Hbio cell replication dna n shi

What are the three components of a DNA nucleotide?

Deoxyribose sugar, phosphate group and nitrogenous base.

What are the four DNA bases and their complementary pairs?

A pairs with T; C pairs with G.

What type of bond holds complementary bases together?

Hydrogen bonds.

What type of bond forms the DNA sugar-phosphate backbone?

Covalent phosphodiester bonds.

What does it mean that DNA is double-stranded and antiparallel?

DNA consists of two complementary strands running in opposite directions.

What is a gene?

A section of DNA containing information used to produce a functional product.

What is a chromosome?

A long DNA molecule associated with proteins and containing many genes.

What are homologous chromosomes?

A maternal and paternal chromosome pair containing the same genes at corresponding locations.

What is the difference between chromosomal and mitochondrial DNA?

Chromosomal DNA is mainly nuclear and arranged into linear chromosomes; mitochondrial DNA is found in mitochondria and is generally circular.

How is mitochondrial DNA generally inherited?

Primarily from the mother.

What is the role of DNA in cells?

DNA stores genetic information and provides instructions for producing functional products such as proteins.

What is the genetic code?

The rules by which mRNA codons specify amino acids or stop signals.

What is a codon and where is it found?

A three-base sequence on mRNA that specifies an amino acid or stop signal.

What is an anticodon and where is it found?

A three-base sequence on tRNA complementary to an mRNA codon.

What does AUG do?

It codes for methionine and commonly acts as the start codon.

What is semi-conservative DNA replication?

Each new DNA molecule contains one original strand and one newly synthesised strand.

When does DNA replication occur?

During S phase of interphase.

What does helicase do?

Breaks hydrogen bonds and separates the two DNA strands.

What does primase do?

Adds an RNA primer to provide a starting point for DNA synthesis.

What does DNA polymerase do?

Adds complementary DNA nucleotides to the growing strand.

What does DNA ligase do?

Joins DNA fragments, particularly Okazaki fragments.

Why are Okazaki fragments produced?

DNA polymerase can only synthesise DNA in one direction, so the lagging strand is produced discontinuously.

What is the basic sequence of DNA replication?

Helicase separates strands → primase adds primers → DNA polymerase adds nucleotides → ligase joins fragments.

What would happen if DNA polymerase stopped working?

New DNA strands could not be properly synthesised.

What is transcription?

The process of producing mRNA using a DNA strand as a template.

Where does transcription occur and what enzyme is involved?

Transcription occurs in the nucleus and uses RNA polymerase.

What base does RNA use instead of thymine?

Uracil (U).

If the DNA template is TAC GGA, what is the mRNA sequence?

AUG CCU.

What is translation?

The process of using mRNA information to assemble a polypeptide at a ribosome.

Where does translation occur?

At ribosomes in the cytoplasm.

What are the roles of mRNA, tRNA and ribosomes?

mRNA carries codons; tRNA carries specific amino acids and has complementary anticodons; ribosomes read mRNA and facilitate peptide-bond formation.

What bond joins amino acids together?

Peptide bonds.

What happens at a stop codon?

Translation terminates and the polypeptide is released.

What is protein synthesis?

The production of proteins through transcription and translation.

What is gene expression?

The process by which information in a gene is used to produce a functional product.

Why can two cells with the same DNA have different structures and functions?

They express different genes and therefore produce different proteins.

What is epigenetics?

Changes in gene expression or regulation that occur without changing the DNA base sequence.

What is the general effect of histone acetylation?

It loosens chromatin, making DNA more accessible and generally increasing transcription.

What is the general effect of DNA methylation?

It generally reduces transcription and gene expression when associated with gene regulatory regions.

What is the difference between an epigenetic change and a mutation?

An epigenetic change alters gene regulation without changing the DNA sequence; a mutation changes the DNA sequence.

What are the main stages of the cell cycle?

G1 → S → G2 → M.

What happens during G1, S and G2?

G1 = growth/normal functions; S = DNA replication; G2 = preparation for division and checking DNA.

What happens during M phase?

Mitosis followed by cytokinesis.

What is the purpose of mitosis?

Growth, tissue repair and replacement of cells.

What are the four stages of mitosis?

Prophase → Metaphase → Anaphase → Telophase.

What happens during prophase?

Chromosomes condense, the nuclear envelope breaks down and the spindle forms.

What happens during metaphase?

Chromosomes line up at the equator.

What happens during anaphase?

Sister chromatids separate and move to opposite poles.

What happens during telophase?

Nuclear envelopes reform around the chromosomes.

What is cytokinesis?

Division of the cytoplasm to form separate daughter cells.

What is the result of mitosis?

Two genetically similar daughter cells with the same chromosome number as the parent cell.

