When During the Cell Cycle is a Cell’s Dna Replicated

Learning Objectives

1. Describe the chromosomal makeup of a jail cell using the terms chromosome, sister chromatid, homologous chromosome, diploid, haploid, and tetrad
2. Recognize the function and products of mitosis and meiosis
3. Compare and dissimilarity the behaviors of chromosomes in mitosis and meiosis
4. Recognize when cells are diploid vs. haploid
5. Predict DNA content of cells in different phases of mitosis and meiosis
6. Call up and describe the phases of the cell cycle
7. Relate the cell cycle stages to changes in DNA content

The Cell Sectionalisation Bicycle

Cells reproduce genetically identical copies of themselves by cycles of cell growth and sectionalisation. The prison cell wheel diagram on the left shows that a cell division cycle consists of iv stages:

• G1 is the period afterwards cell partitioning, and before the kickoff of DNA replication. Cells grow and monitor their environment to determine whether they should initiate another round of prison cell division.
• South is the period of DNA synthesis, where cells replicate their chromosomes.
• G2 is the period between the cease of DNA replication and the start of jail cell division. Cells check to make sure DNA replication has successfully completed, and make whatsoever necessary repairs.
• Chiliad is the bodily menses of cell division, consisting of prophase, metaphase, anaphase, telophase, and cytokinesis.

Chromosomes

Chromosomes were get-go named by cytologists viewing dividing cells through a microscope. The modernistic definition of a chromosome now includes the function of heredity and the chemical composition. A chromosome is a Deoxyribonucleic acid molecule that carries all or part of the hereditary information of an organism. In eukaryotic cells, the DNA is packaged with proteins in the nucleus, and varies in structure and advent at different parts of the prison cell cycle.
Chromosomes condense and become visible by light microscopy as eukaryotic cells enter mitosis or meiosis. During interphase (G1 + S + G2), chromosomes are fully or partially decondensed, in the course of chromatin, which consists of DNA wound around histone proteins (nucleosomes).

In G1, each chromosome is a single chromatid. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sis chromatids, where each chromatid contains a linear Deoxyribonucleic acid molecule that is identical to the joined sister. The sister chromatids are joined at their centromeres, as shown in the paradigm below. A pair of sis chromatids is a single replicated chromosome, a single package of hereditary information.

Ploidy

Humans are
diploid, meaning we have two copies of each chromosome. Nosotros inherited one copy of each chromosome from other mother, and one re-create of each from our father. Gametes (sperm cells or egg cells) are
haploid, meaning that they have just one complete fix of chromosomes.
Chromosomes that do not differ between males and females are chosen
autosomes, and the chromosomes that differ betwixt males and females are the sexual activity chromosomes, X and Y for most mammals. Humans about commonly have 22 pairs of autosomes and 1 pair of sexual practice chromosomes (XX or XY), for a full of 46 chromosomes. We say that humans have
2N
= 46 chromosomes, where
N
= 23, or the haploid number of chromosomes.
Cells with complete sets of chromosomes are called
euploid; cells with missing or extra chromosomes are called
aneuploid. The most mutual aneuploid condition in people is variation in the number of sex chromosomes: XO (having merely one copy of the X), XXX, or XYY. Having no X chromosome results in early on embryonic expiry.
The two copies of a item chromosome, such equally chromosome 1, are called
homologous.The karyotype image above shows the homologous pairs for all the autosomes. Homologous chromosomes are not identical to each other, unlike sister chromatids. They frequently have different variants of the same hereditary information – such as blueish eye color vs dark-brown centre color, or blood type A versus blood blazon B.
Mitosis

Mitosis produces two girl cells that are genetically identical to each other, and to the parental cell. A diploid cell starts with 2N chromosomes and 2X Deoxyribonucleic acid content. Later on DNA replication, the cells is however genetically diploid (2N chromosome number), but has 4X DNA content because each chromosome has replicated its DNA. Each chromosome at present consists of a joined pair of identical sis chromatids. During mitosis the sister chromatids separate and get to reverse ends of the dividing cell. Mitosis ends with 2 identical cells, each with 2N chromosomes and 2X Deoxyribonucleic acid content. All eukaryotic cells replicate via mitosis, exceptgermline
cells that undergo meiosis (encounter below) to produce
gametes
(eggs and sperm).

