Chapter 3: DNA Structure and function Flashcards
Where is DNA found in eukaryotes?
DNA occurs bound to proteins in chromosomes within the nucleus of eukaryotic cells and the nucleus is enclosed in a nuclear membrane to protect its interior.
Where is DNA found in prokaryotes?
DNA occurs as unbound circular DNA in the nucleoid region of the cytosol. the nucleoid region isn’t bound by a nuclear membrane, so the DNA is not contained.
Where else is DNA found in eukaryotes?
Unbound, circular dna is also found oin the mitochondria and chloroplasts of eukaryotic cells
What are nucleotides made up of?
A five-carbon (pentose) sugar known as deoxyribose sugar, a phosphate group and a nitrogenous base.
Simple definition of a nucleotide.
A nuceltoide is the basic structural unit of DNA.
Explain the structure of a nucleotide.
Each phosphate group is attached to 2 sugar molecules by ‘ester’ bonds and is then called a phosphodiester bond.
Explain the ester bonds within nucleotides.
The five carbon atoms in each sugar moplecule which form a ring, are numbers 1’ to 5’. one of these ester bonds is formed with hte 3’ carbon of one sugar ring and the other is formed with hte 5’ carbon of the next ring.
What is the sugar-phosphate backbone?
The chain of alternating sugar molecules and phosphate groups is called the sugar-phospate backbone.
How is the structure of RNA different to that of DNA?
RNA has a similar structure to DNA, expect deoxyribose sugar is replaced with ribose sugar.
What is a strand of nucleotides directionally described?
Using the phase 5’ to 3’. the 5’ end starts with a phosphate and the 3’ end finishes with a sugar. DNA & RNA synthesis occurs in the 5’ to 3’ direction.
Describe the structure of the DNA molecule?
The shape of DNA molecule is a double helix. the term ‘dobule’ refers to the two strands, which are joined by the weak hydrogen bonds between complementary pairs of nitrogenous bases. talk about complementary base pairing then talk about what the term ‘helix’ means.
What does complementary base pairing mean?
Adenine always pairs with thymine, and cytosine always pairs with guanine.
What does the term ‘helix’ mean in terms of the structure of the DNA molecule?
The term ‘helix’ describes the helical (spiral) molecular shape: the two linear strands run in opposite directions to each other (i.e. are anti-parallel) and are twisted into a helix.
Draw a dna molecule.
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What is a gene?
A section of DNA that codes for a species protein (or polypeptide).
Is DNA important in relation to genetics?
Genes may code for more than one kind of polypeptide, and that genes intewract with one another, causing changes in their expression, DNA therefore controls the growth and developement of an organism
Is DNA important in relation to genetics?
Genes may code for more than one kind of polypeptide, and that genes intewract with one another, causing changes in their expression, DNA therefore controls the growth and developement of an organism
Why does DNA enable certain traits to be passed on to the next generation?
Because it carries information coded in the segments of its molecule known as genes.
Why does the structure of DNA allow DNA replication to occur?
The structural properties of the DNA molecule: its nucleotide composition, paring and hydrogen bonding; and the fact that DNA strands can function as template strands.
Describe the structure of the DNA molecule in a simple way.
The DNA helix is a double-stranded molecule. the two strands are held together by hydrogen bonding between complementary nitrogenous bases
As well as nitrogenous bases, nucleotides have a sugar-phosphate backbone, in which the sugar molecules are linked by phosphodiester bonds.
What is the product of DNA replication?
Two identical, double-helix DNA molecules, each consisting of one parental strand and one new strand.
When does DNA replication occur?
During the S phase of interphase during the cell cycle.
Why is DNA replication referred to as semi-conservative?
Because one of the two strands is conserved, or retained, from one generation to the next, while the other strand is new.
What is the genome sequence ?
The sequence of consecutive DNA ‘letters’ spanning all the chromosomes of a cell from start to finish.
What is Coding DNA?
Some sections of the sequence that code for proteins are called coding DNA. they are also called genes. the coding DNA specifics sequences of amino acids.
What do proteins do?
Nearly all cell functions, proteins are the link between the stored genetic code, the genotype and the observable traits, called the phenotype.
What is a function of non-coding DNA?
