DNA, Genes & Chromosomes

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Apuntes de la lección

Prokaryotic DNA

Prokaryotic cells contain a single, circular DNA molecule that is not associated with proteins.
This circular DNA is sometimes called a nucleoid.
Prokaryotes often have additional small, circular DNA molecules called plasmids.
Plasmids contain a few genes and are more accessible for proteins needed for gene expression.
Plasmid genes are often required quickly, frequently, or in emergencies, such as antibiotic resistance genes.

Eukaryotic DNA

Eukaryotic DNA is linear and found in the nucleus, where it is very long.
DNA is associated with histone proteins (positively charged, globular) that help organise and condense it.
Other proteins associated with DNA include enzymes for DNA copying and repair.
The tightly coiled combination of DNA and proteins is called chromatin, which makes up chromatids and chromosomes.
Chromatids are organised into sections called genes.
The ends of chromatids are sealed with protective structures called telomeres.

Mitochondria & Chloroplast DNA

Mitochondria and chloroplasts contain their own DNA, called mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA).
This DNA is similar to prokaryotic DNA: it is short, circular, and not associated with proteins.
Mitochondrial DNA is located in the matrix of mitochondria.
Chloroplast DNA is located in the stroma of chloroplasts.

Genes

A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule.
Functional RNA molecules include mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
mRNA carries the base sequence used by ribosomes to form polypeptide chains.
tRNA carries amino acids to the ribosome during protein synthesis.
rRNA forms part of the structure of ribosomes.
The exact sequence of amino acids determines the shape and behaviour of a protein, so genes control protein structure and function.

Genetic Loci

The position of a gene on a chromosome is called its locus (plural: loci).
Each gene can exist in two or more different forms called alleles.
Different alleles of a gene have slightly different nucleotide sequences.
Alleles of the same gene always occupy the same locus on a chromosome.

The Triplet Code

The DNA nucleotide base code is a three-letter, or triplet, code; each triplet codes for one amino acid.
There are four bases (A, G, C, T), so there are 64 possible triplets (4³), but only 20 amino acids commonly occur in proteins.
Because multiple triplets can code for the same amino acid, the code is said to be degenerate; this can limit the effect of mutations.
Some triplets code for start (TAC – methionine) and stop signals, telling the cell where genes start and stop.
The code is non-overlapping, ensuring the cell reads the DNA correctly and produces the correct amino acid sequences.
The genetic code is universal: almost every organism uses the same code, so the same triplet codes for the same amino acid in all living organisms (with a few rare exceptions).

Non-Coding DNA

The eukaryotic genome contains many non-coding sections of DNA.
Non-coding DNA can be found between genes as non-coding multiple repeats (the same base sequences repeated multiple times).
Non-coding DNA can also be found within genes as introns, which sit between coding regions called exons.
During transcription, eukaryotic cells transcribe the whole gene (introns and exons) to produce pre-mRNA.
Before pre-mRNA exits the nucleus, introns are removed and exons are joined together in a process called splicing.

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Preguntas de práctica

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1.Which of the following is a difference between prokaryotic and eukaryotic DNA?
Easy
- AProkaryotic DNA is circular; eukaryotic DNA is linear.
- BProkaryotic DNA is linear; eukaryotic DNA is circular.
- CProkaryotic DNA is associated with histones; eukaryotic DNA is not.
- DProkaryotic DNA is longer than eukaryotic DNA.
2.Prokaryotic DNA is associated with histone proteins.
Easy
True or false?
3.Which type of DNA is found in the matrix of mitochondria?
Medium
- ANuclear DNA
- BMitochondrial DNA
- CChloroplast DNA
- DPlasmid DNA
4.Which of the following are features of mitochondrial DNA? (select all that apply)
Medium
- AShort
- BCircular
- CAssociated with histones
- DLinear
- ENot associated with protein
5.Match the DNA type with its location.
Medium
- Mitochondrial DNA
- Chloroplast DNA
- Nuclear DNA
- Mitochondrial matrix
- Stroma
- Nucleus
6.What is the term for the position of a gene on a chromosome?
Easy
- AAllele
- BLocus
- CChromatid
- DTelomere
7.Which property of the genetic code is illustrated by the fact that the codons ACU and ACG both code for the amino acid threonine?
Medium
- AUniversal
- BNon-overlapping
- CDegenerate
- DStart and stop signals
8.The genetic code is described as universal because almost every organism uses the same code.
Easy
True or false?