How many chromosomes are in a normal human somatic cell?

46 chromosomes.

What is a stem cell?

An undifferentiated cell capable of self-renewal and differentiation.

What is differentiation?

The process by which an unspecialised cell develops specialised structures and functions.

What does stem-cell potency mean?

The range of different cell types a stem cell can potentially produce.

What is the order of stem-cell potency from greatest to least?

Totipotent → pluripotent → multipotent → unipotent.

What can a totipotent stem cell produce?

All cell types, including extra-embryonic tissues.

What can a pluripotent stem cell produce?

Almost all body cell types, but not the complete set needed to form an entire organism.

What can a multipotent stem cell produce?

A limited range of related cell types.

How does gene expression relate to stem-cell differentiation?

Different genes are expressed, producing different proteins and causing cells to become specialised.

What is cancer?

Uncontrolled abnormal cell growth and division resulting from accumulated genetic changes.

How can mutations cause cancer?

Mutations can disrupt genes controlling cell division, DNA repair or cell death.

What is the difference between benign and malignant tumours?

Benign tumours generally remain localised; malignant tumours can invade tissues and spread.

What is metastasis?

The spread of malignant cancer cells from the original site to other parts of the body.

Give two cancer risk factors and two prevention strategies.

Risk factors include UV radiation and tobacco smoke; prevention includes UV protection and avoiding tobacco smoke.

Why is early cancer detection important?

It allows treatment to begin earlier, potentially before the cancer progresses or spreads.

What is the purpose of meiosis?

To produce haploid gametes for sexual reproduction and generate genetic variation.

How many DNA replications occur before meiosis?

One.

How many divisions occur in meiosis?

Two: Meiosis I and Meiosis II.

What is the final result of meiosis?

Four genetically varied haploid cells.

How many chromosomes are in a human gamete?

23.

What happens during Prophase I?

Homologous chromosomes pair and crossing over occurs.

What happens during Metaphase I?

Homologous chromosome pairs line up randomly at the equator.

What separates during Anaphase I?

Homologous chromosomes separate while sister chromatids remain together.

What separates during Anaphase II?

Sister chromatids separate.

Is DNA replicated between Meiosis I and II?

No.

What is crossing over?

The exchange of DNA between non-sister chromatids of homologous chromosomes.

When does crossing over occur?

Prophase I.

How does crossing over increase variation?

It creates new combinations of alleles on chromosomes.

What is independent assortment?

The random orientation and separation of homologous chromosome pairs during Meiosis I.

When does independent assortment occur?

Metaphase I.

How does independent assortment create variation?

It produces different combinations of maternal and paternal chromosomes in gametes.

What is non-disjunction?

Failure of homologous chromosomes or sister chromatids to separate correctly during meiosis.

What can non-disjunction produce?

Gametes with abnormal chromosome numbers, such as n + 1 or n − 1.

What is trisomy?

Having three copies of a particular chromosome instead of two.

What is monosomy?

Having one copy of a particular chromosome instead of two.

What is an example of trisomy?

Down syndrome, caused by trisomy 21.

What are the three major sources/consequences of meiotic variation?

Crossing over creates new allele combinations; independent assortment creates different chromosome combinations; non-disjunction creates abnormal chromosome numbers.

What is the main difference between mitosis and meiosis?

Mitosis has one division producing two genetically similar cells and maintains chromosome number; meiosis has two divisions producing four genetically varied haploid cells.

What separates during mitotic Anaphase compared with meiotic Anaphase I?

Mitosis separates sister chromatids; Meiosis I separates homologous chromosomes.

Compare the purpose of mitosis and meiosis.

Mitosis is mainly for growth, repair and replacement; meiosis produces gametes for sexual reproduction.

Why is meiosis important for sexual reproduction?

It halves chromosome number so fertilisation can restore the diploid number.

Why does meiosis create genetic variation?

Because of crossing over and independent assortment; non-disjunction can also alter chromosome number.

A cell has highly condensed chromatin around a gene. What would you expect?

Lower transcription because the DNA is less accessible.

A gene becomes methylated without changing its DNA sequence. What type of change is this?

An epigenetic change, generally associated with reduced gene expression.

A human gamete has 24 chromosomes. What could have caused this?

Non-disjunction during meiosis.

If a 24-chromosome gamete fuses with a normal 23-chromosome gamete, how many chromosomes will the zygote have?

47 chromosomes.

Why can a DNA mutation have no effect on the amino acid sequence?

The genetic code is degenerate, so different codons can specify the same amino acid.

What is the sequence of genetic information from DNA to protein?

DNA → transcription → mRNA → translation → polypeptide/protein.

What is the sequence of the major cell-division processes?

Cell cycle → DNA replication → mitosis or meiosis → cytokinesis.