Some non-coding DNA is transcribed into function non-coding RNA molecules, such as transfer RNAs and regulatory RNAs.
what are the essential materials needed for protein synthesis?
Enzymes, codons, nucleic acids, animo acids
what is genetic code?
A term used for the way that the four nitrogenous bases of DNA, adenine, thymine, cytosine, and guanine are ordered.
what happens to genetic code?
The base order is ‘read’ by cellular machinery and turned into a protein via protein synthesis
what is cellular machinery?
Consists of biological machines that work to manufacture biological molecules
What is transcription machinery?
Consists of RNA polymerase and binding factors/proteins
What is translation machinery?
The translation machine is the ribosome.
What is a codon?
Each set of three DNA nucleotides in a row counts as a triplet and codes for an mRNA is a codon
What is the process of transcription?
The synthesis of mRNA using the stored DNA code. the synthesised mRNA is a chain of RNA nucleotides complementary to the DNA strand, except uracil is the base pair of adenine in RNA.
What is the process of translation?
The synthesis of a polypeptide using the information in the mRNA. the RNA nucleotide code is translated into an amino acid sequence.
When can DNA leave the nucleus of a eukaryotic cell?
Only during cell division.
Why can’t DNA just leave the nucleus during protein synthesis?
Because DNA can only leave during cell division, it stays in the nucleus for future cell division (mitosis or meiosis). so the DNA must be transcribed into a mRNA, while inside the nucleus. the mRNA can fit through the nuclear pores because it is a short, single-stranded molecule.
What is chromatin?
The combination of DNA and histone proteins
What is a chromosome made up of?
A double stranded DNA molecule wrapped around its associated histone proteins
What are homologous chromosomes?
The first 22 chromosomes, code for somatic cells
What types of division must occur for cell division?
Nuclear division, and cytoplasmic division
List the six phases of the cell cycle.
G1 phase, G0 phase, G2 phase, S phase, Mitosis, C phase
What happens during fertilisation?
Male and female haploid sex cells fuse to produce a diploid zygote.
What does complementary base pairing ensure?
That the ‘rungs’ of each side of the ladder are of the same length, and the proportions of bases are maintained.
Define RNA.
A type of nucleic acid that is responsible for interpreting genetic information (from DNA) into proteins.
Similarities between DNA and RNA.
Both consist of a chain of nucleotides that each contain a sugar, phosphate group, and a nitrogenous base.
Differences between DNA and RNA.
DNA has a deoxyribose sugar, RNA has a ribose sugar
DNA is double-stranded, RNA is single-stranded
DNA and RNA have most of the same bases, except RNA never has thymine, it instead pairs adenine with uracil.
Explain two of the differences between DNA within prokaryotic and eukaryotic organisms.
Overall shape: DNA is usually circular in prokaryotes, however it is linear in eukaryotes. this is because prokaryotes evolved first, independently of eukaryotes.
Genome sizes: The genome of prokaryotes is significantly smaller than eukaryotes, i.e. the prokaryote genome only contains a single copy of each gene. this results in fewer non-coding sequences for prokaryotes.
Compare chromatin and chromosomes.
Chromatin has thin, thread-like DNA, whereas chromosomes have compact, tightly-wound DNA
Chromatin’s function is for gene transcription, chromosomes function is for cell division.
Define genome.
The genome is an organism’s complete set of genetic material. It contains all of the information its cells need for growth, survival and replication.
Outline the structure of DNA.
DNA is a double-stranded helix
Strands run in opposite directions (antiparallel)
It consists of repeating nucleotide units
Each nucleotide consists of a sugar, phosphate and a nitrogenous bases
Adenine pairs with thymine, cytosine pairs with guanine
Outline the structure of mRNA.
mRNA is a nucleotide chain
Single-stranded
No base pairing
Nucleotides consist of a ribose sugar, a phosphate and a nitrogenous base
Nitrogenous bases are adenine, guanine, cytosine and uracil
List the differences between DNA and RNA.
DNA contains a deoxyribose sugar, RNA contains a ribose sugar
DNA is double-stranded, RNA is single-stranded
DNA is found in the nucleus, RNA is found in the nucleus and the cytoplasm
DNA contains thymine, RNA contains uracil
DNA contains genes, RNA transfers genetic code
DNA is synthesised from itself, RNA is synthesised from DNA
Relationship of the 4 nitrogenous bases.