• prophase – chromosomes condense; each chromosome consists of a pair of identical sis chromatids joined at the centromere.
• metaphase – chromosomes line upwardly at the middle of the cell, forth the aeroplane of cell division, pushed and pulled by microtubules of the spindle apparatus
• anaphase –
  
  
  sister chromatids
  divide
  
  and migrate towards opposite ends of the cell
• telophase – chromatids cluster at opposite ends of the cell and begin to decondense
• cytokinesis – the membrane pinches in to divide the two daughter cells

Hither is a simplified diagram illustrating the overall process and products of mitosis:

Questions or points to ponder or note near the effigy above (answers at bottom of page):

1. are the two daughter cells the same or different from each other, and from the parent prison cell at the start?
2. why does the cartoon analogy of the chromosomes change (from a single rod to joined double rods) later Dna replication, and again (back to single rods) during mitosis?
3. does the figure evidence 2 different chromosomes or a single pair of homologous chromosomes?
4. tin can haploid cells undergo mitosis? what about triploid cells (cells that have 3N chromosomes)?

This animation below shows the packaging of DNA and condensation of chromosomes as a jail cell undergoes mitosis.


The video narration has a major error at time 1:22: chromosomes exist throughout the entire cell bicycle (at all times in a cell’due south life); they are
visible in their condensed form
only during mitosis and meiosis.

Meiosis

This is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells. Information technology starts with a diploid jail cell that has undergone chromosomal Dna replication: 2N chromosomes, 4X DNA content. Two successive divisions, with no additional DNA replication, results in 4 haploid gametes: 1N chromosomes, 1X DNA content.
NOVA has a skillful interactive side-by-side comparison of mitosis and meiosis on this page: How cells carve up
Meiosis sets the stage for Mendelian genetics. Students need to know that near of the genetics activeness occurs in the
commencement meiotic partition:

• homologous chromosomes pair up
  and marshal end-to-end (synapsis) in prophase I
• crossing over occurs between homologous chromosomes in prophase I,
  before
  chromosomes line up at the metaphase plate
• homologous chromosomes separate
  to daughter cells (sister chromatids do not carve up)
  
  in the first sectionalisation, creating haploid (1N) cells
  
  
• the separation of each pair of homologous chromosomes occurs independently, so all possible combinations of maternal and paternal chromosomes are possible in the two daughter cells – this is the ground of Mendel’southward Law of Independent Assortment
• the
  first sectionalisation
  is when daughter cells become functionally or genetically haploid

The last point appears to be the near difficult for students to grasp. Consider the Ten and Y chromosomes. They pair in prophase I, and so separate in the first sectionalisation. The daughter cells of the starting time meiotic partitioning accept either an X or a Y; they don’t have both. Each cell now has just one sex chromosome, like a haploid cell.
Ane fashion of thinking about ploidy is the number of
possible
alleles for each gene a cell tin can take.  Correct after meiosis I, the homologous chromosomes accept separated into different cells.  Each homolog carries i copy of the gene, and each gene could be a unlike allele, simply these two homologs are now in two dissimilar cells.  Though it looks like in that location are 2 of each chromosome in each prison cell, these are
duplicated
chromosomes; ie, it is one chromosome which has been copied, then at that place is but one possible allele in the cell (just 2 copies of it).
The second meiotic division is where sis (duplicated) chromatids separate. It resembles mitosis of a haploid cell. At the start of the second division, each prison cell contains 1N chromosomes, each consisting of a pair of sister chromatids joined at the centromere.
Here is a simplified diagram illustrating the overall process and products of meiosis:

And here is a video that walks through the steps of meiosis:


It is very important that you recognize how and why cells become haploid after meiosis I.