Más preguntas

9.Match each DNA-containing structure with where its DNA is found.
Easy
- Mitochondria
- Chloroplasts
- Prokaryotic cell
- Matrix
- Stroma
- Nucleoid
10.Which of the following statements about plasmids are correct? (select all that apply)
Medium
- APlasmids are small circular DNA molecules.
- BPlasmids contain a few genes.
- CPlasmids are always associated with histone proteins.
- DPlasmids often carry genes for antibiotic resistance.
- EPlasmids are long linear DNA molecules.
11.Which of the following are functions or properties of histones in eukaryotic chromosomes? (select all that apply)
Hard
- AThey are large, positively charged globular proteins.
- BThey organise and condense DNA so it fits into the nucleus.
- CThey are enzymes used in copying and repairing DNA.
- DThey form part of the telomeres that seal the ends of chromatids.
- EThey are small circular DNA molecules.
12.Place the following steps of eukaryotic gene expression in the correct order.
Medium
- The whole gene, including introns and exons, is transcribed to produce pre-mRNA.
- Introns are removed and exons are joined together by splicing.
- The mRNA exits the nucleus and is translated into a polypeptide.
13.Which of the following are features of the genetic code? (select all that apply)
Medium
- AIt is a triplet code.
- BIt is degenerate.
- CIt is non-overlapping.
- DIt is universal.
- EIt is read in overlapping pairs of bases.
14.Which of the following statements about mitochondrial DNA (mtDNA) are correct? (select all that apply)
Hard
- AIt is short and circular.
- BIt is not associated with protein.
- CIt is linear and associated with histones.
- DIt is located in the mitochondrial matrix.
- EIt is identical in structure to nuclear DNA.
15.Which of the following are types of non-coding DNA found in eukaryotic cells? (select all that apply)
Medium
- AIntrons
- BExons
- CNon-coding multiple repeats
- DTelomeres
- EPlasmids
16.Which of the following molecules are functional RNA molecules? (select all that apply)
Medium
- AmRNA
- BtRNA
- CrRNA
- DDNA polymerase
- EHistone proteins
17.Which of the following are properties of prokaryotic DNA molecules? (select all that apply)
Medium
- AShort
- BLinear
- CAssociated with histones
- DCircular
- ENon-coding regions present
18.Which of the following statements about the genetic code are correct? (select all that apply)
Medium
- AThere are 64 different triplets possible.
- BThere are 20 amino acids commonly occurring in biological proteins.
- CMultiple triplets can code for the same amino acid.
- DSome triplets code for start and stop signals.
- EThe code is overlapping.
19.Which of the following are differences between mitochondrial DNA and nuclear DNA? (select all that apply)
Hard
- AMitochondrial DNA is circular; nuclear DNA is linear.
- BMitochondrial DNA is not associated with histones; nuclear DNA is associated with histones.
- CMitochondrial DNA is shorter than nuclear DNA.
- DMitochondrial DNA is found in the nucleus; nuclear DNA is found in the matrix.
- EMitochondrial DNA contains introns; nuclear DNA does not.
20.Which of the following statements about genes and alleles are correct? (select all that apply)
Medium
- AA gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide.
- BA gene can also code for a functional RNA molecule.
- CAlleles of a gene occupy different loci on a chromosome.
- DDifferent alleles have slightly different nucleotide sequences.
- EEach gene can exist in two or more different forms called alleles.
21.Which of the following are components of a eukaryotic chromosome? (select all that apply)
Medium
- ADNA
- BHistone proteins
- CEnzymes used in copying and repairing DNA
- DTelomeres
- EPlasmids
22.Which of the following statements about the triplet code are correct? (select all that apply)
Medium
- AEach triplet codes for one amino acid.
- BThe code is read as a series of non-overlapping triplets.
- CSome triplets code for start and stop signals.
- DThe triplet TAC codes for methionine.
- EThe code is read as overlapping triplets.
23.Which of the following are roles of non-coding DNA in eukaryotic cells? (select all that apply)
