Biology UNIT 4 LG1 & LG 2

What is DNA and what is its function?

A double-stranded nucleic acid that stores genetic information.

What is a nucleotide?

A sugar + phosphate + nitrogenous base.

What are the four DNA bases and how do they pair?

A pairs with T; G pairs with C. Base pairs are held by hydrogen bonds.

What does antiparallel mean?

DNA strands run in opposite directions: 5′→3′ and 3′→5′.

What are a gene, allele and gene locus?

Gene = DNA sequence coding for a functional product; allele = alternative form of a gene; locus = gene's position on a chromosome.

What are homologous chromosomes and sister chromatids?

Homologous chromosomes = chromosome pair, one from each parent, with the same genes but potentially different alleles. Sister chromatids = identical copies joined at the centromere.

What are centromeres and telomeres?

Centromere = joins sister chromatids. Telomeres = repetitive DNA at chromosome ends that protect chromosomes.

What are histones, nucleosomes and chromatin?

Histones = proteins DNA wraps around; nucleosome = DNA wrapped around histones; chromatin = DNA + proteins that package DNA.

What are circular chromosomes and plasmids?

Circular chromosomes are circular DNA molecules found in prokaryotes and mitochondria/chloroplasts. Plasmids are small circular DNA molecules separate from the main chromosome.

What are introns, exons and the promoter region?

Introns = non-coding regions removed from pre-mRNA; exons = regions retained in mature mRNA; promoter = DNA region where transcription begins and transcription factors/RNA polymerase bind.

Why does DNA replicate and what is semiconservative replication?

DNA replicates before cell division. Each new DNA molecule contains one original strand and one newly synthesised strand.

What do helicase and DNA polymerase do?

Helicase breaks hydrogen bonds to separate DNA strands. DNA polymerase adds complementary nucleotides to the new strand.

What are the leading and lagging strands?

Leading strand = synthesised continuously. Lagging strand = synthesised discontinuously in Okazaki fragments.

What are Okazaki fragments and what does DNA ligase do?

Okazaki fragments are short DNA sections on the lagging strand. DNA ligase joins them together.

Why is DNA synthesis directional?

DNA polymerase adds nucleotides only to the 3′ end, so new DNA is synthesised 5′→3′.

What is a mutation and what can cause one?

A change in DNA. It can result from DNA replication errors or physical/chemical environmental factors.

What are point and frameshift mutations?

Point mutation = change involving one nucleotide/base pair. Frameshift mutation = insertion/deletion that shifts the reading frame.

What are silent, missense and nonsense mutations?

Silent = no amino-acid change; missense = changes an amino acid; nonsense = creates a premature stop codon.

Why can frameshift mutations have major effects?

They change the reading frame, altering downstream codons and potentially many amino acids.

What are the two stages of protein synthesis?

Transcription and translation.

What happens during transcription and what enzyme is involved?

A gene's DNA sequence is copied into mRNA in the nucleus by RNA polymerase.

What happens during RNA processing?

Introns are removed, exons joined, and a 5′ cap and poly-A tail are added to produce mature mRNA.

What are codons and anticodons?

Codon = three bases on mRNA coding for an amino acid or stop signal. Anticodon = complementary three-base sequence on tRNA.

What are tRNA and ribosomes?

tRNA carries specific amino acids to the ribosome. Ribosomes read mRNA and assemble the polypeptide.

What are the start and stop codons?

Start = AUG (methionine). Stop = UAA, UAG and UGA.

What is translation?

A ribosome reads mRNA codons and links amino acids together to form a polypeptide.

What is the overall flow of protein synthesis?

DNA

How can a DNA mutation affect a polypeptide?

The DNA change can alter mRNA codons, changing the amino-acid sequence and potentially protein structure/function.

What is gene expression and why do cells express different genes?

Gene expression is using a gene's information to produce a functional product. Different genes are expressed in different cells, producing different proteins and specialised functions.

What is gene regulation and what are transcription factors?

Gene regulation controls when, where and how much a gene is expressed. Transcription factors are proteins that bind DNA near promoters to regulate transcription.

What is epigenetic regulation?

Regulation of gene activity without changing the DNA sequence.

What are euchromatin and heterochromatin?

Euchromatin = loosely packed, accessible DNA where genes are generally active. Heterochromatin = tightly packed, less accessible DNA where genes are generally inactive.

How does histone modification affect gene expression?

Chemical changes to histones alter how tightly DNA is packed, affecting whether genes can be expressed.

How does DNA methylation generally affect gene expression?

It reduces gene expression by making transcription less likely.

How does gene regulation allow cell differentiation?

Different genes are switched on/off, causing cells to produce different proteins and develop different structures and functions.

What are HOX genes and how do they regulate morphology?

HOX genes are master regulatory genes that produce transcription factors controlling other genes involved in body development, structure and positioning.

How do you determine the effect of a mutation using the genetic code?

Identify the DNA change