondividing cell (picture)

...

Nondividing cell

Chromatin is condensing into chromosomes and centrioles have duplicated in preparation for mitosis

Prior to cell division, how are the chromosomes and why? What happens once the DNA is replicated?

They are not condensed and thus not visible. Leaving the chromosomes in an uncondensed state makes it easier to copy the DNA but makes the chromosomes very stringy and fragile. Once the DNA is replicated, the chromosomes must condense so that they are not broken as they are divided up into the two daughter cells

What happens to the centrioles in prophase?

They replicate and move to opposite ends of the cell. Once they have migrated to the poles of the cell, they begin to produce a spindle apparatus consisting of spindle fibers that radiate outward, forming asters.

What are the spindle fibers made of and where do they attach?

They are made of microtubules that ultimately attach to each chromosome at the kinetochore; the kinetochore appears at the centromere of each chromosome

Prophase (picture)

...

Prophase

Duplicated chromosomes are visible. Centrioles begin moving apart; nuclear envelope is fragmenting and will disappear

In metaphase, where is each chromosome attached?

Each chromosome is attached to a spindle fiber at the kinetochore; the chromosomes are aligned along the center of the cell at the metaphase plate

Metaphase (picture)

...

Metaphase

Chromosomes (each consisting of two sister chromatids) are at the metaphase plate (center of fully formed spindle apparatus)

What happens to the centromere during anaphase?

The centromere splits, allowing each chromatid to have its own centromere

At this point, what happens to the chromatids?

The chromatids can be separated from each other and are pulled toward opposite poles of the cell, separating the chromosomes into two distinct piles, one for each daughter cell

Anaphase (picture)

...

Anaphase

Daughter chromosomes (each consisting of one chromatid) are moving toward the poles

What happens to the spindle apparatus in telophase?

It is no longer needed because the chromosomes have been divided into two groups, and it disappears

What happens to the chromosomes in telophase?

A new nuclear membrane forms around each set of chromosomes and the chromosomes uncoil back to their original state

Telophase (picture)

...

Telophase

Daughter cells are forming as nuclear envelopes appear. Chromosomes will decondense

Cytokinesis process

The cytoplasm is divided between the cells; a cleavage furrow forms, which pinches the cells apart from each other. The result is two daughter cells ready to begin interphase of their cell cycles

How many times does a normal cell divide before its telomeres shorten to the point where the chromosome is risking damage?

50

Cancer cells

Have the ability to bypass apoptosis when their telomeres become too short; also develops by a failure of a variety of mechanisms used to regulate progression through the cell cycle. Checkpoints exist throughout the cycle to ensure that cell division does not occur unless necessary. When these checkpoints are bypassed, cell division happens continually, ultimately producing a mass of unnecessary cells termed a tumor

Protooncogenes

The gene products from theres are one of several mechanisms to regulate the cell cyle

Oncogene

Mutation to a protooncogene causes activation of this; the gene product of the oncogene does not properly regulate the cell cycle, which can lead to cancer. Certain viruses are known to be oncogenic viruses, meaning that the viral DNA inserts into human chromosomes, disrupting protooncogenes

Meiosis process

Begins with a diploid parent cell in the reproductive system that has completed interphase and then follows stages similar to mitosis, twice. Result=4 haploid gametes that are genetically diverse

What are the two major changes between meiosis I and mitosis?

The first involves genetic recombination between homologous pairs, and the second involves the alignment of chromosome pairs during metaphase of meiosis I

What is the unique event that occurs in prophase I?

Crossing over

Prophase I

Homologous pairs of chromosomes associate and twist together in synapsis. This configuration consists of two replicated chromosomes (a total of 4 chromatids) and is often called a tetrad. At this point, crossing over can occur

Crossing over

Pieces of chromatid break off and exchange with another. It can occur in more than one location and can unlink genes that were previously linked on the same chromosome. It is an important source of genetic diversity, creating new combinations of alleles that were not seen previously

Difference between metaphase and metaphase I?

In metaphase, chromosomes aligned single file along the center of the cell. In metaphase I, the chromosomes align as pairs along the center of the cell

The alignment of pairs in metaphase I is important because?

It is the critical factor in creating haploid daughter cells; the alignment of each member of the homologous pair during metaphase I is random so that each daughter cell has a unique combination of maternal and paternal alleles

Anaphase I

The homologous pairs separate from each other during anaphase I and are pulled to the poles of the cells. This separation is referred to as disjunction

How are the chromosomes is telophase I?

They are still in their replicated form, consisting of two chromatids

Meiosis II

Only necessary to split the chromatids present in the daughter cells produced during meiosis I; there is no interphase because the chromosomes are already replicated; prophase II, metaphase II, anaphase II, telophase II

Nondisjunction

Mistake in meiosis; chromosomes fail to separate properly; leads to gametes that have the wrong number of chromosomes. If those gametes are fertilized, the resulting embryo will have the wrong diploid number. Example: Down's syndrome

Down's syndrome

Often is the result of a nondisjunction in the female gamete. If a female egg contains 24 chromosomes instead of the expected 23 and is fertilized by normal sperm, the resulting embryo will have 47 chromosomes. This condition is referred to as trisomy (trisomy of human chromosome 21)

Monosomy

A gamete is missing a chromosome as the result of a nondisjunction and is fertilized by a normal gamete, the result is an embryo with 45 chromosomes

What happens to most embryos with trisomies and monosomies?

They do not survive development

Chromosomal damage: Deletion

Occurs when a portion of a chromosome is broken off and lost during meiosis. Although the total number of chromosomes is normal, some alleles have been lost

Chromosomal damage: Duplication

Occurs when a chromosome contains all of the expected alleles and then receives a duplication of some alleles

Chromosomal damage: Inversion

Occurs when a portion of a chromosome breaks off and reattaches to the same chromosome in the opposite direction

Chromosomal damage: Translocation

Occurs when a piece of a chromosome breaks off and reattaches to another chromosome

In men, how many gametes become sperm? In women?

All 4 in men, only 1 in women (released every 28 days during ovulation)

Polar bodies

The three gametes that do not become functional eggs in women

Time at which the process begins
1. Spermatogenesis
2. Oogenesis

1. At puberty
2. Before a female is born (during development)

Time at which the process ends
1. Spermatogenesis
2. Oogenesis

1. Theoretically never
2. At menopause

Time needed to complete meiosis
1. Spermatogenesis
2. Oogenesis

1. 65-75 days
2. Many years

Number of gametes made
1. Spermatogenesis
2. Oogenesis

1. Unlimited numbers are possible
2. The number of follicles is set at birth in females

Fates of the daughter cells
1. Spermatogenesis
2. Oogenesis

1. All four are sperm
2. One is the egg and the other three are polar bodies

Age of the gametes
1. Spermatogenesis
2. Oogenesis

1. Not applicable- old sperm are degraded
2. Women are born with a set number of follicles which are the same age as the woman

Presence of arresting stages in meiosis
1. Spermatogenesis
2. Oogenesis

1. None
2. Meiosis I starts before birth and then arrests. Meiosis I resumes only after puberty. Only one cell is selected to complete meiosis I per month. Meiosis II happens only if fertilization occurs

Any genes present on mitochondrial DNA are inherited by the daughter cells during what?

During cytokinesis of mitosis or meiosis

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