Hard
- AIntrons are removed during splicing.
- BNon-coding multiple repeats are found between genes.
- CExons are joined together after introns are removed.
- DNon-coding DNA codes for the amino acid sequence of a polypeptide.
- ENon-coding DNA is transcribed into pre-mRNA.
24.Which of the following are features of eukaryotic DNA? (select all that apply)
Medium
- AIt is linear.
- BIt is associated with histone proteins.
- CIt is circular.
- DIt contains non-coding regions.
- EIt is not associated with any proteins.
25.Which of the following are found in prokaryotic cells? (select all that apply)
Medium
- ASingle circular chromosomal DNA
- BPlasmids
- CHistone-associated linear DNA
- DNucleoid
- EIntrons
26.A DNA molecule is measured to be 2 nm wide. In an electron micrograph, the same molecule appears 1.5 mm wide. What is the magnification of the electron micrograph?
Medium
- A×750
- B×750,000
- C×7,500
- D×75,000
27.A gene has the DNA base sequence TAC. What is the corresponding mRNA codon?
Easy
- AATG
- BAUG
- CTAC
- DUAC
28.The DNA of a prokaryote is best described as:
Easy
- Alinear and associated with histones
- Bcircular and not associated with proteins
- Clinear and not associated with proteins
- Dcircular and associated with histones
29.During transcription, eukaryotic cells transcribe the whole gene, including introns and exons, to produce pre-mRNA.
Easy
True or false?
30.Match each DNA component with its correct description.
Medium
- Exon
- Intron
- Telomere
- Plasmid
- Coding region of a gene
- Non-coding region within a gene
- Protective structure at the end of a chromatid
- Small circular DNA molecule in prokaryotes
31.Place the following events in the correct order for the expression of a eukaryotic gene.
Medium
- Transcription of the whole gene to form pre-mRNA
- Splicing to remove introns and join exons
- mRNA exits the nucleus
- Translation of mRNA into a polypeptide
32.A sequence of DNA bases from a eukaryotic cell is: AAA CAG GAC ATC AAT TGG AGC ATA GGT CCA. What is the maximum number of amino acids for which this piece of DNA could code?
Medium
- A10
- B30
- C20
- D60
33.A student prepares a stained root tip squash and observes the cells under a light microscope. Which feature would allow the student to identify a prokaryotic cell rather than a eukaryotic cell in the sample?
Medium
- ADNA that is not associated with histone proteins
- BDNA that is linear and associated with histones
- CDNA contained within a membrane-bound nucleus
- DDNA organised into chromatids sealed by telomeres
34.A scientist extracts DNA from a cell and finds that the DNA molecules are short, circular and not associated with protein. The DNA is located within a double-membraned organelle that contains stacks of thylakoids. Where is this DNA located?
Medium
- AThe stroma of a chloroplast
- BThe matrix of a mitochondrion
- CThe nucleoid of a prokaryote
- DThe nucleus of a eukaryotic cell
35.A sample of DNA was hydrolysed and the resulting bases were separated by chromatography. The scientist counted 640 thymine bases and 360 guanine bases in the sample. How many cytosine bases would be expected in the same sample?
Hard
- A360
- B640
- C1000
- D280
36.A gene from a human was inserted into a bacterial plasmid and the bacterium successfully produced the human protein. Which property of the genetic code makes this possible?
Medium
- AThe code is universal.
- BThe code is degenerate.
- CThe code is non-overlapping.
- DThe code contains stop signals.
37.A length of pre-mRNA transcribed from a eukaryotic gene contains 900 nucleotides. After splicing, 300 nucleotides have been removed. Assuming the mature mRNA is translated from the first start codon with no untranslated regions, what is the maximum number of amino acids in the resulting polypeptide?
Hard
- A200
- B300
- C100
- D600
38.A student is preparing a slide of onion root tip cells to observe chromosomes. Which of the following steps are required to produce a valid root tip squash? (select all that apply)
Hard
- AStaining the root tip so that the chromosomes become visible
