Chapter 5.1 Flashcards
Chromosome Structure
- Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)
- The main proteins present are the large positively charged globular proteins called histones, their role is to organise and condense the DNA tightly so that it fits into the nucleus
- The other proteins are enzymes used in copying and repairing the DNA —The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of
S-Phase (interphase)
- (S phase) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere
- The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
- It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical
- Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes
What is “important or essential” to make sure that cell division takes place during interphase
- It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical
- Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes -called telomeres
telomeres
The ends of the chromatids in chromosomes are ‘sealed’ with protective structures
The Importance of Mitosis:
The process of mitosis is of great biological significance and is fundamental to many biological processes:
- Growth of multicellular organisms
- replacement of cells and repair of tissues
- Asexual reproduction
The Importance of Mitosis: Growth of multicellular organisms
- The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell
- This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
- Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants
The Importance of Mitosis: Replacement of cells and repair of tissues
- Damaged tissues can be repaired by mitosis followed by cell division
- As cells are constantly dying they need to be continually replaced by genetically identical cells
- In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut
- Some animals can regenerate body parts
The Importance of Mitosis: Asexual reproduction
-Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent -For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring -For multicellular organisms (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries
Mitosis
is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus
The Cell Cycle
is the regulated sequence of events that occurs between one cell division and the next The cell cycle has three phases:
- interphase
- nuclear division (mitosis)
- cell division (cytokinesis) =Mitosis is part of a precisely controlled process known as the cell cycle
The length of the cell cycle is very variable
depending on environmental conditions, the cell type and the organism
-For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)
what causes the change in phases
The movement from one phase to another is triggered by chemical signals called cyclins
during Interphase
the cell increases in:
- mass, size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
- Interphase consists of three phases:
—G1 phase-Cells make the RNA, enzymes and other proteins required for growth
—S phase-It is at some point during the G1 phase a signal is received telling the cell to divide again -The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)– S stands for synthesis (of DNA) -The S phase is relatively short -
—G2 phase the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired -Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
-Interphase = G1 + S + G2
Nuclear division (mitosis)
- Follows interphase
- Referred to as the M phase
– M stands for mitosis
-Cell growth stops during the M phase
Cytokinesis
- Follows M phase
- Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells
- In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed
telomers are made of
non-coding DNA (DNA that does not contain genes) that is made up of short base sequences that are repeated many times (multiple repeat sequences) In telomeres
-one strand is rich in the base guanine (G) and the other strand is rich in the complementary base cytosine (C)
-The main function of telomeres
is to ensure that the very ends of the DNA molecules are included in DNA replication during mitosis (the copying enzyme responsible for DNA replication is unable to run right to the very end of the DNA molecule and stops a little short of the end)
- If this end part of the DNA molecule contained an important gene, that piece of genetic information would be lost during DNA replication
- In each subsequent cell division, a little more genetic information would be lost
- Telomeres therefore act as a ‘buffer’ region of non-essential DNA and ensure that no important coding sections near the ends of the DNA molecules are left out of the replication process -This ensures no genes are lost during cell division (the loss of vital genes can even result in cell death) and allows for continued replication of a cell
To avoid the risk of losing genes most cells have an enzyme called
telomerase that adds additional bases at each end (the telomeres)
-Some cells (generally specialised cells) do not have telomerase to ‘top up’ their telomeres and therefore after a certain number of cell divisions the cell dies, this has been connected with the ageing process
Stem Cells
is a cell that can divide (by mitosis) an unlimited number of times
- Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)
- This ability of stem cells to differentiate into more specialised cell types is known as potency
potency
ability of stem cells to differentiate into more specialised cell types
There are three types of potency
- Totipotency
- Pluripotency
- Multipotency
There are three types of potency: Multipotency
-Multipotency
– multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent Multipotent adult stem cell
There are three types of potency: Totipotency
-Totipotency
– totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta). The zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage of human embryo development
There are three types of potency: Pluripotency
-Pluripotency
– pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)
