What Is Interphase? - Definition & Stages

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  • 0:04 What is Interphase?
  • 0:37 What Happens During G1?
  • 2:09 What Happens During S?
  • 3:01 What Happens During G2?
  • 3:52 Lesson Summary
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Lesson Transcript
Jeffrey Sack

Jeff is a Biology teacher and has a Doctorate in Educational Leadership

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

This lesson will define the stage of interphase during the cell cycle. The different phases of interphase and the events that occur during each will also be discussed.

What Is Interphase?

Before a cell can reproduce, it has to perform a variety of activities to get ready. The stage of the cell cycle when a cell is preparing itself to duplicate is called interphase. Since so many things are happening in the cell at this time, most of the cell's life is spent in this stage. While preparing to reproduce, the cell makes more cytoplasm (the gel-like substance found inside the cell membrane that bathes the organelles) and increases its supply of proteins. When it's ready, it goes through three sub-phases of interphase: G1, S, and G2.

Sub-phases of interphase
Interphase I

What Happens During G1?

During the G1, or gap 1, sub-phase of interphase, the cell is getting ready to move into the S sub-phase. The chromosomes in the nucleus have not yet replicated and the cell is growing. It experiences a significant increase in size. If the cell did not grow before it divided, it would ultimately become too small to function. Each cell division would make it smaller and smaller until there was nothing left of it.

G1 can vary in its length of time, depending upon the type of cell. Some cells, such as rapidly dividing embryonic cells, do away with it altogether. This is because these cells are more concerned with how fast they can divide than making sure all of the new cells work properly. If some of these cells fail to function because they did not grow enough, there are enough of them so that the others can take up the slack.

Other cells stay in this sub-phase for weeks or years. If they are in this sub-phase for long periods of time, they enter what is called the G0 phase. This is basically a state of suspended animation. To get out of G0, special internal and external signals are needed. Once received, the cell will re-enter G1.

There is a transition step between G1 and S called the restriction point. This is the time when the cell has committed to duplicate its DNA and engage in cell division. Once it reaches this point, the cell will proceed regardless of the environmental conditions. This guarantees another cell cycle after the current one is completed.

What Happens During S?

The S sub-phase of interphase is when all of the cell's genetic material is duplicated. Each chromosome is copied and results in a pair of sister chromatids. Remember that a chromosome is a strand of DNA wrapped around proteins. These will then separate when the cell finally divides. One half will go into each new cell.

Chromosome replication happens when a DNA strand unwinds and the strands separate. Each of these strands now serves as a template for the new strands of DNA that will be formed. Free-floating nucleotides in the nucleus attach themselves to the exposed nitrogen bases of the template strand. This results in the new chromosomes being half of the original parent strand and half of the new strand. They are almost identical to the original strands. This entire process is driven by several enzymes, proteins that speed up reactions.

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Additional Activities

Introduction to the Activity

In the video that you viewed, you learned that a cell's chromosomes are in their nonreplicated state during the G1 (or G0) subphase of interphase. At this time, each chromosome is one DNA double helix wrapped around proteins. During the S subphase, each chromosome within the cell's nucleus is replicated so that by the completion of this subphase, each chromosome consists of two identical DNA molecules joined together at a structure called the centromere. Recall that the two identical DNA molecules are called sister chromatids. These 'twin sisters' will hang out together as the cell finishes preparations for division in the G2 phase and remain together until partway through the division phase, M-phase. As the cell finishes the G2 phase and enters M-phase, the replicated chromosomes undergo compaction. The chromosomes will soon align in the middle of the cell so that the sister chromatids of each can be moved to opposite sides (poles) of the cell.

In the activity below, you will draw the changes in chromosomes during interphase, and as the cell enters M-phase.

Activity: Changes in Chromosome Structure during Interphase and M-phase of the Cell Cycle

The diagram below shows the nucleus of a cell in G1 of interphase. The cell has four chromosomes which based on size, are two pairs of chromosomes. One member of each pair is depicted in red to represent the chromosome that came from the individual's mother. The other member of each pair is depicted in blue to represent the chromosome that came from the individual's father. The two members of each pair are called homologous chromosomes because they have the same genes.

A cell nucleus in G1 of Interphase. The cell has two pairs of chromosomes, or four total.

  1. Draw the appearance of the chromosomes just after the S phase is completed. Label the sister chromatids, centromeres, and homologous chromosomes in your drawing. How many total DNA molecules are present in the cell you drew?
  2. Draw the appearance of the chromosomes just after G2 is complete and the cell has started M-phase, again labeling sister chromatids, centromeres, and homologous chromosomes. How many DNA molecules are present now? How many chromosomes are present?

Answer Key (for teachers):

The chromosomes are replicated in S phase and each consists of two sister chromatids in G2 and the beginning of M-phase. At the beginning of M-phase, the chromosomes have compacted and begun to align at the center of the cell.

Following the S phase, there are 8 DNA molecules as part of 4 replicated chromosomes

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