- BWarming the root tip in hydrochloric acid to soften the tissue
- CSquashing the root tip to spread the cells into a single layer
- DUsing a coverslip and pressing firmly to separate the cells
- EAdding a drop of water to dissolve the histone proteins
39.A scientist wishes to isolate mitochondrial DNA (mtDNA) from a sample of eukaryotic cells. Which of the following statements about the DNA obtained are correct? (select all that apply)
Medium
- AIt is short and circular.
- BIt is not associated with histone proteins.
- CIt is found in the mitochondrial matrix.
- DIt is linear and found in the nucleus.
- EIt contains introns that must be spliced out.
40.Plasmids are small circular DNA molecules that often carry genes for antibiotic resistance.
Easy
True or false?
41.State what a gene codes for.
Easy
42.The genetic code uses four different DNA bases. State the maximum number of different DNA triplets that can be made using these four bases.
Medium
43.Name the two non-coding regions of eukaryotic nuclear DNA.
Easy
The two non-coding regions are ____ and ____.
44.Name the cellular structure that contains DNA in a prokaryotic cell, in addition to plasmids.
Medium
45.A polypeptide is 250 amino acids long. Calculate the minimum number of DNA bases needed to code for it.
Medium
46.Complete the sentences about the genetic code.
Medium
The genetic code is described as ____ because more than one triplet can code for the same amino acid. It is described as ____ because almost all organisms use the same code.
47.Papain is an enzyme that is 345 amino acids long. Calculate the minimum number of DNA bases needed to code for papain.
Medium
48.Myoglobin is a eukaryotic protein consisting of a single polypeptide chain of 153 amino acids. Calculate the minimum number of DNA bases needed to code for myoglobin.
Medium
49.In an electron micrograph, a DNA molecule appears 1.5 mm wide, however the actual width of a DNA molecule is 2 nm. What is the magnification?
Hard
50.Complete the sentences about the DNA found in prokaryotic cells.
Easy
Prokaryotic cells contain a single ____ chromosomal DNA molecule, and they usually also contain small circular DNA molecules called ____.
51.Complete the sentence about the structure of eukaryotic chromosomes.
Medium
In eukaryotic chromosomes, DNA is tightly coiled around large, positively charged globular proteins called ____.
52.Complete the sentences about mitochondrial and chloroplast DNA.
Medium
Mitochondrial DNA is located in the ____ of mitochondria, whereas chloroplast DNA is found in the ____ of chloroplasts.
53.Describe two differences between the DNA in prokaryotic cells and the DNA in eukaryotic cells.
Medium
54.Describe how the structure of a eukaryotic chromosome is organised, including the roles of histones and telomeres.
Hard
55.Explain why the genetic code is described as being universal.
Medium
56.State where telomeres are found within a chromosome as well as their function.
Hard
57.A scientist treated a culture of eukaryotic cells with a drug that prevents the removal of introns from pre-mRNA. Explain how this would affect the protein produced from a gene in these cells.
Hard
58.The genetic code uses four different DNA bases. State the maximum number of different DNA triplets that can be made using these four bases.
Medium
- A76
- B58
- C70
- D64
59.A polypeptide is 250 amino acids long. Calculate the minimum number of DNA bases needed to code for it.
Medium
- A750 bases
- B900 bases
- C675 bases
- D825 bases
60.Papain is an enzyme that is 345 amino acids long. Calculate the minimum number of DNA bases needed to code for papain.
Medium
- A1139 bases
- B1243 bases
- C931 bases
- D1035 bases
61.Myoglobin is a eukaryotic protein consisting of a single polypeptide chain of 153 amino acids. Calculate the minimum number of DNA bases needed to code for myoglobin.
Medium
- A505 bases
- B459 bases
- C413 bases
- D551 bases
62.In an electron micrograph, a DNA molecule appears 1.5 mm wide, however the actual width of a DNA molecule is 2 nm. What is the magnification?
Hard
- A750000 x
- B825000 x
- C900000 x
- D675000 x