To confirm for yourself that you lot understand meiosis, work through i or more than of these interactive tutorials:

• The U. Arizona Cell Biology Project’s Meiosis tutorial has a click-through animation of meiosis, with 10 thought-provoking problem questions.
• Jung Choi’southward interactive wink tutorial, programmed by Pearson, uses human chromosome 7, with wild type and cystic fibrosis alleles for CFTR, to track segregation through meiosis, with and without crossing over: Meiotic Segregation tutorial

Chromosomes, chromatids, what is the difference and how many chromosomes are in that location at different times of the cell cycle and after mitosis and meiosis?

Chromosomes by definition contain the Deoxyribonucleic acid that makes up the fundamental genome of the prison cell. In a prokaryote, the genome is usually packaged into ane circular chromosome consisting of a circular Dna molecule of a few 1000000 base pairs (Mbp). In eukaryotes, the genome is packaged into multiple linear chromosomes, each consisting of a linear DNA molecule of tens or hundreds of Mbp. Chromosomes exist at all different phases of the cell cycle. They condense and become visible to light microscopy in prophase of mitosis or meiosis, and they decondense during interphase, in the grade of chromatin (DNA wrapped around nucleosomes, like “beads on a string”).
The chromosome number, N, in eukaryotes, refers to the number of chromosomes in a haploid cell, or gamete (sperm or egg prison cell).
Diploid cells (all the cells in our body except our gametes) have 2N chromosomes, because a diploid organism is created by marriage of 2 gametes each containing 1N chromosomes. In terms of chromosome number (ploidy), it’southward useful to think of chromosomes equally packages of genetic information. A pair of sis chromatids is i chromosome because it has genetic information (alleles) inherited from only i parent. A pair of homologous chromosomes, each consisting of a single chromatid in a girl cell at the end of mitosis, has alleles from the father and from the female parent, and counts as 2 chromosomes.
This chromosome number stays the same after chromosome replication during S phase:
each chromosome entering cell division now consists of a pair of sister chromatids joined together at the centromere. Then in mitosis, the sister chromatids of each chromosome separate, so each daughter prison cell receives one chromatid from each chromosome. The issue of mitosis is two identical daughter cells, genetically identical to the original cell, all having 2N chromosomes. So during a mitotic cell cycle, the Dna content per chromosome doubles during S phase (each chromosome starts as i chromatid, and so becomes a pair of identical sis chromatids during S stage), but the chromosome number stays the aforementioned.
A chromatid, so, is a single chromosomal Deoxyribonucleic acid molecule. The number of chromatids changes from 2X in G1 to 4X in G2 and back to 2X, just the number of chromosomes stays the same.
The chromosome number is reduced from 2N to 1N in the first meiotic division, and stays at 1N in the 2nd meiotic division.
Because homologous chromosomes separate in the first division, the daughter cells no longer accept copies of each chromosome from both parents, and so they have haploid genetic information, and a 1N chromosome number. The second meiotic division, where sister chromatids carve up, is like mitosis.
Chromosome number stays the same when sister chromatids separate.

Using the data above, compare these 2 simplified diagrams of mitosis and meiosis to visualize why cells are haploid after meiosis I. Specifically, compare the chromosomes in cells at the end of mitosis vs the terminate of meiosis I, recognizing that the diagram of mitosis tracks only a
single
pair of homologous chromosomes, whereas the diagram of meiosis tracks
two
pairs of homologous chromosomes (one long chromosome and short chromosome):

The video below is geared toward a high school audience, but it does present a helpful style for recognizing how many chromosomes are nowadays in a cell (and thus the ploidy level of that cell).  While watching, encounter if you tin can recognize why the products of meiosis i are haploid cells:

Answers to questions virtually the mitosis figure:

1. The 2 girl cells are the same as each other, and same equally the parental cell
2. Each rod represents a chromatid, and Dna replication results in two sis chromatids joined at their centromeres. Mitosis separates the sister chromatids.
3. A unmarried pair of homologous chromosomes. Cerise and blue are chromosomes inherited from the male and female parents.
4. Any cell tin dissever by mitosis – haploid, triploid, even aneuploid cells.