Chapter 9- DNA determines the structure and function of cells Flashcards
define DNA
short for deoxyribonucleic acid, is found in the cells of all organisms and contains genetic information which determines the structure of the cell and the way it functions.
where are most DNA molecules found?
in the nucleus of the cell and are therefore called nuclear DNA.
what is mitochondrial DNA?
this is the small amount of DNA that is found in the mitochondria (mtDNA).
mtDNA makes up less than 1% of the DNA found in humans.
what are the two main types of nucleic acid found in the body?
DNA and RNA.
explain the structure of DNA
DNA is a polymere, a molecule made up of many repeating small units. the small repeating units are called nucleotides.
Each nucleotide is composed of a sugar molecule (deoxyribose in DNA), a phosphate group and a nitrogenous base. There are 4 different nitrogenous bases in DNA; adenine, thymine, guanine and cytosine.
The sugar molecule of one nucleotide bonds to the phosphate group of another, forming a long chain of alternating sugars and phosphates, with side chains of bases.
in DNA how do strands join
by specific bases being attracted to each other by weak hydrogen bonds. the two strands of DNA twist into a spiral shape called double helix.
what determines genetic code?
the order in which nitrogenous bases occur in the DNA molecule.
a code of only 4 letters would not seem to allow many different combinations, but each gene consists of up to 2 million pairs of bases.
how long is a DNA strand in humans?
estimated between 2-3 metres.
what are DNA strands wrapped around?
a group of eight special proteins called histones to form a nucleosome.
define histone
a special protein around which DNA is coiled to form chromatin
explain the difference between chromatin in a non-dividing and dividing cell
in a non-dividing cell, the coiled DNA forms a tangled network called chromatin.
in a dividing cell the coiled chromatin becomes even more tightly coiled, becoming ‘super-coiled’ structures called chromosomes.
how many chromosomes are in a normal human cell?
46
what are chromosomes made up of?
each chromosome is made up of sections of DNA that code for particular protein.
each of these sections is called a gene.
what are some important differences between DNA found in the nucleus and mtDNA?
nuclear DNA is in the form of very long strands that are bound to proteins, the histones.
mtDNA is in the form of small circular molecules that are not bound to proteins.
mtDNA is passed from mother to offspring and nuclear DNA is a combination of both the mother and father.
how many molecules of mtDNA are found in each mitochondrion?
10
explain genes within mtDNA
it has 37 genes, all essential for the function of mitochondrion.
24 of the genes contain the code for making transfer RNA (tRNA) molecules, which are involved in protein synthesis.
the remaining 13 genes contain instructions for making some of the enzymes necessary for the reactions of cellular respiration.
explain DNA replication
the first stage is when the two strands of the DNA molecule are separated by the enzyme helicase. each strand of the separated section contains half he original information. each strand serves as a template for the nucleotides that will form the new strand.
As the base adenine can only pair with thymine and the base cytosine can only pair with guanine, the new strand that forms is identical to the original.
what two enzymes are important in DNA replication?
DNA polymerase - enzyme that joins nucleotides together
DNA ligase - enzyme that joins the short sections of DNA together
what is protein synthesis
the formation of DNA in the cell
instructions for protein synthesis are provided by genetic code
what controls the order of amino acids?
the sequence of bases in DNA controls the order of amino acids, and therefore the type of protein that is produced.
what is RNA?
RNA stands for ribonucleic acid and it is another type of nucleic acid.
like DNA, RNA is composed of a chain of nucleotides.
outline four differences between RNA and DNA
- the sugar molecule is ribose, not deoxyribose. ribose has one more oxygen atom than deoxyribose.
- RNA is single stranded, while DNA is double stranded.
- RNA has bases cytosine, guanine, adenine and uracil. the structures of thymine and uracil are very similar, meaning that uracil is complementary to adenine.
- the RNA strand is able to fold onto itself, forming hydrogen bonds between complementary bases.
what are the different types of RNA?
messenger RNA (mRNA)
Ribosomal RNA (rRNA)
Transfer RNA (tRNA)
what is messenger RNA’s (mRNA) role in protein synthesis?
mRNA is made in the nucleus and takes the genetic code into the cytoplasm allowing the genetic code to be ‘read’ by ribosomes.
what is ribosomal RNA’s (rRNA) role in protein synthesis?
rRNA makes up approx. 60% of the mass of ribosomes, with the other 40% being protein. the rRNA ensures the correct alignment mRNA, tRNA and ribosome. it also has an enzymatic role in the formation of peptide bonds between amino acids.
what is transfer RNA’s (tRNA) role in protein synthesis?
tRNA is a small molecule of RNA, containing only 70-90 nucleotides. each tRNA is able to carry specific amino acid and therefore provides a vital role in protein synthesis.
what determines the proteins that are produced
the sequence of bases
provide examples of base sequences and the proteins they produce
CAG codes for amino acid valine
TTA codes for leucine
CCC codes for proline
define transcription
the process by which the genetic instructions are copied (transcribed) from the DNA to the mRNA molecule.
what is the transcription triggered by?
by chemical messengers that enter the nucleus from the cytosol and bind to the DNA at the relevant gene.
what happens when the transcription is triggered?
it causes an enzyme called RNA polymerase to begin the process of making mRNA.
explain the 5 steps of transcription
- DNA unzips and RNA polymerase attaches to the template strand at a ‘start’ command.