Apuntes de repaso

## DNA, Genes & Chromosomes ### Prokaryotic DNA - Single, **circular** DNA molecule (nucleoid); not associated with proteins. - Also have **plasmids**: small circular DNA molecules with a few genes (e.g. antibiotic resistance). ### Eukaryotic DNA - Long, **linear** DNA molecules in the nucleus, associated with **histones** (positively charged globular proteins) that condense DNA. - DNA + proteins = **chromatin**; chromatids/chromosomes are made of chromatin. - Ends of chromatids are sealed by **telomeres**. ### Mitochondrial & Chloroplast DNA - **mtDNA** in mitochondrial matrix; **cpDNA** in chloroplast stroma. - Similar to prokaryotic DNA: short, circular, not associated with protein. ### Genes - A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule (mRNA, tRNA, rRNA). - One gene → one mRNA → one polypeptide. ### Genetic Loci - **Locus** = position of a gene on a chromosome. - **Alleles** = different forms of a gene; slightly different nucleotide sequences but same locus. ### The Triplet Code - Three-letter code: each triplet codes for one amino acid. - 4 bases → 4³ = **64** possible triplets; only 20 amino acids → code is **degenerate** (multiple triplets per amino acid). - Start (TAC – methionine) and stop signals; code is **non-overlapping**. - **Universal**: almost all organisms use the same code (rare exceptions). ### Non-Coding DNA - Non-coding DNA between genes (multiple repeats) and within genes as **introns** (coding regions = **exons**). - During transcription, whole gene → pre-mRNA; introns removed and exons joined by **splicing** before translation. ### Key calculations - Minimum DNA bases = number of amino acids × 3. - Magnification = image size ÷ actual size (convert units). ### Exam tips - Distinguish DNA triplets from mRNA codons (complementary base sequence). - Know differences between prokaryotic and eukaryotic DNA for compare questions.

Prokaryotic DNA: Single, circular DNA molecule (nucleoid) not associated with proteins; plasmids may also be present.
Plasmid: Very small circular DNA molecule containing a few genes, often including antibiotic resistance genes.
Eukaryotic DNA: Long, linear DNA molecules in the nucleus, associated with histone proteins.
Histone: Large, positively charged globular protein that organises and condenses DNA so it fits into the nucleus.
Chromatin: The tightly coiled combination of DNA and proteins that makes up chromatids and chromosomes.
Telomere: Protective structure that seals the ends of chromatids in chromosomes.
Mitochondrial DNA (mtDNA): Short, circular DNA not associated with protein, located in the mitochondrial matrix.
Chloroplast DNA (cpDNA): Short, circular DNA not associated with protein, located in the stroma of chloroplasts.
Gene: A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule.
Locus: The position of a gene on a chromosome.
Allele: Different forms of a gene with slightly different nucleotide sequences but occupying the same locus.
Triplet code: The three-letter DNA base code within a gene; each triplet codes for one amino acid.
Degenerate: Multiple triplets code for the same amino acid, limiting the effect of mutations.
Universal: Almost every organism uses the same genetic code, so the same triplet codes for the same amino acid.
Non-overlapping: Each base is read only once in the triplet code, ensuring correct amino acid sequences.
Intron: Non-coding region within a gene that is removed during splicing.
Exon: Coding region within a gene that is joined to other exons during splicing.
Splicing: Process where introns are removed and exons joined before pre-mRNA exits the nucleus.

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