As the % of Adenine increases, the % of Guanine decreases.
The percentage of A = T is related to the percentage of G = C due to the base pairing rule. This means as A goes up, T will also go up because A = T and A + T + C + G should equal 100%. Therefore as A increases, the amount of G should decrease as there is a smaller proportion available.
Why is the semi-conservative nature of DNA replication beneficial?
It reduces the chances of errors.
Two structural features of DNA that allow for replication.
Base pairing
Hydrogen bonding
How does base pairing allow for DNA replication?
DNA polymerase can only add free nucleotides to their complementary base, which means the original DNA molecules acts as a template. This ensures that the DNA is replicated into two exact copies of the original molecule, which is important for cell functioning.
How does hydrogen bonding allow for DNA replication?
Complementary nitrogenous bases are joined by weak hydrogen bonds. this means helicase enzymes are able to separate the double helix into two single strands, exposing bases to complementary free nucleotides for attachment by DNA polymerase.
Explain the breakdown and formation of chemical bonds during DNA replication.
DNA helicase unzips DNA, breaking the hydrogen bonds between complementary base pairs
DNA polymerase adds free nucleotides to single DNA strands, forming hydrogen bonds between complementary base pairs
DNA polymerase seals the DNA backbone, forming bonds between the sugar and phosphate groups of the newly added nucleotides
What is chromatin?
Chromatin consists of DNA wrapped around proteins. This helps to keep the DNA organised!
What happens After the parent cell has divided in two?
After the parent cell has divided in two, a membrane forms around each set of chromosomes. This marks the end of mitosis.
(a) Explain the relationship between DNA, genes and chromosomes.
- Chromosomes are made up of DNA that is coiled tightly around histones (1).
- DNA is made up of four nucleotides (ATGC) (1)
- Genes are sequences of DNA that code for a certain protein (1)
(f) Name 2 processes that need to occur before the mRNA can leave the nucleus.
- Introns spliced out
- Methylated cap added to the 5’ end
- Poly-A tail added to 3’end
There are 64 three-letter codes in the genetic code table but only 20 amino acids. Explain how having many more codes than amino acids could be of benefit to the process of protein synthesis.
- If a mistake occurs there is a greater chance of translating for the same amino acid.
- If this occurs, it will have no effect on the final protein or its function.
compare mitosis and meiosis based on the following points:
Number of chromosomes in the parent cell
Number of cell divisions
Location
Type of cell produced
Mitosis
DIPLOID/2n/46
1
AUTOSOMAL CELLS/ BODY CELLS
SOMATIC/DIPLOID
Meiosis
DIPLOID/2n/46
2
TESTES/ OVARIES/GONADS
GAMETES/HAPLOID
what happens during these parts of the cell cycle
G1
S
G2
CELL GROWTH before DNA REPLICATION
DNA REPLICATION
CELL PREPARES FOR DIVISION
Are all cell cycles the same time length? Give examples with your answer?
- NO -
- SKIN CELLS HAVE A RAPID CYCLE- LOTS OF RENEWAL 1 mark
- NERVE CELLS GO INTO 0 PHASE- NO REPLICATION 1 mark
proof a trait is autosomal recessive
- II2 has the feature while neither parent has it (1)
- Male I1 does not have it but daughter does shows it is autosomal and recessive (1)
- Female with condition must have father and sons with condition for it to be X linked therefore it is not X linked
Polyploidy is a chromosomal mutation. Explain this mutation in detail.
- Occurs when cell division fails- half of the gametes are 2n/diploid (they should be haploid), the other half have NO chromosomes. (1)
- IF a diploid gamete fuses with normal haploid gamete = 3n or triploid (3 copies of chromosomes) individual occurs. (1)
- IF a diploid and diploid gamete fuses – 4n or Tetraploid (4 copies of chromosomes) individual occurs. (1)
- Adv’s- in fruit crops the fruit are bigger and hardier. Advantage in commercial crops (1)
- Disadv’s- reduced fertility/ Polyploidy is LETHAL in humans. (1)