- RNA polymerase reads one triplet at a time, adding complementary nucleotides to form codons (uracil instead of thymine).
- A ‘stop’ command (ATT) is reached and RNA polymerase detaches.
- Newly formed mRNA strand detaches from the template strand and exists through nuclear pores.
- DNA ligase reforms weak hydrogen bonds when the specific DNA is no longer needed.
explain the 4 steps of translation
- mRNA enters the cell cytosol and is attached to a ribosome.
- The ribosome reads a codon at a time and recruits the tRNA molecule with the complimentary anticodon.
- The tRNA molecule carries an amino acid, which forms a peptide bond with the AA before it.
- The chain of peptides detaches and folds in a particular way to form a protein.
define the cells cycle
the events that take place from one cell division to the next.
what phases does interphase include?
G1 phase, S phase and G2 phase
what is G1 phase?
the first growth stage. the cell produces new proteins, grows and carries out its normal tasks for the body.
this phase ends when the cells DNA begins to duplicate.
what is S phase?
synthesis phase. the DNA molecules in the cell nucleus form exact copies of themselves.
what is G2 phase?
the second growth phase. short phase where the cell prepares for division.
what is mitotic phase and what phases does it include?
when the cell divides into two daughter cells. it includes:
- prophase
- metaphase
- anaphase
- telophase
explain the 5 events in prophase
- nuclei disappear
- nuclear membrane breaks down
- centrioles migrate to opposite poles
- chromosomes appear as pairs of chromatids
- spindle fibres form
explain the 2 events in metaphase
- chromosomes line up at the equator of the cell
2. the centromere of each pair is attached to a spindle fibre
explain the 2 events of anaphase
- centromeres divide
2. chromosomes move to opposite ends of the spindle
event the 5 events of telophase
- spindle disappears
- nuclear membranes and nucleoli form
- centrioles divide
- chromosomes uncoil and disappear
- cytokinesis begins
define mitosis
the division of a nucleus of a cell in which the two daughter nuclei have the same number and type of chromosomes as the parent nucleus.
what is differentiation?
the process by which cells become specialised.
as cells undergo division by mitosis, different genes become activated, making the cells differentiate into specialised cells that can perform particular functions.
examples of specialised cells
stomach cells that can secrete enzymes.
muscle cells that can contract.
red blood cells that can carry oxygen.
what are stem cells?
all cells that undergo cell differentiation are known as stem cells.
they’re not specialised for any role and can undergo repeated division through mitosis.
in right conditions, they can differentiate into specialised cells.
what do stem cells have the potential to do?
to develop into any cell type, meaning they could possibly provide an unlimited source of cells for repair of tissues such as bone, skin, muscle, liver or blood.
explain stem cell classification
stem cells can be classified based on where they originate or the type of cells they form.
totiponent - eg the early embryo before the formation of the inner cell mass. (first group of cells in an embryo that can go on to form any cell).
pluripotent - eg the inner cell mass of an embryo. (creates all the tissues for the body).
multipotent - eg embryonic stem cells, adult stem cells, cord blood stem cells.
what is gene expression?
the process of copying information from DNA onto mRNA and then translating the message into a series of amino acids to form a protein.
in reference to gene expression, what do genes contain?
instructions for making mRNA, but at any given time a cell is making mRNA from only a fraction of its genes.
eg genes for the production of insulin are activated in some cells in the pancreas but not in bone or muscle cells.
what are some factors that determine whether a gene is being expressed (whether it’s on or off)?
- age of the cell
- time of day
- signals from other cells
- environment of the cells
- whether or not the cell is dividing
what is DNA replication?
DNA producing an exact copy of itself.
what is chromatin?
the tangled network of DNA in the nucleus of a cell that isn’t dividing.
explains ways that gene expression can modify the way in which DNA is wrapped around the nucleus
by changes in the amino acids of histone proteins, modifying the shape of the histone or DNA molecule remodelling.
what are some histone modifications?
acetylation = the addition of an acetyl group to the histone protein. acetylation enhances gene expression.
methylation = the addition of methyl groups to DNA. it inhibits gene expression by restricting access to RNA polymerase.
what is epigenetic’s?
some of the factors that make genes more or less likely to be expressed are inherited.
a persons genome vs epigenome
genome = the heredity information encoded in their DNA.
epigenome = the sum of all factors that determine when, where and which genes are ‘switched on’.
